Here we present a roadmap on Photonic metasurfaces. This document consists of a number of perspective articles on different applications, challenge areas or technologies underlying photonic metasurfaces. Each perspective will introduce the topic, present a state of the art as well as give an insight into the future direction of the subfield.

Sebastian A. Schulz,* Rupert F. Oulton,** and Mitchell Kenney***

*sas35@st-andrews.ac.uk

**r.oulton@imperial.ac.uk

***Mitchell.Kenney@nottingham.ac.uk

Over the last decade, the topic of metasurfaces has flourished like no other field in optics. This is primarily due to the appeal of compact flat optical components with wide design flexibility inherent to these artificial two-dimensional materials. As can be seen in this roadmap, metasurfaces can be made from a wide range of materials, from metals and other plasmonic materials to dielectrics and even flexible and conformable platforms. In all cases, a metasurface's overall properties follow not only from the those of the constituent materials but also from the shape and arrangement of meta-atoms, engineered on nanoscopic dimensions. The broad range of metasurface applications available stem from the rich physics that emerges from the arrangement and geometry of the meta-atoms; by arranging these into periodic or disordered distributions, that display local or non-local responses, as well as exhibiting conventional or topological characteristics, these “design parameters” allow us to influence light's amplitude, phase, polarization, and nonlinearity.

This document offers a comprehensive overview of the physics and applications of photonic metasurfaces which sets the context for discussion of future research directions. As commissioning editors, we have invited 20 perspectives from the community, where each has given equal importance to reviewing the historic development of the field, presenting highlights of current research and then pointing to future challenges and outlook. Andrea Alu sets the scene with a general introduction (I). The roadmap is then organized into three sections of perspectives: ten perspectives on the applications of photonic metasurfaces; three perspectives on the fabrication and material platforms; and finally, six perspectives on the emerging topics and the theory of metasurfaces. The roadmap explore several key application areas for metasurfaces, including metasurfaces for: the control of light emission (II); nonlinear optics (III); biosensing (IV); passive radiative cooling (V); photocatalysis (VI); orbital angular momentum and wavefront control (VII); holography and structural color (VIII); augmented reality (IX); quantum state generation (X); and finally metasurface based lasers (XI). The section on fabrication and material platforms includes solution derived metasurface fabrication (XII); semiconductor based metasurfaces (XIII); and flexible metasurfaces (XIV). The final six perspectives explore metasurfaces for or with time modulation (XV); disorder (XVI); epsilon-near-zero characteristics (XVII); deep learning (XVIII); complex-valued singularities (XIX); and, last but not least, multipolar characteristics (XX).

In contrast to previous reviews (e.g., Refs. 1–3) and roadmaps,4 which focus exclusively on either past or future development of the field, we have aimed instead to provide a comprehensive overview that gives equal weight to past, present and future of the selected topics. This gives the necessary context for the discussion of future research directions and outlook whilst also providing a valuable up to date resource by indexing the rapidly amassed literature in this field.

This roadmap is truly the culmination of a world-wide effort of leading contributors from across the metasurface community. We are grateful for their time, effort, and dedication to this compilation of perspectives. We hope that this Roadmap on Photonic Metasurfaces will provide you—the reader—with an up to date reference of the current status, understanding, and direction of photonic metasurface research; whether you are already an experienced researcher looking for a contemporary summary of the research and direction of the field or whether you are looking for a comprehensive introduction to this fascinating area.

Andrea Alù

aalu@gc.cuny.edu

The field of photonics relies on gaining control over light flows to advance a wide and rapidly growing range of technologies, including energy, sensing, imaging, and computing. Photonics research has been recently unveiling exciting opportunities both in the context of basic science and of its engineering and technological impact. A major role in this recent progress has been played by ultrathin surfaces engineered at the nanoscale, known as metasurfaces,5 which have been enhancing the way we tailor optical wavefronts over an ultrathin platform, opening a paradigm of compactification of photonic devices for extreme light control. In turn, metasurfaces have also been unveiling unique forms of light–matter interactions emerging from their subwavelength light confinement and extreme light control at the nanoscale. The recent surge of interest for metasurfaces in the photonics community has been driven by the introduction of the “generalized Snell's laws of refraction,”5,6 according to which a transverse gradient of phase discontinuities imparted by a tailored array of nanostructures can offer enhanced control over wave transmission through a planar interface. These concepts have been building on well-established technologies in the realm of microwave components7,8 and “flat” diffractive optical elements,9,10 established fields of research for several decades. Over the years, more sophisticated metasurface designs have been demonstrating further mastery over all properties of light, enhancing efficiency, bandwidth, polarization, and wavefront control through metasurfaces,11–16 which, in turn, have been translated into ultrathin devices for lensing, holograms and lasing, among many other applications.

While the early demonstrations of metasurfaces were mostly based on plasmonic nanostructures, which enable field enhancement and light focusing at deeply subwavelength scales, the associated material loss have driven the field toward alternative materials, including high-index dielectrics and doped semiconductors. Today, the possibilities offered by metasurfaces to efficiently control light with nanoscale resolution over an ultrathin platform have translated into myriads of opportunities, not only limited to high-profile academic research, but also to commercial companies.17,18 The progress in this area has been truly impressive, leveraging the combination of theoretical advances in our understanding of the interactions of light with nanostructures, and progress in nanofabrication of a wide range of materials, also over large area. In turn, the discoveries driven by photonic metasurface research have also translated back to lower frequencies, where the fields of reflect- and transmit-arrays, as well as frequency-selective surfaces, originally demonstrated wavefront and spectrum control for radio-waves, driving the interest in a new wave of hardware platforms known as reconfigurable intelligent surfaces for wireless communications.19 

The first demonstrations of optical metasurfaces, e.g., Fig. 1(a), were limited in terms of efficiency and of functionalities, due to a variety of challenges. For the most part, they relied on metallic resonant elements, associated with significant absorption, and on polarization conversion mechanisms, which tend to be inefficient and limited by symmetries, to control the amplitude and phase of the scattered fields. The overall efficiency and the type of transformations that could be implemented on the incoming wavefront were significantly limited in these first demonstrations.14 By contrast, over the years new generations of photonic metasurfaces have flourished into a plethora of exciting functionalities for holography,20 multi-functional and multi-wavelength operation,21 lensing and imaging,22 and several other impressive demonstrations of complex wavefront shaping using ultrathin optical devices, becoming competitive with bulky optical technologies in terms of several performance metrics. Figure 1 shows a few demonstrations of the state of the art of this technology, in which tailored nanostructured apertures are able to imprint the desired wavefront to the electromagnetic fields at the wavelength of interest. Figure 1(b), for instance, shows high-efficiency holograms created by tailored nanostructured surfaces, and Fig. 1(c) an ultrathin metalens that can focus light at multiple frequencies.

FIG. 1.

Metasurfaces for wavefront control. (a) Generalized laws of refraction for beam steering through phase gradients (reproduced with permission from Yu et al., Science 334, 333 (2011). Copyright 2011 AAAS;5 (b) metasurface holograms with high efficiency (from Ref. 20); (c) multi-wavelength, polarization-insensitive metalenses (from Ref. 21). All images are reprinted (adapted) with permission from the respective Journal and copyright remains with the original publisher.

FIG. 1.

Metasurfaces for wavefront control. (a) Generalized laws of refraction for beam steering through phase gradients (reproduced with permission from Yu et al., Science 334, 333 (2011). Copyright 2011 AAAS;5 (b) metasurface holograms with high efficiency (from Ref. 20); (c) multi-wavelength, polarization-insensitive metalenses (from Ref. 21). All images are reprinted (adapted) with permission from the respective Journal and copyright remains with the original publisher.

Close modal

Several design principles have been explored to tailor the optical wavefront, spanning from coupled resonances locally tuned to control the amplitude and phase23 to polarization conversion5 and geometric phase concepts,24 each coming with their own advantages and tradeoffs. For instance, coupled resonances tend to introduce unwanted frequency dispersion, modulating the scattering amplitude as we tailor the local phase of each metasurface elements. Polarization conversion can powerfully tailor the phase of the scattered fields, but its efficiency is typically limited. Geometric phase concepts can partially address these challenges, but they are conventionally limited to circularly polarized responses.

While all these design principles offer a powerful playground for wavefront control, metasurfaces often face fundamental constraints in terms of efficiency and bandwidth of wavefront manipulation, which stem from their ultrathin footprint and inherent symmetry constraints. Over the years, several of these bounds have been unveiled,11,14,25,26 indicating that extreme compactification of optical devices comes at a price, which needs to be taken into account when compared to other more bulky solutions. For instance, it is hard to imagine that a fully metasurface-based approach may be able to compete with a sophisticated EOS camera lens, but metasurfaces may be able to replace specific components. Hybrid approaches leveraging metasurfaces appear to be certainly competitive, and metasurfaces may be excellent at specific tasks, or for targeted wavelengths of operation.

While the conventional approach to metasurface design has been the one of locally patterning the aperture with nanoscale resonators that control point by point the response, a few more recent approaches have been opening new design degrees of freedom. For instance, metagratings27,28 have been demonstrating that extreme control over the optical wavefront can be achieved with high efficiency, for instance yielding near-grazing beam steering. Metagratings are based on grating resonances that stem from an overarching long-range periodicity of the structure, while the individual metasurface elements are designed to control the way the incident wavefront locally couples to the available diffraction orders. These principles have been further empowered by nonlocal metasurfaces, in which the coupling among distant elements across the metasurface aperture is not fought against, but rather leveraged through more sophisticated design principles that exploit lattice resonances locally perturbed by symmetry-breaking defects.29 Nonlocal metasurfaces provide a more sophisticated control over the spectral response, and open new opportunities for wavefront manipulation, including wavefront selectivity,30 and multi-functionality.31 

The recent progress in metasurface design principles, fabrication and also tunability of their response has led to a surge of exciting applications. Figure 2 summarizes a few relevant highlights of metasurfaces that realize complex structured wavefronts, relevant for e.g., optical communications and holography,32 for eye tracking applications,33 for quantum technologies,36 for catalysis and chemical reactions,34 and for augmented reality (AR).35 These breadth of applications holds the promise of a vibrant future for metasurfaces, opening exciting prospects for the near future.

FIG. 2.

(a) Metasurface for complex wavefront shaping to impart orbital angular momenta to the impinging light (reproduced with permission from Ren et al., Nat. Commun. 10, 2986 (2019). Copyright 2019 Springer Nature Publishing Group;32 (b) nonlocal metasurface for eyetracking applications (from Ref. 33); (c) concept of metasurfaces to facilitate chemical processes (from Ref. 34); (d) metasurface for augmented reality applications (from Ref. 35). All images are reprinted (adapted) with permission from the respective Journal and copyright remains with the original publisher.

FIG. 2.

(a) Metasurface for complex wavefront shaping to impart orbital angular momenta to the impinging light (reproduced with permission from Ren et al., Nat. Commun. 10, 2986 (2019). Copyright 2019 Springer Nature Publishing Group;32 (b) nonlocal metasurface for eyetracking applications (from Ref. 33); (c) concept of metasurfaces to facilitate chemical processes (from Ref. 34); (d) metasurface for augmented reality applications (from Ref. 35). All images are reprinted (adapted) with permission from the respective Journal and copyright remains with the original publisher.

Close modal

The opportunities for photonic metasurfaces have been growing by the day, fostered by continuous progress on multiple fronts, which drives the future of this research area. From the modeling perspective, enhancing the ultimate performance of metasurfaces requires sophisticated modeling tools that can capture and optimize their design. Given that most metasurfaces are non-periodic, rational design tools, fast simulation and optimization techniques have been emerging as exciting prospects, for instance in the context of adjoint methods, topology optimization and machine learning. Physics-driven design and optimization approaches, such as the use of singularities in the complex frequency plane, topological concepts, bound states in the continuum (BIC) and exceptional points are emerging directions that unveil new optical phenomena and add new designer tools for photonic metasurfaces.37–40 

Emerging applications of nonlocal metasurfaces include the possibility of filtering and processing images in momentum space, realizing Fourier and even nonlinear operations over an ultrathin, efficient platform. The idea is to leverage the engineered nonlocality in metasurfaces to perform mathematical operations on the incoming images,41–43 and even realize optical analog computers that can solve complex mathematical problems44–46 [Figs. 3(a) and 3(b)]. Tailored nonlocalities in metasurfaces can also add spatial and temporal coherence, ideally suited to pattern and control thermal emission and photoluminescence. These ideas are paving the way to the realization of ultrathin surfaces that emit tailored wavefronts with desired amplitude, phase, and polarization without the need for an external coherent source driving them.47,48 Figure 3(c) shows a thermal metasurface, based on an underlying periodic lattice that can control the degree of temporal and spatial coherence endowed to thermal emission, and whose local perturbations control the polarization and wavefront shape of thermal emission. Future efforts may be able to demonstrate ultrathin patterned surfaces that embed their own optical sources and pattern them with extreme flexibility.

FIG. 3.

(a) Edge detection metasurfaces for image processing and biomedical applications (from Ref. 42); (b) analog optical computing based on nonlocal metasurfaces (from Refs. 44 and 46); (c) nonlocal metasurface to manipulate thermal emission (from Ref. 47); (d) metasurfaces integrating 2D materials (from Ref. 49); (e) spatiotemporally modulated metasurface to extend the degree of control over wavefront manipulation to space-time diffraction (from Ref. 50). All images are reprinted (adapted) with permission from the respective Journal and copyright remains with the original publisher.

FIG. 3.

(a) Edge detection metasurfaces for image processing and biomedical applications (from Ref. 42); (b) analog optical computing based on nonlocal metasurfaces (from Refs. 44 and 46); (c) nonlocal metasurface to manipulate thermal emission (from Ref. 47); (d) metasurfaces integrating 2D materials (from Ref. 49); (e) spatiotemporally modulated metasurface to extend the degree of control over wavefront manipulation to space-time diffraction (from Ref. 50). All images are reprinted (adapted) with permission from the respective Journal and copyright remains with the original publisher.

Close modal

On the material front, the use of two-dimensional (2D) materials integrated with metasurfaces, such as graphene51 and transition metal dichalcogenides49 [Fig. 3(d)], and even direct patterning of bulk 2D materials, holds the promise for exciting opportunities both in the context of basic science and applications. These materials offer interesting forms of light–matter interactions in the form of plasmon, exciton, phonon polaritons, in which light and matter are so intertwined to form quasi-particles. Combined with metasurface concepts, these phenomena can further boost light control, and impart exotic photonic features to polaritonic responses. Polaritonic materials can be also ideally suited to boost optical nonlinearities, opening exciting opportunities to extend metasurface operations and wavefront control to nonlinear optical processes, such as wave mixing, frequency generation, limiting, up- and downconversion.52,53 These materials can also be exciting prospects to efficiently integrate and pattern optical gain in metasurfaces, paving the way to a plethora of interesting non-Hermitian wave phenomena.38,39,54

Nonlinearities can also offer powerful tools to reconfigure and modulate in time the metasurface response, which becomes crucial to make an impact in many technologies. Beyond the importance of manipulating in real-time the spatial degrees of freedom of the incoming wavefront, creating enhanced forms of spatial light modulators,55 but also their temporal and frequency content. Suitable temporal modulation schemes can break reciprocity and efficiently mix frequencies,56 as well as induce nontrivial parametric phenomena, including Doppler shifts, and active beam steering,50 largely expanding the reach and opportunities offered by optical metasurfaces, as schematically shown in Fig. 3(e). For various applications, real-time dynamic programmability may become necessary moving forward the field of metasurfaces, for instance in the context of image processing, dynamic scene creation, and holograms. The integration of 2D and polaritonic materials may enable faster and more efficient forms of modulation.

Finally, photonic metasurfaces may truly flourish once its realization becomes fully compatible with large-area and inexpensive fabrication techniques, pushing metasurfaces from proof of concept implementations to foundry-level production. Roll-to-roll, nanoimprinting, and self-assembly techniques appear promising in this context, and efforts to develop design principles compatible with the constraints of these techniques in terms of materials and disorder tolerance are becoming necessary. Overall, photonic metasurfaces have a bright future ahead, continuously evolving along the years with new concepts, new material playgrounds and a highly interdisciplinary broad research community. As the field continues to mature, the exotic wave phenomena at the basis of photonic metasurfaces are empowering photonic technologies and exciting several industries.

Our work on these topics has been supported by the Simons Foundation and the Air Force Office of Scientific Research.

Ayesheh Bashiri, Zlata Fedorova, Radoslaw Kolkowski, A. Femius Koenderink, and Isabelle Staude*

*isabelle.staude@uni-jena.de

Optical metasurfaces are two-dimensional (2D) arrangements of subwavelength scale building blocks known as meta-atoms with engineered scattering properties. In contrast to the passive metasurfaces discussed in most other sections of this Roadmap, which manipulate the light propagating from a distant light source, light-emitting metasurfaces incorporate nanoscale light sources in their architecture, allowing the coupling of the emission from the sources to the far-field.57 Light-emitting metasurfaces offer various functionalities including photoluminescence enhancement,58 tailored emission directionality,48,59,60 improved quantum efficiency,61 color conversion,62 and controlled degree of coherence,63 thus offering important opportunities for compact and efficient light sources such as lasers (see also Sec. XI), LEDs,64 and single-photon sources65 for quantum technologies (see also Sec. X). Other applications range from displays and optical communication to biomedical applications, and energy harvesting. Note that, in this section, the term “light emission” specifically refers to the fluorescent and photoluminescent sources, while thermal emission is addressed in Secs. I and V.

Light-emitting metasurfaces can be categorized into two main platforms: plasmonic and high-refractive-index all-dielectric. Plasmonic metasurfaces are composed of metallic meta-atoms, where the large confinement of electromagnetic fields in their surroundings can result in a strong reduction of the radiative lifetime of the emitters coupled to the metasurface.66 However, the applicability of these metasurfaces is limited due to their intrinsic Ohmic losses. All-dielectric metasurfaces, on the other hand, can exhibit low absorption losses and support both electric and magnetic multipolar Mie-type modes without demanding complex geometries.67 Multipolar superposition then allows for tailoring directional properties, e.g., through the Kerker effect, which facilitates applications in beam steering, lensing, and beam shaping.

Various types of emitters have been considered as active components of light-emitting metasurfaces, such as quantum dots (QDs), dyes, and semiconductors. Recently, many studies concentrated on the coupling of optical metasurfaces with 2D materials. In the family of 2D materials, layered systems of transition metal dichalcogenides (TMDs)68 have attracted special attention due to their unique optoelectronic properties, including strong photoluminescence, excitonic response at room temperature, and circular dichroism caused by their valley-selective optical transitions.69 

The key mechanism for controlling light emission by metasurfaces is the engineering of the local density of photonic states (LDOS) through either the localized resonances provided by individual meta-atoms or the collective resonances originating from their overall arrangement or both. In periodic metasurfaces, collective resonances can be regarded as photonic Bloch bands (as in photonic crystals) transformed by the resonant meta-atoms into surface lattice resonances (SLRs).70 They also include dark modes and quasi bound states in the continuum (quasi-BICs), in which radiative loss is suppressed by destructive interference.71 Collective resonances can provide high LDOS at the band edges, together with control of spatial coherence and directivity of emission.59,72 Engineering the individual meta-atoms allows to further tailor the light emission properties, employing both local and nonlocal effects. For example, asymmetric meta-atoms may allow a controlled fraction of the light to be radiated away by turning the perfectly nonradiating BICs into quasi-BICs.73 Moreover, spatial variation of the meta-atom design can allow for the emission of light fields with shaped wavefronts through metasurface defined spatially varying phase or amplitude profiles.48,60,74

Although most of the light-emitting metasurfaces demonstrated so far facilitate tailoring of spontaneous emission in the weak coupling regime, their potential for strong coupling and lasing has also been demonstrated.75,76 The latter is discussed in detail in Sec. XI. Semiclassical strong coupling (ensemble of many emitters) is achieved when the coupling rate of emitters to the photonic resonances exceeds the photonic loss rate, and the rate of spontaneous decay is gauged by the emission spectral bandwidth. This can be achieved by aligning the active material's emission bandwidth with metasurface resonances featuring sharp line widths while aiming to maximize the oscillator strength of the dipole transition. Low-threshold lasers, on the other hand, require high-quality (high-Q) optical cavities and precise spatial and spectral overlap of their modes with the gain material. This can be achieved by engineering the parameters of the metasurface integrated with the gain medium.

The relevant performance factors for light-emitting metasurfaces depend on the specific target application. For instance, to impact high-power LED lighting one must contend with 0.5–1 A/mm2 current density (blue emitting LEDs), or equivalently for phosphor converting layer of order 1 W/mm2 emitted power, and 1 W/mm2 thermal load due to the Stokes shift. This places huge challenges on photostability and absorption coefficients. Another application is light sources for projection or image-projecting metasurfaces. In contrast to general lighting, such applications require a small etendue. Metasurface-based displays must emit a set of selected wavelengths, which can be achieved, e.g., by energy transfer between different types of emitters. For applications like, e.g., AR glasses, it is not overall emission power output that matters, but the combination of emissive behavior with transparency for most of the ambient light. Some factors are relevant for multiple applications, e.g., brightness, photostability, power efficiency, fabrication reliability, scalability, and cost. In addition, a frequently desired property is post-fabrication tunability. Modulating the emission and/or achieving multiple and adjustable functionalities in one design are the key features of light-emitting metasurfaces that can bring them closer to real-world applications.77 

Figure 4 summarizes many of the aspects of light-emitting metasurfaces discussed in this section.

FIG. 4.

Summary of the key aspects of light-emitting metasurfaces discussed in this chapter including material platforms, physical mechanisms, desired functionality, performance requirements, as well as future goals.

FIG. 4.

Summary of the key aspects of light-emitting metasurfaces discussed in this chapter including material platforms, physical mechanisms, desired functionality, performance requirements, as well as future goals.

Close modal

1. Emission enhancement

The emission from nanoscale light sources incorporated into metasurfaces can be tailored through a modification of excitation rate (for optical pumping), of radiative decay rate, and of directionality of the emission.78 Metasurfaces can strongly confine the excitation field in the emissive layer, resulting in an efficient incoupling and further enhancement of the emission. Radiative decay rate enhancement is associated with the Purcell effect and achieved through modifications of the LDOS by resonant metasurfaces. For classical cavities, it is well known that the Purcell factor scales approximately with the ratio of the quality factor Q and the mode volume V. Simple Mie-resonant all-dielectric metasurfaces exhibit Q-factors on the order of 10–100 and moderate Purcell factors.79 However, they contribute effectively to the radiative decay rate enhancement due to their negligible absorption losses. This is especially beneficial for emitters with intrinsically high quantum yields. In contrast, plasmonic metasurfaces can offer much smaller mode volumes, resulting in larger Purcell factors,80 at the cost of enhancing both the radiative and nonradiative decay rate, thus reducing the quantum yield for highly efficient emitters. Hence, they can only be usefully employed for boosting the quantum yield of rather inefficient emitters78 or should be operated in the regime of high-Q SLRs, where the hybridization between plasmon and diffraction resonances is tuned to trade in plasmonic confinement for the quality factor. Indeed, collective metasurface resonances such as SLRs or quasi-BICs generally support significantly high Q-factors (in particular for low-loss dielectric implementations) at the cost of relatively high mode volumes. The high Q-factors make them suitable for lasing, entangled-photon generation, and luminescence enhancement.71 For example, a 40-fold enhancement by quasi-BIC for color centers in silicon metasurfaces has been reported81 [see Figs. 5(a)–5(d)]. For germanium QDs, over three orders of magnitude luminescence enhancement was obtained in dielectric Fano-resonant metasurfaces.58 Efficient lasing and strong coupling enabled by collective metasurface resonances have been demonstrated, e.g., in a plasmon-exciton-polariton laser, exhibiting a reduced threshold in the strong coupling regime.82 

FIG. 5.

(a) Schematic illustration of a silicon metasurface supporting high-Q quasi-BIC resonances; (b) SEM images of the fabricated metasurface; (c) illustration of the carbon G-centers on the side walls of the etch holes; (d) emission from the carbon G-centers resonantly enhanced by the quasi-BIC [(a)–(d) from Ref. 81]; (e) illustration of chiral emission due to circularly polarized states (CPS) originating from BIC in a metasurface with broken symmetry; (f) distribution of polarization vectors in the momentum space, with a pair of CPS shown as red and blue dots (topological charges ±1/2); (g) SEM images of the fabricated metasurface; (h) enhanced circularly polarized emission from polycarbonate (PC) film doped with 2-methyl-6–(4-dimethylaminostyryl)-4H-pyran (DCM) deposited on the metasurface [(e)–(h) from Ref. 87]; (i) schematic of a single layer TMD integrated with an achiral dielectric metasurfaces for controlling valleytronic emission; (j) detected valley resolved photoluminescence of excitons and trions upon left-handed and right-handed circularly polarized laser excitation for structure shown in (i); (k) measured trion and exciton degree of polarization, for structure shown in (i) [(i)–(k) from Ref. 69)]; (l) schematic of a polymer layer containing Eu3+ compound integrated with a broken symmetry TiO2 metasurface for tailoring directionality and fluorescence enhancement of MD transitions; (m) measured fluorescence spectra for the metasurface scheme shown in (l) (red curve) normalized to the spectra of the substrate (blue curve); (n) measured back focal plane images of (n) MD and (o) ED transitions coupled to the metasurface shown in (l) [(l)–(o) from Ref. 72]. All images are reprinted (adapted) with permission from the respective Journal and copyright remains with the original publisher.

FIG. 5.

(a) Schematic illustration of a silicon metasurface supporting high-Q quasi-BIC resonances; (b) SEM images of the fabricated metasurface; (c) illustration of the carbon G-centers on the side walls of the etch holes; (d) emission from the carbon G-centers resonantly enhanced by the quasi-BIC [(a)–(d) from Ref. 81]; (e) illustration of chiral emission due to circularly polarized states (CPS) originating from BIC in a metasurface with broken symmetry; (f) distribution of polarization vectors in the momentum space, with a pair of CPS shown as red and blue dots (topological charges ±1/2); (g) SEM images of the fabricated metasurface; (h) enhanced circularly polarized emission from polycarbonate (PC) film doped with 2-methyl-6–(4-dimethylaminostyryl)-4H-pyran (DCM) deposited on the metasurface [(e)–(h) from Ref. 87]; (i) schematic of a single layer TMD integrated with an achiral dielectric metasurfaces for controlling valleytronic emission; (j) detected valley resolved photoluminescence of excitons and trions upon left-handed and right-handed circularly polarized laser excitation for structure shown in (i); (k) measured trion and exciton degree of polarization, for structure shown in (i) [(i)–(k) from Ref. 69)]; (l) schematic of a polymer layer containing Eu3+ compound integrated with a broken symmetry TiO2 metasurface for tailoring directionality and fluorescence enhancement of MD transitions; (m) measured fluorescence spectra for the metasurface scheme shown in (l) (red curve) normalized to the spectra of the substrate (blue curve); (n) measured back focal plane images of (n) MD and (o) ED transitions coupled to the metasurface shown in (l) [(l)–(o) from Ref. 72]. All images are reprinted (adapted) with permission from the respective Journal and copyright remains with the original publisher.

Close modal

2. Shaping the emitted light

Other crucial enabling features of metasurfaces are their areal nature and the diversity of far-field spatial character that their modes can be engineered to support, in particular for spatially varying architectures. As such, the radiative decay rate enhancement can also serve to tailor the emission directionality by channeling the emitted light into carefully tailored spatial modes. This is usually achieved by adjusting the meta-atom geometry or their arrangement,83 which can span from a basic periodic lattice to a complex, spatially inhomogeneous distribution, as well as the substrate/superstrate material. Tailoring directionality is key for realizing light-emitting devices with efficient outcoupling,84 emission into a reduced solid angle for augmented/virtual reality (AR/VR) applications35 and generating arbitrary emission patterns,48,60,85 e.g., for projectors or light-field displays. The combination of the high Q-factors and topological properties of quasi-BICs can also be harnessed for beam shaping, e.g., for generation of vortex laser beams.86 

3. Dynamic tuning of emission

Many envisioned applications of light-emitting metasurfaces require dynamic, controllable, and reversible modulation of light emission. Such post-fabrication tunability can be achieved using active materials, which change their optical properties under electrical, optical, or thermal stimuli. Recently, sub-picosecond modulation of photoluminescence has been achieved in semiconductor metasurfaces through optically induced free-carrier effects,88 which constitutes the current state of the art in modulation speed. On the other hand, large modulation depths can be provided by liquid crystals (LCs) or changing the metasurface geometry, e.g., by stretchable deformation or using micro-electro-mechanical systems (MEMS). The use of both approaches for actively tuning spontaneous emission has already been demonstrated,89,90 making them promising future platforms for dynamic light-emitting metasurface devices.

4. Chiral light control

Moreover, an important area of application for light-emitting metasurfaces is the creation and manipulation of circularly polarized light—a property that is often desired in classical and quantum optical information processing, communication, sensing, and displays. Several strategies have emerged in this field. In one scenario, circular polarization of the emission is enforced by the metasurface design, while the emitter can be arbitrary. This can be achieved, e.g., by using chiral meta-atoms, photonic spin-valley locking91 or optical Rashba effect, where lack of inversion symmetry causes splitting of optical spin-polarized states in momentum space. Alternatively, the utilization of chiral quasi-BICs was introduced as a compelling method for narrowband directional chiral light emission87 [see Figs. 5(e)–5(h)]. In the second scenario, chiral metasurfaces are combined with emitters of circularly polarized light. Prominent examples include the usage of chiral metasurfaces for circular dichroism enhancement or selective coupling with valley-polarized excitons in 2D TMDs. The latter is often motivated by the demands of valleytronics. As to the valleytronic applications of chiral metasurfaces, they, however, face notable limitations as they often promote only one specific valley and would require nanoantenna geometry changes for switching. This leads us to the third scenario, where achiral metasurfaces enable control over the circularly polarized emission. Specifically, achiral metasurfaces were proven to allow for control of the lifetime, the degree of valley polarization, and the spectral shape of chiral 2D TMD emission.69,92 An example of chiral 2D TMDs coupled to the achiral metasurfaces is shown in Figs. 5(i)–5(k). However, a comprehensive understanding of the coupling mechanisms in such complex hybrid systems, which would be essential for testing their applicability for valleytronics, is still missing. Notably, coherent coupling between excitons and meta-atom resonances offers new avenues for the tunability of chiral properties, including via the magneto-optical effect.93 

5. Methods: Design and fabrication

Various numerical methods, including rigorous coupled-wave analysis (RCWA), finite element method (FEM), and finite-difference time-domain (FDTD), are used to simulate light-emitting metasurfaces.94,95 RCWA is well-suited for periodic metasurfaces with low refractive index contrast excited by plane waves, and directly computes reflected and transmitted diffraction orders, but requires extra steps for near-field reconstruction. FEM and FDTD can simulate both periodic structures with plane wave excitation and finite structures with open boundary conditions. Importantly, for light-emitting metasurfaces, point dipole excitation mimicking a nanoscale emitter is possible for these techniques.96 Emission calculations for infinite periodic systems are not as straightforward, since periodic boundary conditions replicate a single emitter in the unit cell to an infinite coherent periodic set. For such systems a Floquet transformation is required, also known in the RF antenna community as the so-called “array scanning method.”97 Alternatively, reciprocity-based methods that relate fluorescence to absorption can be used, which are particularly powerful to simulate extended ensembles of incoherent emitters.98 

Among the nanofabrication techniques, electron beam lithography (EBL) and focused ion beam milling (FIB) provide state-of-the-art nanoscale precision while preserving high flexibility, and were generally empowering research on metasurfaces over the last decade.99 The key challenge for the fabrication of light-emitting metasurfaces is the integration of nanoscale emitters into the metasurface architecture. Typically, this is achieved either by fabricating the metasurfaces directly from light-emitting materials or by hybridizing passive metasurfaces with active materials post-fabrication. A typical materials choice for the first approach are direct bandgap semiconductors, including epitaxially grown structures incorporating quantum wells or quantum dots.100 The latter approach can be achieved, e.g., by spin-coating the metasurfaces with nanoemitter or dye-containing polymers100,101 or by depositing or chemically binding emitters such as QDs on the surface. 2D or layered active semiconductors are obtained from bulk crystals via exfoliation or grown directly by chemical vapor deposition (CVD). Among the integration techniques, one distinguishes dry and wet transfer as well as various strategies of direct integration.68 

While the field of light-emitting metasurfaces has witnessed significant progress to date, several challenges remain to be overcome to further develop the metasurface platforms for future practical applications. Here, we provide some suggestions for progress in this regard.

1. Materials and their integration

As light-emitting metasurfaces consist of two primary components, namely the metasurface itself and the nanoscale emitter, robust material platforms for both, as well as effective integration techniques are required. Importantly, high radiation efficiencies can only be obtained by using dielectric materials with minimal losses. With much of the recent research on passive metasurfaces resorting to a convenient implementation in silicon, metasurfaces made from wide-bandgap materials such as titanium dioxide (TiO2), gallium phosphide (GaP), zirconium dioxide (ZrO2), or lithium niobate (LiNbO3) can be game-changers for the visible spectral range, which is of particular interest for applications of light-emitting metasurfaces.

Regarding the emitters, high quantum efficiency along with strong optical and thermal stability is desired. Importantly, for both metasurface and emitters, fitting the industrial needs and compatibility with electrical devices must be considered. Here, electrical driving schemes are of particular interest, considering that most research works on light-emitting metasurfaces still rely on optical pumping.

As such, many hybrid material platforms and integration schemes remain to be explored. For example, fluorescent ions such as trivalent lanthanides could be doped into the constituent materials of the metasurface during the fabrication process or implanted afterwards using ion implantation.

2. Fabrication challenges

Apart from materials and integration, important challenges remain in terms of nanofabrication. First, Q-factors in low-loss quasi-BIC metasurfaces are currently limited by fabrication imperfections such as roughness.102 Thus, it is highly desirable to further improve the structure quality to unlock a range of effects dependent on high Q-factors. Second, another grand challenge in terms of fabrication, especially regarding application perspectives, is scalability. While this holds for most metasurface applications, the challenge is even greater for light-emitting metasurfaces due to the need to integrate the active material. For example, although deep-UV photolithography has been proposed as a viable solution for complementary metal–oxide–semiconductor (CMOS)-compatible wafer-scale metasurface production,103 light-emitting materials are usually not CMOS-compatible, requiring new approaches to be developed. For example, a combination of nanoimprint lithography and selective area sublimation has been proposed for large-scale production of GaN light-emitting metasurfaces.104 Other alternatives may include, e.g., laser-induced transfer, suitable for creating large-scale periodic arrays of light-emitting nanoparticles.105 Third, the precise and deterministic placement of the active material only at desired locations within the metasurface geometry still presents a challenge. This can be, e.g., the gap between two meta-atoms where the fields are significantly enhanced, asymmetric locations within the unit cell for beam-steering effects, or positions featuring specific local field characteristics such as chiral density. Suitable placement strategies may be adopted from the field of nanoantennas, with approaches relying on two-step electron-beam lithography showing particular potential for metasurfaces. The large areas that the metasurface spans also suggest that nanoimprint methods may be used to print emissive materials in a registry with meta-atoms if sufficiently precise spatial registration can be achieved.

3. Emission stability

A critical issue concerning a wide range of emitters, spanning from fluorescent dyes to QDs and perovskites is photobleaching, where the excess energy delivered during the excitation process results in the reduction of the emitted light intensity from the emitters over time. Photobleaching is attributed to both pumping conditions and intrinsic properties of the emitters. Therefore, an adequate emitter choice is crucial for minimizing sample photodamage. In this regard, interesting opportunities may be offered by highly stable emitters like nitrogen vacancies and nanodiamonds.106 

4. Magnetic dipole transitions

The light–matter interaction in the optical frequency range mainly refers to the interaction of the electric field with electric dipolar (ED) electronic transitions. However, certain solid-state emitters, such as trivalent lanthanides and transition-metal-doped crystals, exhibit robust magnetic dipolar (MD)-dominated electronic transitions as a consequence of a selection rule forbidden ED transition.107 Due to a strong optical magnetic response, all-dielectric metasurfaces were suggested as exquisite candidates for tailoring the fluorescence properties of MD transitions, opening an exciting pathway toward exploring magnetic light–matter interactions at the nanoscale57,72 [see Figs. 5(l)–5(o)]. A particular challenge associated with commonly studied MD transitions is their slower radiative decay rate and consequently lower emission intensity compared to their ED counterparts. However, in a recent study, a significantly bright and fast optical-frequency MD radiation is identified in 2D hybrid organic-inorganic perovskites (HOIPs) such as butylammonium lead iodide (BA2PbI4).108 It is shown that the MD radiative rate in such systems is up to 3 orders of magnitude faster than previously established MD transitions. Additionally, similar to trivalent lanthanides, 2D HOIPs exhibit spectrally separated ED and MD radiations. MD radiation properties, e.g., emission directionality of such emitters, can be engineered by integrating them with suitable nanophotonic platforms. Particularly, metasurfaces supporting high-Q resonances are elegant tools for enabling directional color and/or polarization routing of the emission from MD and ED transition channels. This opens new perspectives on realizing spectrometer-free, low-loss, and CMOS-compatible nanophotonic devices for imaging, sensing, and probing.

5. 2D materials and valleytronics

When dealing with 2D materials as active components, their ultrathin nature presents both advantages and drawbacks. Notably, it entails limited overlap with the excitation field, resulting in a low signal in absolute units. To address this, employing high-Q metasurfaces is needed to boost local pump field enhancements, while also being key for superior Purcell enhancements. A significant limitation of all valleytronic devices is the cryogenic temperature requirement due to the presence of phonon-assisted intervalley scattering that prohibits valley polarization under normal conditions. A promising avenue toward realizing non-zero valley polarization at room temperature involves the utilization of TMD-containing van-der-Waals heterostructures. These artificial materials can support a flurry of interesting effects and applications, one of them being that valley polarization can be augmented through mechanisms such as charge or spin transfer and other proximity effects. Adding to the general fabrication challenge, for realistic device applications methods for scalable and high-quality fabrication of 2D materials and heterostructures have to be developed. Here, the CVD-based phase engineering techniques can pave the way for novel heterostructures and junctions with unique electronic phases and customized properties.

6. Emerging trends in metasurface design

Regarding the design of light-emitting metasurfaces, a grand challenge is posed by the high computational demand of suitable approaches like those based on the reciprocity principle. These demands are typically higher than for other metasurface applications: the emission process needs multi-wavelength (optically pumped) and multi-physics analysis (including, e.g., electrical and thermal effects), coherent or incoherent summing over many source positions, and when translated by reciprocity into a scattering problem it involves a continuum of wave vectors. Finally, emissive devices such as LEDs often rely on metasurfaces in complex vertically stratified stacks. Generally in metasurface research, there is a growing preference for optimization algorithms over human-driven approaches due to their capability of efficient systematic investigation of parameters, as detailed in Secs. XVIII and XX. New, highly performing algorithms may thus prove especially useful for active architectures and help to come up with spatially variant architectures of unprecedented complexity or help to tackle specific design issues associated with the integration of emitters. For example, quasi-BICs can be engineered to suppress absorption losses, enabling, e.g., plasmonic-dielectric metasurfaces with theoretically arbitrary high Q-factors,109 which can potentially lead to high-performance metasurface designs incorporating light-emitting and thus inevitably also absorbing materials.

7. High-power applications

To be competitive as commercial light sources, metasurfaces must reach specific performance values. A challenge for high power applications, such as LEDs is the required combination of high emitter density, high emitter efficiency, and resistance to photobleaching and thermal load. Inherent to the process of emission is that the Stokes shift energy is dissipated as heat, often meaning 0.5–1 eV of dissipated energy for every emitted photon. For example, a metasurface-based LED should withstand of order 1 A/mm2 input current while maintaining power-to-photon conversion and power handling capacity sufficient for providing 1 W/mm2 emission intensity. These goals require proper material engineering to obtain sufficiently bright and photostable emitters—the associated dissipated power, or in the case of a color converting layer, the Stokes-energy loss in such systems is also of order 1 W/mm2. For metasurface lasers, it is important to realize that a very high benchmark is set by III–V surface-emitting lasers that provide continuous-wave output power on the level of 100 W and brightness >1 GW/cm2/sr at power-to-photon conversion >0.5 W/A, while maintaining decent beam quality (M2). This typically requires large-area single-mode operation, which is particularly challenging for metasurfaces. However, the above figures have recently been approached by photonic crystal surface-emitting lasers.110 

8. Tunable light sources

In addition to simple light sources, many of the emerging applications of light-emitting metasurfaces, such as LiFi, LIDAR, AR/VR, require a reliable way of tuning the emission. In that regard, LCs and MEMS are the most mature technologies, which are also CMOS-compatible and therefore suitable for large-scale and low-cost production.89,90 Both approaches can offer a large modulation depth while providing a modulation speed that is sufficient for most commercial uses. However, direct integration of MEMS and LCs for specific applications of light-emitting metasurfaces remains to be developed.

9. Topological, non-Hermitian, and temporal effects

In addition to the real-world applications, the research on light-emitting metasurfaces continues to explore unconventional directions such as nontrivial topology,111 non-Hermitian physics/parity-time (PT) symmetry,112 and temporal photonics.50 Topological robustness enables various systems (including, e.g., photonic topological insulators, BICs) to exhibit fragile properties regardless of imperfections, which has been studied especially in the context of large-area single-mode lasing.113 Photonic systems with gain and loss can support non-Hermitian features known as exceptional points (EPs), which can be used to control the directionality and polarization of emitted light.54 In temporal photonics, sufficiently fast time modulation can give rise to exotic physical phenomena related to light emission and lasing, such as new mechanisms of light amplification.114 Currently, the concepts of topological, non-Hermitian, and temporal photonics remain mainly at the level of fundamental research. However, in the future, some of these concepts may revolutionize the field of light-emitting metasurfaces, enabling unprecedented performance and novel functionalities.

Light-emitting metasurfaces represent a cutting-edge area of research in nanophotonics, offering dynamic control over the properties of emitted light from the incorporated nanoscale light sources. Furthermore, thanks to recent developments in the field of nanofabrication and material processing, light-emitting metasurfaces hold great promise for a wide range of practical applications from advanced displays and communication systems to imaging devices, sensors, and beyond. Although challenges such as energy efficiency and integration with existing technologies need to be addressed to reach their full potential, the ongoing research and development in this area lead us to expect a bright future for advanced light-emitting metasurface devices with novel functionalities as well as their integration into real-world applications. Finally, metasurfaces also offer a unique platform for future scientific applications investigating fundamentally new directions in the area of light emission.

I.S., A.B., and Z.F. acknowledge financial support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through the International Research Training Group (IRTG) 2675 “Meta-ACTIVE,” Project No. 437527638, and through the Emmy Noether Program, Project No. STA 1426/2-1. The work of A.F.K. is part of the Dutch Research Council (NWO) and was performed at the research institute AMOLF. R.K. acknowledges the support of the Research Council of Finland (Grant Nos. 347449 and 353758).

Xiaofei Xiao, John Yang, and Rupert. F. Oulton*

*r.oulton@imperial.ac.uk

The field of nonlinear optics, emerging in the 1960s alongside lasers, explores light–matter interactions under extreme light intensities.115 Effects, such as harmonic generation, three- or four-wave mixing and parametric amplification, have expanded the operational bandwidth of coherent light sources from the extreme ultraviolet to terahertz and produced methods to measure and control light on ultrafast time scales. Applications extend to quantum optics, where parametric downconversion and spontaneous four-wave mixing produce correlated and entangled photon pairs for quantum enhanced sensing, imaging and computing.116–121 

Nonlinear processes such as harmonic generation and four-wave mixing can be remarkably efficient when beams being mixed propagate in phase over many millimeters of interaction in a nonlinear crystal. Phase-matching brings material and technical restrictions as it comes at the cost of reduced tuning range and material flexibility. For example, phase-matching may only be achieved at specific wavelengths and temperatures and the large interaction lengths require low absorption. For some photonic applications, new nonlinear materials that can be integrated with the most relevant photonic platforms are important, such as polymers122–124 and 2D materials.125–128 Nonlinear optical metasurfaces also hold promise for addressing these challenges [Figs. 6(a) and 6(b)]. First, optical nanostructures can be designed at the nanoscale to enhance local fields and meet symmetry requirements to produce strong nonlinear effects, as we discuss in detail below. Second, nonlinear metasurfaces do not use phase-matching but instead rely on their high intrinsic nonlinearity to produce a useful nonlinear effect. This alleviates tuning, bandwidth and material absorption restrictions. Third, nanostructured metasurfaces can be integrated with a wide variety of nonlinear materials to further strengthen their intrinsic nonlinearity. Finally, their engineered nature allows for nonlinear wavefront control. These design features have led to a burgeoning field of nonlinear metasurfaces.

FIG. 6.

The variety of mechanisms used in nonlinear optical metasurfaces. (a) Comparison of nonlinear optical medium based on conventional bulk crystals and meta-optics.142 (b) Representative nonlinear optical processes where meta-optics may have unique advantages.142 (c) Illumination of strong enhancement of SHG based on plasmonic resonances in an aluminum nanoantenna array.143 (d) Illustration of the continuous geometry-phase-controlled nonlinear harmonic responses of individual metal nanostructures in plasmonic metamaterials.144 (e) Schematic of enhanced THG in Silicon nanoparticles driven by magnetic response.145 (f) Enhanced THG in individual Ge nanodisks excited at an anapole state.146 (g) Illustration of THG in Si nanodisk trimers and its SEM image.147 (h) Schematic of efficient THG enhanced by metal-dielectric hybrid nanoantennas.148 (i) Enhanced high-harmonic generation from an all-dielectric metasurface made of dipolar bar antennas and disk resonators on a sapphire substrate. Inset: level scheme for the mode coupling in a schematic three-level Fano-resonant system.149 (j) Illustration of the Fano-assistant THG from silicon quadrumers of four a-Si:H nanodisks with SEM image of the sample in the bottom left corner.150 (k) Illustration of continuous wave SHG enabled by quasi-BIC on gallium phosphide metasurfaces.151 (l) Illustration of the BIC-assisted SHG in a single nanoresonator under azimuthally polarized vector beam excitation. Inset: SEM and schematic of an individual dielectric nanoresonator.152 (m) Illustration of enhanced SHG based on SLR in plasmonic nanoparticle arrays. Inset: SEM image and dimensions of each structure.138 (n) Schematic of high-harmonic generation from an ENZ-assisted sample composed of a MgO substrate, CdO thin film and gold capping layer.153 (o) Large optical nonlinearity in ENZ material (A 23-nm-thick indium tin oxide (ITO) layer) enhanced by coupling with plasmonic (gold) nanoantennas.154 (p) Schematic of efficient four-wave mixing based on nanofocusing in a silicon hybrid gap plasmon waveguide. Inset: electromagnetic mode distributions and the chemical formula for MEH-PPV.124 (q) Illustration of dual-wavelength antenna and frequency upconversion using BPT molecules in a 1.3 nm nano-gap between Au nanoparticle and disk. Inset: schematic of upconversion process, AFM (disk) and SEM (nanoparticle) images.155 (r) Schematic of radial BIC-enhanced SHG in a MoSe2 monolayer covering the ring structures.156 (s) Illumination of SHG in a leaky cavity by considering the input (output) coupling coefficient and the spatial overlap between the dominant modes.157 (t) The interplay of symmetry and scattering phase in SHG from gold nanoantennas.158 All images are reprinted (adapted) with permission from the respective Journal and copyright remains with the original publisher.

FIG. 6.

The variety of mechanisms used in nonlinear optical metasurfaces. (a) Comparison of nonlinear optical medium based on conventional bulk crystals and meta-optics.142 (b) Representative nonlinear optical processes where meta-optics may have unique advantages.142 (c) Illumination of strong enhancement of SHG based on plasmonic resonances in an aluminum nanoantenna array.143 (d) Illustration of the continuous geometry-phase-controlled nonlinear harmonic responses of individual metal nanostructures in plasmonic metamaterials.144 (e) Schematic of enhanced THG in Silicon nanoparticles driven by magnetic response.145 (f) Enhanced THG in individual Ge nanodisks excited at an anapole state.146 (g) Illustration of THG in Si nanodisk trimers and its SEM image.147 (h) Schematic of efficient THG enhanced by metal-dielectric hybrid nanoantennas.148 (i) Enhanced high-harmonic generation from an all-dielectric metasurface made of dipolar bar antennas and disk resonators on a sapphire substrate. Inset: level scheme for the mode coupling in a schematic three-level Fano-resonant system.149 (j) Illustration of the Fano-assistant THG from silicon quadrumers of four a-Si:H nanodisks with SEM image of the sample in the bottom left corner.150 (k) Illustration of continuous wave SHG enabled by quasi-BIC on gallium phosphide metasurfaces.151 (l) Illustration of the BIC-assisted SHG in a single nanoresonator under azimuthally polarized vector beam excitation. Inset: SEM and schematic of an individual dielectric nanoresonator.152 (m) Illustration of enhanced SHG based on SLR in plasmonic nanoparticle arrays. Inset: SEM image and dimensions of each structure.138 (n) Schematic of high-harmonic generation from an ENZ-assisted sample composed of a MgO substrate, CdO thin film and gold capping layer.153 (o) Large optical nonlinearity in ENZ material (A 23-nm-thick indium tin oxide (ITO) layer) enhanced by coupling with plasmonic (gold) nanoantennas.154 (p) Schematic of efficient four-wave mixing based on nanofocusing in a silicon hybrid gap plasmon waveguide. Inset: electromagnetic mode distributions and the chemical formula for MEH-PPV.124 (q) Illustration of dual-wavelength antenna and frequency upconversion using BPT molecules in a 1.3 nm nano-gap between Au nanoparticle and disk. Inset: schematic of upconversion process, AFM (disk) and SEM (nanoparticle) images.155 (r) Schematic of radial BIC-enhanced SHG in a MoSe2 monolayer covering the ring structures.156 (s) Illumination of SHG in a leaky cavity by considering the input (output) coupling coefficient and the spatial overlap between the dominant modes.157 (t) The interplay of symmetry and scattering phase in SHG from gold nanoantennas.158 All images are reprinted (adapted) with permission from the respective Journal and copyright remains with the original publisher.

Close modal

In this perspective, we present the state-of-the-art in nonlinear photonic metasurfaces, by reviewing the rich variety of mechanisms to engineer a nonlinear response at the nanoscale. Before considering future directions for this field, we present a short study of how the metasurface interaction area affects nonlinear mixing efficiency as a function of incident beam powers and areas. To conclude, we identify the most useful metrics for assessing nonlinear metasurface performance and future direction this field might take.

Resonances play a crucial role in nonlinear optical metasurfaces, since the linear optical response often dictates the nonlinear one.129 The relationship between linear and nonlinear susceptibilities is well known,115 but are not often utilized in phase-matched processes due to the constraints of absorption on interaction length. However, thin metasurfaces can utilize resonance to produce strong nonlinearities. Through structural resonances, metasurface can also enhance the electric and magnetic fields of incident light. These are usually categorized into local and collective resonances.130 Local resonances, such as surface plasmons,131 Mie modes of metallic and dielectric nanoparticles,132,133 and Fano resonances,134,135 utilize nanostructures that operate independently across the metasurface, while collective resonances, such as guided-mode resonances,136,137 surface lattice resonance,70,138,139 and bound states in the continuum140,141 (BIC), access mutual coherences between nanostructures.

1. Local resonance—surface plasmons

Localized surface plasmon resonances (LSPR) are collective oscillation of free electrons in metal nanoparticles (typically gold or silver) excited by incident light, where resonance is controlled via size, shape, and particle composition. When integrated into metasurfaces, these resonances enhance second harmonic generation (SHG)143 [as shown in Fig. 6(c)] as well as a range of mixing effects53,158–160 including third harmonic generation (THG), four-wave mixing, and sum frequency generation, due to their ability to provide strong local light field enhancement. Wavefront control has also been shown [Fig. 6(d)] using the globally varied local nonlinear harmonic response of gold nanoantennas of C3 and C4 rotational symmetries, respectively.144 Although metal provide excellent confinement, the high nonlinearity of the metal is difficult to use, since metals do not easily admit electric fields—this means interaction volumes are relatively small. Simple plasmonic resonances also provide relatively weak local field enhancement and structures with sharp features and gap that do enhance fields have low damage threshold.132,161 For example, typical quantum efficiencies of SHG are 10 9.

2. Local resonance—Mie and multipolar modes

Metasurfaces made of high refractive index dielectrics, such as silicon, germanium, and gallium phosphide, have emerged recently as a competitive alternative to plasmonic metasurfaces, due to low material loss, larger interaction volumes and the capability to engineer their magnetic response. Their efficiencies compared to plasmonic metasurfaces show improvement by several order of magnitude, mainly due to the higher damage threshold (see Sec. III C).151,162 These metasurfaces use Mie resonances to control the nonlinear interaction. Although they exhibit some ability to enhance optical fields, the larger interaction volume provides the largest benefit.

Individual Mie resonance modes,132,135,145 such as electric, magnetic dipoles, and higher-order Mie resonances, have been explored individually to enhance nonlinear conversion efficiency beyond plasmonic metasurfaces.132,133,163,164 For example, Fig. 6(e) shows that THG conversion efficiency in silicon nanodisks is enhanced by magnetic dipole resonances by two orders of magnitude compared to bulk silicon.145 

To increase the performance further, resonant coupling is often used. Interference of multiple Mie resonances may result in far-field destructive interference, consequently enhancing the nonlinear effect. For example, the anapole state, associated with the destructive interference of dipole and toroidal moments, suppresses scattering to produce near-field enhancements.165 Both fundamental and a higher-order anapole states in individual all-dielectric nanodisks have been implemented.146,166,167 Figure 6(f) shows a fundamental anapole state in a single Ge nanodisk for THG.146 Here, a minimum in extinction cross section corresponds to a maximum electric energy within the material, producing THG quantum efficiencies as large as 10 6. Coupled resonators also provide stronger nonlinear responses through their mutual coupling.168–171 For example, Fig. 6(g) shows a nanotrimer of three silicon nanocylinders that improves THG via the interference of electric and magnetic dipoles.147 

The nonlinearity of nanostructures can be further improved by combining the confinement capabilities of metallic resonances and the interaction volume and radiative control of dielectric resonances.172,173 For example, the plasmonic resonance of a Au nanoring around a Si nanodisk supporting an anapole state148 [Fig. 6(h)] shows THG quantum efficiency as high as 10 4.

3. Local resonance—Fano interference

Fano resonance arises from the interference between a broad continuum of states and a narrow discrete resonance.134 In metasurfaces, this can be realized through the coupling of plasmonic elements with a continuum of modes, leading to asymmetric line shapes and sharp spectral features. The intense and localized electromagnetic fields associated with Fano resonances can greatly boost nonlinear frequency mixing processes.149, Figure 6(i) shows enhanced high-harmonic generation from an all-dielectric metasurface,149 where non-perturbative high-harmonic generation is enhanced in a Fano-resonant Si metasurface. Figure 6(j) shows magnetic-Fano-resonance enhanced THG signal via interplay between collective and individual optically induced magnetic responses in quadrumers made of identical dielectric nanoparticles.150 Plasmonic nanostructures show similar nonlinear enhancements.169,174 We note nonlinear generation enhanced by Fano resonances are usually anisotropic with respect to the excitation polarization and have narrow operation bands due to their resonant nature.

4. Collective resonance—bound states in the continuum

Collective high-Q resonances in all-dielectric metasurfaces, such as bound states in the continuum (BICs) can offer large improvements in nonlinear metasurface response. BICs are eigenstates of a system that exist within the continuum but do not radiate energy into the surrounding space. They were initially introduced in quantum mechanics and later extended to photonics.71,140,141,175,176 Mathematical BICs may have vanishing resonant linewidths. In practice, due to finite sample size, material absorption, small amount of radiation leakage, and structural imperfections, BICs manifest with reasonably large Q factors, known as quasi-BICs. The long lifetimes of quasi-BICs are highly desirable for concentrating light in time and thus the nonlinear optical process.177,178 Figure 6(k) shows SHG in a dielectric (gallium phosphide) metasurface enhanced by two asymmetric cylinders with quasi-BIC resonances.152 

Quasi-BICs can also be engineered as individual local resonators by exploiting the interference of several Mie modes, similar to the anapole state discussed earlier.146,166,167 For instance, AlGaAs cylinders placed on an engineered three-layer substrate (SiO2/ITO/SiO2) show suppressed radiative losses, as shown in Fig. 6(l), boosting SHG efficiency.151 

5. Collective resonance—surface lattice resonance

Surface lattice resonances (SLR) also achieve high Q-factors. When a grating Rayleigh anomaly condition crosses a LSPR, a sharp spectral feature occurs, due to the hybridization of the surface and localized resonances.70 The SLR feature is controlled via nanostructure shape and lattice parameters. The effect of SLRs at the fundamental139 and the generated nonlinear modes138 has been studied for boosting the nonlinear optical response.179, Figure 6(m) shows SLRs used to enhance SHG from a plasmonic nanoparticle array made of split-ring resonators138 where the SHG intensity improved more than a thirty-fold. Nonlinear metasurfaces based on both localized and collective plasmonic resonances have also enabled the generation of controllable terahertz waves,180,181 using mechanisms like ponderomotive acceleration of photo-ejected elections182 and optical rectification.183 

6. Material resonance—epsilon near zero materials

At the boundary between metals and dielectrics, the nonlinearity of materials with epsilon-near-zero (ENZ) points have also been studied184–188 (see Sec. XVII). Where ENZ occurs, the surface normal electric field of incident light diverges when the imaginary part of the permittivity approaches zero. This produces field enhancements inversely proportional to ENZ film thickness189 that boost nonlinear effects.184–188 Their unique nonlinear optical response and the required deep sub-wavelength thickness of the ENZ film also makes them appealing for high-harmonic generation.153 Indium-doped cadmium oxide thin films (75 nm) show harmonic generation up to the ninth order,153 as shown in Fig. 6(n). More on these materials is presented in Sec. XVII of this roadmap. Figure 6(o) shows a metasurface combining plasmonic antennas with an ENZ material.154 Here, the narrow spectral range and ENZ position of the underlying ENZ material could be engineered to significantly boost the nonlinear optical response.

7. Integrating metasurfaces with nonlinear materials

In attempts to maximize the nonlinearity of metasurfaces, researchers have integrated nonlinear materials with metasurfaces to exploit both material and structural resonances. For example, a 100-path spontaneous parametric downconversion photon-pair source was created by integrating a metalens array (10 × 10 array of GaN nanopillars) with a β-barium borate (BBO) nonlinear crystal.190 Another example uses low-loss nonlinear polymer within nanoplasmonic systems.124,191–193 Figure 6(p) shows efficient four-wave mixing over micrometer-scale interaction lengths at telecommunications wavelengths on silicon can be achieved by combining an integrated plasmonic gap waveguide with a nonlinear organic polymer. Similar ideas have been used to focus light into a nanoscale cavity to enable frequency upconversion from Mid infrared to visible,155,194 via molecular vibrations [Fig. 6(q)]. Highly resonant BIC metasurfaces may also be integrated with highly nonlinear materials. Figure 6(r) shows a radial BIC was used to boost SHG from monolayers of transition metal dichalcogenide.156 This brings two advantages: the two-dimensional materials are straightforward to integrate within metasurfaces and their atomic thickness minimizes the deformation of the resonance.

8. Nonlinear metasurface selection rules

Finally, we briefly consider the tensorial nature of non-linear interactions and their corresponding selection rules. Local field enhancement and resonances are not alone sufficient for strong nonlinear effects to be achieved. The non-linear polarization created by pump light in a metasurface must also be able to radiate to the far field to complete the nonlinear process. This requires that the nonlinear tensor components of the metasurface be matched to the desired input and output beams. Moreover, the interaction of the input and output beams with the metasurface should be optimized by both impedance matching, and spatial mode overlap at the various interacting frequencies.151,195,196 Ideally, the suppression of all radiation channels apart from those for power injection and extraction has been proposed to increased coupling to and from the metasurface,157 as schematically shown in Fig. 6(s). Indeed, this has been experimental investigated both in plasmonic158 [as shown in Fig. 6(t)] and all-dielectric145,197,198 metasurfaces.

Conventional nonlinear materials produce large conversion efficiencies by mediating nonlinear processes over a large interaction volume.115 Thus, large area metasurfaces should produce the highest nonlinear generation rates. To elaborate on this, consider the following analysis of nonlinear frequency mixing at a metasurface.

An nth order nonlinear process is affected by up to n input beams. The ith Gaussian optical pulse intensity in time (t) and space (r) at a metasurface (z = 0), I i ( r , t ) = P i / A i e π r 2 / A i t 2 / τ i 2, where Pi is the peak power, A i = π w i 2 / 2, is the beam area (wi is beam waist) and τi is the pulse duration. This arrangement is shown in Fig. 7(a). The process generates a nonlinear polarization, P ¯ n = χ ¯ ¯ ( n ) i = 1 n E ¯ i ( r , t ), where χ ¯ ¯ ( n ) is the nth order nonlinear susceptibility tensor and E ¯ i ( r , t ) are the mixed electric fields.115 In most optical metasurfaces, the interaction length is λ, so that the generated beam intensity is I n = α n i = 1 n I i ( r , t ), where αn is introduced to represent the intrinsic nonlinearity of the metasurface. Here, we assume only simple in-plane tensor connections between the input and output beams. (Out-of-plane components generate non-Gaussian beams.) Substituting for the intensities of the mixing and output beams, we find,
P n A n e π r 2 / A n t 2 / τ n 2 = α n i = 1 n P i A i e π r 2 / A i t 2 / τ i 2 .
(1)
FIG. 7.

Nonlinear generation rate and efficiency. (a) Schematic of a nonlinear process at a metasurface involving multiple incident beams and a generated output beam. The conversion process is determined by incident beam powers and the peak intensities that can be achieved at the metasurface. (b) Map of SHG quantum efficiency in log scale as a function of normalised pump power and mode area. Linear contours passing through origin have gradient of intensity. The broken line represents the damage threshold intensity. (c) Map of SHG average power as a function normalised power and mode area. SHG power is maximised at the highest powers along the damage threshold intensity line. The SHG efficiency and average power are normalised to the maximum value.

FIG. 7.

Nonlinear generation rate and efficiency. (a) Schematic of a nonlinear process at a metasurface involving multiple incident beams and a generated output beam. The conversion process is determined by incident beam powers and the peak intensities that can be achieved at the metasurface. (b) Map of SHG quantum efficiency in log scale as a function of normalised pump power and mode area. Linear contours passing through origin have gradient of intensity. The broken line represents the damage threshold intensity. (c) Map of SHG average power as a function normalised power and mode area. SHG power is maximised at the highest powers along the damage threshold intensity line. The SHG efficiency and average power are normalised to the maximum value.

Close modal
To simplify further, we assume the incident beams have comparable mode area, A0, and pulse duration, τ0, so that n τ n = τ 0 and n A n = A 0 at the metasurface. Since we assess nonlinear generation with time averaging detectors, we use the average beam power as a measure of nonlinear conversion, P ¯ n = P n Ω τ n, where Ω is the laser repetition rate. Thus, we find the average emission power in terms of the average incident beam powers, P ¯ i,
P ¯ n = α n n n [ A 0 Ω τ 0 ] n 1 i = 1 n P ¯ i .
(2)
This expression is valid for nth-harmonic generation (nHG), where i = 1 n P ¯ i = P ¯ 0 n. In the case of four wave mixing, for example,
P ¯ fwm = α 3 3 3 [ A 0 Ω τ 0 ] 2 P ¯ p 2 P ¯ s ,
(3)
where P ¯ p and P ¯ s are average powers in the pump and signal beams, respectively. Average photon generation is improved for pulsed pumping over CW by a factor [ n Ω τ 0 ] 1 n, but the advantage does not apply to peak powers. In this analysis, we assume that Ω τ 0 is fixed, and we instead concentrate on the influence of beam power and area. We also focus on SHG for simplicity. SHG efficiency is assessed against two common metrics in the literature. The first efficiency, η p = P ¯ shg / P ¯ 0 2, is chosen since it is independent of pump power by definition, where η p = α n / [ 2 2 A 0 Ω τ 0 ]. The second efficiency, η q = P ¯ shg / P ¯ 0 = η p P ¯ 0, is a power dependent quantum efficiency.

These SHG efficiencies are both influenced by mode area, A0, and pulse duration, τ0. Clearly, the best conversion rates occur when the incident average pump power is high and the mode area and pulse duration are small. Inevitably, SHG is limited by the damage threshold intensity of a metasurface, ID. In our experiments with plasmonic metasurfaces using near diffraction limited beams from either mode-locked oscillators (Ω = 80 MHz, τ 0 = 150 fs) or continuous wave lasers, we have found damage occurs at about 5 mW average power. For a diffraction limited area of 0.25   μ m 2 , I D 10 11 W cm–2 for pulsed operation and I D , c w 10 6 W cm–2 for CW. Damage here is due to sample heating, and so the lower repetition rates of amplified laser systems can allow higher intensities. Dielectric metasurfaces have slightly higher damage threshold intensities than plasmonic one, but are not immune to damage at intensities I D > 10 12 W cm–2 where ionization effects occurs. We note that at these intensities, non-perturbative nonlinear process start to emerge, and the preceding analysis requires reevaluation.

The quantum efficiency and average SHG power are plotted in Figs. 7(b) and 7(c) as a function of normalized power and mode area. Along the power axis, normalization against both ID and diffraction limited mode area, AD, leaves the axis dimension-less. Likewise, we have also normalized the beam area against AD. [We assumed a pump wavelength, λ = 1 μ m with a diffraction limited mode area: A D = π ( 0.41 λ ) 2.]

The power independent efficiency, ηp, might seem like the most appropriate metric, but it predicts optimal conversion efficiency at low beam area. Indeed, this is where the nonlinear effect is strongest, since the beam intensity is maximized. However, the scaling of SHG with pump power squared suggests that large mode areas and high pump powers should produce optimal SHG. The quantum efficiency proves to be a more useful metric; although it varies with pump power, it can be evaluated in a way that is independent of both beam area and pulse duration, η q = α n P ¯ 0 / [ 2 2 A 0 Ω τ 0 ] = α n I 0 / [ 2 2 ]. As shown in Fig. 7(b), contours of ηq are linear and pass through the origin, so the gradient gives the intensity. If the damage threshold of the metasurface is known, its limiting performance can be completely specified at any power and beam area.

Figure 7(c) shows a plot of average SHG power as a function of average pump power and beam area. The plot confirms that optimal SHG occurs when the pump power is maximized and the mode area is minimized to operate at the damage threshold intensity. This power-area scaling presents challenges for applications of metasurfaces. The write field of electron beam lithography is typically 100 × 100 μ m 2, representing an upper limit on metasurface area. A beam area of 10 4 μ m 2, at damage threshold intensity requires an average beam power of 100 W!

Metasurfaces cannot compete with phase-matched nonlinear processes in terms of efficiency, and the power-area analysis above suggests there are a few routes forward. Metasurfaces offer strong resonant nonlinearities, wave-front control and the possibility to integrate new materials. In this final section, we review applications in wave-front control, optical switching, and accessing non-perturbative nonlinear physics at reduced power.

1. Metasurfaces for nonlinear wave-front control

The capability to locally engineer optical nonlinearity in metasurfaces provides the means to engineer local nonlinear phase. Here, sub-wavelength nanostructures are tailored to control the phase and amplitude and direction of the nonlinear polarization.2,199,200 Recently, nonlinear wavefront control has been demonstrated using plasmonic144,201,202 and all-dielectric203,204 nanostructures with specific structural symmetries from the extreme ultraviolet to terahertz.53,180,181,205 Beam shaping based on binary nonlinear phase and holography based on continuous nonlinear phase have been demonstrated by nonlinear optical wavefront engineering techniques.144,206,207 Currently, there are a few studies in this area and with the attention being dedicated to controlling local nonlinear phase, conversion efficiencies are relatively low. Additionally, wavefront engineering techniques for various optical beams,53,208 such as multiple optical vortex beams (Sec. VII), have been realized using linear optical metasurfaces. Transferring these techniques developed for linear optical metasurfaces into the nonlinear regime will open a new avenue for the realization of compact and ultrafast nonlinear optical devices.

2. Metasurfaces for modulation and switching

Although nonlinear metasurfaces open new avenues in flat nonlinear optics, the majority cannot be actively tuned post-fabrication despite this being a key feature in some linear metasurfaces (see Secs. II, XIV, and XV). Dynamic control over nonlinear metasurfaces under external stimuli is highly desirable in future. The mechanisms of the tunability include but are not limited to thermo-optic effects, free-carrier effects, and phase transitions.209 The electrical modulation of a nonlinear response based on electric-field-induced SHG or optical rectification has been investigated in both plasmonic or dielectric metasurfaces.210–214 While this research direction is in its infancy, there are clear opportunities that play to the strengths of metasurfaces: thin films can be electrically accessed and modulated and resonant nonlinearities are more amenable to tuning with stronger modulations in material parameters.

3. Reduced power non-perturbative nonlinear optics

The intrinsic nonlinearity of materials is measured by the nonlinear susceptibility,115  χ ( n ). For example, SHG is governed by the value of χ ( 2 ) and naturally transparent crystals have χ ( 2 )  1–100 pm V–1. When multiplied by incident electric fields of light, the dimensionless product indicates nonlinear strength. For an n-order process, the dimensionless parameter χ ( n ) | E | n 1 is a metric of nonlinearity. For example, SHG with 1 W of CW light over a 10 4   μ m 2 beam area, has χ ( 2 ) | E | 10 8 10 6. Meanwhile, strong nonlinear effects occur for χ ( n ) | E | n 1 1, where the perturbation description of the nonlinear polarization breaks down. Achieving this condition requires either more intense light or a more nonlinear material. For example non-perturbative effects such as high harmonic generation occur at intensities I > 10 13 W cm 2 in gases, and slightly lower in solids. While amplified pulsed lasers can access non-perturbative intensities, lowering the intensity at which non-perturbative effects begin is appealing for reducing cost, complexity, size, weight and power. The increased nonlinearity of metasurfaces are a promising route.

Nanostructures have been used to control non-perturbative high harmonic generation effects both in isolated structures215–219 and metasurfaces.149,220–224 High-harmonic generation has also been shown in ENZ based metasurfaces.165 Such applications play to the strengths of metasurfaces and would enable an expansion of research teams able to study non-perturbative nonlinear physics across different disciplines by mitigating the need for expensive and bespoke laser systems. Metasurface nonlinearity can be extremely large: to our knowledge, χ ( 2 ) = 1.2 μm V–1 is the highest nonlinear response reported for a metallic metasurface coupled to inter-sub-band quantum well transitions in the mid-infrared,52,225 yielding χ ( 2 ) | E | 1 under CW illumination. Such giant nonlinear responses have been seen in metasurfaces combining both electromagnetic and material resonances.

William J. Peveler and Alasdair W. Clark*

*Alasdair.Clark@glasgow.ac.uk

The advancement of technologies for the rapid and sensitive detection of biological macromolecules (e.g., nucleic acids, proteins, glycans) and whole organisms (e.g., viruses, bacteria, and other pathogens), are crucial to furthering our understanding of biological systems, to the development of new diagnostic tests and healthcare tools, to improved drugs and therapeutics, and for ensuring the safety of our food and water supplies. Therefore, as new developments are made in materials science, one of the first application areas of these new materials is often sensors. This is also true of nanophotonic metasurfaces. Indeed, metasurface biosensors have emerged as one of the most notable success stories in the field of nanoscale metamaterial research, transitioning quickly from an academic curiosity to practical, commercial sensing devices that are having impact in medical diagnostics, environmental monitoring, and pharmaceutical research. The term “nanophotonic metasurface” relates to devices comprised of two-dimensional arrangements of nanoscale building blocks (meta-atoms), whose collective optical properties are derived from their rational design and engineered array properties, rather than purely from their chemical composition.199 These surfaces can be constructed using metallic or dielectric meta-atoms which have optical resonance properties defined by their size, shape, composition, spacing, and crucially for biological detection, the refractive index of their local surroundings.226 Therefore, their resonances properties can be altered by individual biomolecules interacting with their surface. The nanoscale nature of the meta-atoms mean they are often of comparable size to these biomolecules, meaning that even single-molecular localization on a meta-atom surface can produce resonance shifts easily measurable in the far-field by simple white light illumination; shifts that can also provide information on interaction kinetics between molecules.227 Resonant meta-atoms are also able to confine light to extremely small volumes, producing many orders of magnitude enhancement to the electric field around certain facets of the nanostructure (e.g., at sharp corners and inside small gaps). Since many vibrational spectroscopy techniques have efficiencies that scale with electric-field strength, the signals recorded from molecules within these enhanced regions is greatly improved. These two aspects of resonant meta-atoms make them excellent candidates for applications where label-free, hyper-sensitive detection is required, and for applications where the interaction strength between pairs of molecules is being assessed. It has been possible to produce structures with bespoke optical properties for decades, but recent strides in fabrication technology have greatly improved the efficiency, speed, and cost-effectiveness of producing metasurface sensors. Coupled with their ease of excitation (e.g., illumination with incandescent light bulbs, LEDS, laser diodes, etc.) and the low complexity of their optical measurement, these advancements pave the way for portable, low-power, point-of-care, and field-deployable sensing solutions. In this brief roadmap we will cover a small number of research developments in the field of metasurface biosensors (some of which appear in Fig. 8), and provide our outlook on what the future of this fast-moving field may hold.

Metasurface biosensors can be broadly categorized in two groups: plasmonic (where meta-atoms are metallic) and dielectric (where meta-atoms are, typically, silicon). Plasmonic metasurfaces exploit localized surface plasmon resonances (LSPR), a nanoscale variant of the long-established biosensor technology of surface plasmon resonance (SPR).1 Instead of generating surface plasmons that propagate along the air-metal interface of bulk, thin-film metals (as in SPR), LSPR is generated by the oscillatory interaction of light with the free electrons in the metal nanostructures. These structures are smaller than the wavelength of the exciting light, producing resonantly driven, highly localized electric fields. In contrast to SPR devices, LSPR metasurfaces require no special coupling schemes, meaning they are trivial to excite, and produce fields with very short decay lengths (only probing their immediate surroundings228—i.e., bound molecules, not molecules in the bulk), meaning they are not thrown off by environmental fluctuations (e.g., temperature). While plasmonic nano-resonators are decades old, new research into dielectric resonators is revealing key advantages. Metallic metasurfaces suffer from high losses, strong dispersion, and poor magnetic response, whereas dielectric resonance, the result of Mie resonances which generate oscillating displacement currents in response to the driving field polarization, exhibit low loss, sharper bandwidths, little heating, and can support strong magnetic dipoles.229 Despite their different resonance mechanisms, the outcomes are similar: nanostructures, resonantly driven by light, with properties tunable by altering their geometry, and which are either sensitive to their surroundings (i.e., the presence of a biomolecule), or that produce a highly localized electric field that can be used to enhance existing sensing techniques. In terms of sensor production, both wide-area, relatively inexpensive techniques, like colloidal lithography, and smaller-area, relatively slow, more expensive techniques like electron-beam lithography are commonly used in the research space. However, advances in rapid nanolithography techniques, such as nano-imprint lithography and extreme UV photolithography, mean that, even for e-beam-developed geometries, there is a clear path to practical, high-throughput production of metasurface biosensors.

The resonances that unlock applications in biosensing produce highly confined field enhancements at the meta-atom surface. Therefore, only molecules resident within this near-field (typically in direct contact with the meta-atoms) are going to influence, or be influenced by, the resonance, and be detected. So, to function as a sensor, mechanisms are required to ensure that analytes are located within the sensing volume long enough to measure. As a result, in almost all the following biosensor examples, bio/chemical modification of the metasurface is required, usually with a receptor molecule that is complementary to the target analyte (e.g., nucleic acids, proteins, antibodies, etc.). Once located there are several techniques that allow molecular presence to be confirmed, some of which can also provide information about molecular structure and binding strength of those analytes. The examples below include technologies developed to detect biomarkers of specific diseases. In those cases, the detection limits achieved are within the range of clinical relevance.

The most widely employed sensing modality involves monitoring the resonance frequency of the metasurface as it interacts with its surroundings—operating on the principle that this frequency is linked to the refractive index immediately adjacent to the nanostructure. Therefore, when biomolecules bind to or approach the nanostructure's surface, they induce an easily observable resonance shift that provides a quantitative measure of molecular presence, binding events, and interaction kinetics.230 

As a widely used technique, there are thousands of examples of different geometries, materials, and operating frequencies that have been explored. What follows is a small selection of recent examples. Gold is the most popular material for these sensors because of its chemical stability, simplicity of modification with bio-receptors, and the ease by which visible resonances can be excited using white light.231 For over two decades, the most popular surface geometry has been circular nanodisks, a geometry that can be produced using many different fabrication techniques. Since controlled array spacing is not necessarily required for many biosensing tasks, a number of wide-area fabrication examples (e.g., colloidal fabrication) exist. Examples include hole-mask colloidal lithography and photolithography to produce gold nanodisks for multiplexed measurement of diagnostic-relevant protein-protein binding affinity at pM concentration.232 Similar fabrication techniques have been used to detect single-base DNA mutations,233 and others have shown that by coating the nanodisks with molecularly imprinted polymers (MIPS) instead of traditional “complementary” receptor molecules, label-free quantification of many polyphenol-protein interactions can be made.234 Other examples of wide-area fabrication for scalable sensor production include nanosphere lithography to produce Au nanopyramids to measure real-time protein (α–Synuclein)-small molecule interactions,235 and the use of nanoimprinting to create indented gold structures (“nanocaves”) for detecting tumor markers in human serum (carcinoembryonic antigen at 5 ng/ml).236 Electron-beam lithography enables greater metasurface control, providing more options for geometry and layout variation (e.g., multi-layered biosensors with internal self-referencing),237 and receptor localization (e.g., receptor “patches” adjacent to the meta-atoms for biomolecule-mediated metasurface reconfiguration).238,239 As such, periodic arrays of e-beam produced gold structures have been used to detect prostate specific antigen (PSA), in serum, at 500 pg/ml.240 In a variation of the periodic nanodisk structure, Shen et al. demonstrated gold mushroom arrays with high-intensity coupled resonance modes (between a gold film and elevated gold disks) for detecting alpha-fetoprotein (a liver cancer and fetal development marker) at 15 ng/ml.241 Similarly, high-intensity fields can be produced in small gaps between discrete structures, as demonstrated with IR-resonant nanorods for detecting human antibody IgG down to 30 pM,242 and to monitor peptide-induced neurotransmitters from synaptic vesicle mimics.243 

Recently, as the benefits of dielectric resonators has become apparent, similar functionality has been demonstrated with dielectric metasurfaces. Silicon nanodisks have been used to monitor biotin-streptavidin interaction at 10 10 M,244 to detect breast cancer biomarkers at 0.7 ng/ml,245 and to identify PSA at 1.6 ng/ml (below diagnostic requirements).246 Low-cost fabrication has also been explored in this space, with PSA detection shown using silicon disks produced via colloidal lithography,247 and, in efforts to produce cost-effective meta-sensors with CMOS camera compatible readouts, Triggs et al. demonstrated chirped silicon-nitride gratings for characterizing antibody binding.248 More complex shapes are also emerging, with crescent structures supporting quasi-BIC (bound states in the continuum)140 having achieved 0.16 nM protein detection,249 and where, notably, Altug's laboratory have manipulated the angles and spacings of germanium and silicon nano-ellipse structures, and combined them with hyperspectral imaging, to fingerprint various biomolecular events at high sensitivities.250,251

Chirality is a fundamental property of many biomolecules, including amino acids and nucleotides, where molecules exist in two forms that are mirror images of each other, known as enantiomers. This characteristic is crucial in biochemistry and pharmacology because enantiomers of the same substance can exhibit vastly different biological activities, with one form potentially beneficial and the other harmful. Chiroptical spectroscopic techniques can identify and characterize chiral biomacromolecules, but their sensitivities can be greater than six orders of magnitude lower than refractometric plasmonic sensors,252 meaning they are largely unsuitable for detection of enantiomers at diagnostically relevant concentrations (which can be pg/ml). The enhanced fields exhibited by resonant metasurfaces can significantly enhance the chiroptical response, enabling more sensitive detection of chirality. By using geometrically chiral plasmonic structures, so-called superchiral fields can be generated,253 optimizing the selective absorption of light by chiral molecules and enhancing CD signals. This provides a more efficient means for the detection, analysis, and separation of enantiomers, offering improved sensors for pharmaceutical development and diagnostics. The quest to generate superchiral fields for these purposes has led to a number of plasmonic metasurfaces which exhibit complex meta-atom geometries. Notable examples include the use of gold shuriken indentations, fabricated by injection molding, to detect pg quantities of helical biopolymers,252 perform multiplexed sensing of proteins and virions,254 and record the chiro-optical response of type II dehydroquinase.255 Another example of a chiral metasurface sensor demonstrated the use of a 3D core-shell nanohelix structure, constructed via ion beam deposition of a Pt core, surrounded by the conductive polymer P-oPD, to achieve detection of DNA binding protein 43 (relevant for neurodegenerative disease) at a concentration of 10 fM.256 

The ability to confine light to incredibly small volumes make metasurfaces ideal for increasing the efficiency of vibrational spectroscopy techniques. Many techniques can benefit from these properties, with Infrared Absorption Spectroscopy, Surface-Enhanced Infrared Absorption (SEIRA), and Terahertz spectroscopy, having benefited from metasurface enhancement (using IR resonating microstructures).257,258 However, the most common use of metasurface-enhanced vibrational spectroscopy is seen in Surface Enhanced Raman Spectroscopy (SERS).259,260 When light passes through a molecule with certain attributes, it may undergo Raman scattering; inelastic scattering that produces a fingerprint of molecular vibrations from that molecule, elucidating chemical composition and structure. However, Raman scattering is inefficient; large concentrations are required to obtain signals. SERS amplifies scattering from molecules adsorbed on nanostructured surfaces, enabling detection of individual molecules and events. Since SERS enhancement scales with electric-field, meta-atoms are engineered to exhibit sharp edges or small gaps to concentrate the electric-field. Sensing requires the excitation laser line crossover with the (typically broad) meta-atom plasmon, making surface tuning simple, even with less-controlled techniques (e.g., colloid-mediated lithography). Indeed, the vast majority of SERS work is performed using cheap, effective, metal colloid. However, commercial and research examples of metasurface SERS systems do exist when signal reproducibility or more sophisticated functionality are required. One such example used coupled gold triangular trimer structures designed to exhibit multiple resonance modes spanning the visible and mid-IR.261 With tip separation of a few nanometers, the trimers promoted large field in these volumes. The authors demonstrated that these multi-resonant structures were multi-functional, and could be used as SERS substrates for detecting DNA hybridization, and as surface enhanced fluorescence and SEIRA substrates.261 DNA hybridization has also been detected via SERS using metasurfaces consisting of polarization-sensitive silver split-ring resonators262,263 and by using the hybridization events to drive individual nanoparticle localization into the center of gold bowtie structures.239 Combining SERS with chiral nanostructures has led to chiral discrimination of amino-acids via SERS.263 Recent work on dielectric metasurfaces has shown that, much like for refractometric sensing, these surfaces may hold advantages over plasmonics for SERS biosensing. While plasmonic meta-atoms can require high laser intensities, leading to heating and molecular damage, dielectric meta-atoms exhibit little absorption, minimizing heat, while retaining nanoscale light confinement. However, although there is activity in dielectric SERS,264 and some examples of dielectric SERS metasurface substrates do exist (e.g., silicon dimers to detect β-carotenal molecules)265 the enhancement factors that can be achieved in Si systems are significantly lower than those seen in plasmonic systems (103 for Si vs 10 6 10 1 4 in metal systems),260 meaning that their impact in biosensing may be limited.

Surfaces that exhibit structured “negative” features, such as engineered holes and voids, also make for effective biosensors.266 Extraordinary Optical Transmission (EOT) through plasmonic nanoapertures is a phenomenon where light passes through holes in metal films at efficiencies higher than classical theories predict. A combined effect of propagating and localized surface plasmon resonances, EOT acts to concentrate and funnel light through the nanoapertures, significantly enhancing the transmitted light intensity, in doing so generating enhanced fields within the aperture itself. Thus, if molecules can be located inside these holes, refractometric and enhanced spectroscopy sensors can be created. Gold nanohole arrays have been shown to be highly sensitive to antibody binding events,267 capable of responding to PSA in a 96-well plate compatible metasurface format,268 used for detecting numerous exosomes for cancer diagnostics (measurements that can also monitor treatment efficacy),269,270 for detecting pancreatic cancer,271 and for identifying virus-like particles and assessing viricidal drug candidates.272 Hole structures also bring the possibility to create nanofluidic flow-through systems, where the apertures serve dual roles as both optical modulators and liquid delivery channels. Examples of these systems include circular gold apertures for label free DNA detection,273 single base DNA discrimination using gold nanopores and electro-plasmonic trapping,274 and bowtie shaped gold pores to detect DNA275 and beta-amylase protein translocation.276 

In all the above examples, recognition of the target and retention in the sensing volume has been achieved using a specific binding partner or interaction. For example, complementary base pairing of DNA, or a specifically chosen antibody or affinity protein for the target. Such approaches are well established and many methods of engineering metasurfaces with suitable chemistries attached have been achieved. However, there are also drawbacks to such an approach that are now becoming more widely recognized.

FIG. 8.

Metasurface biosensing examples. (a) Gold mushroom nanostructures.241 (b) Quasi-BIC mode supporting silicon crescent nanostructures.249 (c) Flow-through nanohole sensor for DNA sensing.273 (d) DNA-directed nanoparticle localization inside gold bowties, as a sensing system.239 (e) Cross-reactive metasurfaces for monitoring water treatment sites.277 (f) Multi-responsive, array-based nanoparticle sensor for micro-organism detection.278 (g) Barcode-based biosensing with elliptical dielectric resonators.251 (h) A SERS-based artificial nose sensors.279 (a) Reprinted with permission from Shen et al., Nat. Commun. 4, 2381 (2013). Copyright 2013 Macmillan Publishers Limited.241 (b) Reprinted with permission from Wang et al., Adv. Funct. Mater. 31, 2104652 (2021). Copyright 2021 Wiley-VCH GmbH.249 (c) Reprinted with permission from Shi et al., Nano Lett. 18, 8003 (2018). Copyright 2018 Authors, licensed under a Creative Commons Noncommercial License (CC-BY-NC-ND).273 (d) Reprinted with permission from Clark et al., Adv. Mater. 26, 4286 (2014). Copyright 2021 Wiley-VCH Verlag GmbH & Co.239 (e) Reprinted with permission from Sperling et al., Environ. Sci. 10, 3500 (2023). Copyright 2020 Authors, licensed under a Creative Commons Attribution 3.0 Unported License.277 (f) Reprinted with permission from Li et al., Anal. Chem. 89, 10639 (2017). Copyright 2017 American Chemical Society.278 (g) Reprinted with permission from Yesilkoy et al., Nat. Photonics 13, 390 (2019). Copyright 2019 Springer Nature Limited.251 (h) Reprinted with permission from Kim et al., Nat. Commun. 11(1), 207 (2020). Copyright 2020 Authors, licensed under a Creative Commons Attribution 4.0 International License.279 

FIG. 8.

Metasurface biosensing examples. (a) Gold mushroom nanostructures.241 (b) Quasi-BIC mode supporting silicon crescent nanostructures.249 (c) Flow-through nanohole sensor for DNA sensing.273 (d) DNA-directed nanoparticle localization inside gold bowties, as a sensing system.239 (e) Cross-reactive metasurfaces for monitoring water treatment sites.277 (f) Multi-responsive, array-based nanoparticle sensor for micro-organism detection.278 (g) Barcode-based biosensing with elliptical dielectric resonators.251 (h) A SERS-based artificial nose sensors.279 (a) Reprinted with permission from Shen et al., Nat. Commun. 4, 2381 (2013). Copyright 2013 Macmillan Publishers Limited.241 (b) Reprinted with permission from Wang et al., Adv. Funct. Mater. 31, 2104652 (2021). Copyright 2021 Wiley-VCH GmbH.249 (c) Reprinted with permission from Shi et al., Nano Lett. 18, 8003 (2018). Copyright 2018 Authors, licensed under a Creative Commons Noncommercial License (CC-BY-NC-ND).273 (d) Reprinted with permission from Clark et al., Adv. Mater. 26, 4286 (2014). Copyright 2021 Wiley-VCH Verlag GmbH & Co.239 (e) Reprinted with permission from Sperling et al., Environ. Sci. 10, 3500 (2023). Copyright 2020 Authors, licensed under a Creative Commons Attribution 3.0 Unported License.277 (f) Reprinted with permission from Li et al., Anal. Chem. 89, 10639 (2017). Copyright 2017 American Chemical Society.278 (g) Reprinted with permission from Yesilkoy et al., Nat. Photonics 13, 390 (2019). Copyright 2019 Springer Nature Limited.251 (h) Reprinted with permission from Kim et al., Nat. Commun. 11(1), 207 (2020). Copyright 2020 Authors, licensed under a Creative Commons Attribution 4.0 International License.279 

Close modal

The first consideration is one of multiplexing and the number of targets that need to be detected. In the paradigm considered above, assuming perfect receptor or sensing element specificity, “n” functional sensing elements are required for n different targets. As the understanding and appreciation of multi-biomarker solutions for disease detection or biological systems measurement grows, particularly with advances in “omic” technologies, n can approach a very large number, rendering it impractical or impossible to build such a large number of sensors.280 Second, again led by the increased amount of -omic data available, there is an understanding that not all biomarkers or target molecules are actually known at the outset. These “unknown-unknowns” might enable greater insight or more accurate diagnosis but simply have not yet been linked to the condition in hand. Finally, there is a question of just how specific a sensor can be, particularly when targeting small molecules or macromolecules with few features to bind.

Thus, the idea of cross-reactive arrays is now gaining traction. Such arrays contain multiple sensing elements, sensitive to multiple or many different targets, and, rather than generating a single target response, generate a pattern or fingerprint when exposed to a sample. The sensing fingerprint is then tied back to the sample contents by statistical analysis and pattern recognition. This cross-reactive approach breaks the n for n paradigm and enables a holistic approach to sample measurement where all biomarkers in the sample have a chance to be measured, not just those known about at the outset.281 The sensor targeting chemistry can be tuned to be more specific or more cross-reactive as the user desires, and a “training and testing” model is used to gather representative data that informs a statistical model or machine learning for further “unknown” sample analysis.282 

Incorporating this approach with metasurfaces allows optically addressable arrays to be created where changes in absorbance, reflectance, optical helicity, or other outputs are transduced to create a fingerprint, with spatial or wavelength based multiplexing possible for each individual part of the sensor array.227 

There are many examples of the method using plasmonic nanoparticles in solution, deposited inside well-plates, or drop-cast onto surfaces as discrete sensing areas that display differential reactivity directly with target compounds or biomarkers to alter their plasmonic color.283 For example, Hormozi-Nezhad and co-workers have made extensive studies in this area, differentiating pesticides,284 measuring urinary biomarkers for neurology,285 and also amyloid proteins among many other bio-targets.286 Biogenic amines as markers of food spoilage can also be detected in this manner,287 as well as the different bacteria themselves.278 In each case, the growth, etching, and aggregation of metallic nanomaterials that leads to their color change, is influenced by the different reactive chemistry included in each part of the array (typically a well in a 96 well plate), and the chemical interactions with the sample under test. It is clear that a reactive plasmonics approach such as this could be further integrated into arrays of plasmonic materials as part of a metasurface, however, this would likely have to be a single-use sensor, so may have limited utility in practical applications.

A second approach is to functionalize the metasurface elements with different cross-reactive chemistries, less specific than an antibody but still imparting a degree of selective binding to nearby biomolecules. We have already exemplified this approach with cross-reactive metasurface arrays applied to the sensing of small molecules in whiskey and water.277,288 In each case, a sensor was produced that consisted of an array of plasmonic metasurfaces (many separate metasurfaces on the same substrate) that were each modified with a self-assembled monolayer (SAM) of a different small molecule, imparting varying local charge, acidity, nucleophilicity and hydrophobicity to each metasurface. The metasurfaces were then exposed as an ensemble to different samples, reacting with molecules in solution to alter the local dielectric environment of each metasurface differently, creating a varying plasmonic fingerprint based on color shift across the array. These fingerprints were analyzed with principal component analysis (PCA) and linear discriminant analysis (LDA) to create tools capable of either discriminating whiskeys based on their chemical content, or treated and untreated water samples based on their mineral and organic carbon content. It is clear how such an approach can now be extended to direct biosensing, and is an area of ongoing research.

In a similar approach, but with a different transduction method, Stevens et al. demonstrated a gold nanopillar metamaterial, where each region (sensing element) again featured a different SAM.279 The metamaterial created strong SERS hotspots between the elements, and the interaction of the SAM with the targets (small biomolecules, extracellular vesicles, cell lysates) ensured different molecules could enter the hotspots, driving differential SERS spectra across the array. Two companion cell lines (one cancerous, one not) were easily identified based on their differential SERS spectra across select array elements, coupled with PCA and LDA analysis.

The coupling of this approach with the exquisite sensitivity of metasurfaces, their greatly improved multiplexing capacities, and advanced fabrication methods should make metasurface sensing arrays a hugely powerful tool in bioanalaysis of the future, capable of operating in both liquid and gas sensing modes to deliver pattern recognition of a wide variety of targets. Key to this success will be the choice of interfacial cross-reactive chemistries to deliver the best interactions with the analytes under study, as well as exploitation of novel transduction modes possible with advanced metasurfaces, to increase the data density that can be collected. As more and more metasurface-based tools become portable, thanks to their compatibility with simple optical platforms and even smartphone read outs, there is huge potential here for delivering ultra-sensitive biomolecule detection with specific or cross-reactive sensing, at the point of need.

George Perrakis*, Anna C. Tasolamprou, and Maria Kafesaki

*gperrakis@iesl.forth.gr

From a thermodynamic point of view, exploiting a 3 K heat sink (ultra-cold universe) of infinite heat capacity, earth objects can reach temperatures below the ambient air temperature ( 300 K). This is the principle behind passive radiative cooling (PRC). Specifically, PRC technology aims to exploit the coldness of the Universe for cooling purposes without any electricity input required.289 It relies on Earth's atmosphere transparency between 8 and 13 μm. This transparency window coincides with the peak of the black-body thermal radiation spectrum of earth objects at typical temperatures ( 300 K). Thus, any earth object facing the sky can release heat to the outer space, in the form of thermal electromagnetic (EM) radiation, lowering its temperature.

Such a passive strategy (i.e., that cools without any electricity input) could significantly impact global energy consumption (15% decrease).290 The use of PRC as a passive cooling mechanism in human infrastructure elements like buildings, for instance, may lead to lower indoor temperature and alleviate the need for excessive air conditioning, with potential for energy savings and improved efficiency.290 Many other devices and technologies, such as solar cells, thermal textiles, and thermophotovoltaics, can highly benefit from PRC.291,292

Despite the practical benefits and high potential impact, PRC has three main limitations.293–298 First, peak cooling demand occurs during daytime, when incident sunlight heats the objects, Fig. 9(a). Therefore, one will need a structure that reflects the entire solar spectrum, Figs. 9(b) and 9(d).296 Second, PRC technology may be counterproductive in year-round assessments, due to provision of undesirable cooling during cold days. Therefore, adaptive PRC may be required, i.e., based on dynamic modulation techniques to tune thermal emission depending on the ambient air temperature.297 Third, current PRC technology is highly unstable, strongly affected by climate or varying weather conditions (e.g., humidity or clouds, resulting to reduction of atmospheric transparency) and by surrounding constructions [e.g., high-rise buildings, providing additional heat or obstructing the heat release, Fig. 9(c)], causing drop of the radiative cooling power. These limitations are a direct consequence of the symmetry of absorption-emission met in most thermal emitters, owing to the fact that PRC configurations are made of reciprocal materials. Specifically, reciprocal materials, characterized by symmetric permittivity and permeability tensors, satisfy Kirchhoff's law of thermal radiation, which states that their spectral, directional emissivity (ε) equals their spectral, directional absorptivity (α). To this end, the two main existing strategies to stabilize PRC (i.e., avoid fluctuations in the radiative cooling power) are (i) high spectral selectivity [maximizing the thermal radiation in the atmospheric transmission window and keeping it minimum everywhere else, Figs. 9(b) and 9(d)]299 to mitigate the sensitivity to humidity, and (ii) enhanced directionality, also to a limited solid angle (beaming), to avoid additional heat gains from the surrounds, Fig. 9(c).300 

FIG. 9.

(a) Solar absorption and thermal radiation properties of conventional surfaces (without metasurfaces cooler). Sub-optimal thermal emission (for λ > 4 μm) and strong parasitic solar absorption are observed resulting to object's temperature larger than the ambient (300 K). (b) With a metasurface cooler on top (yellow layer), the radiative cooling is enhanced, by enhancing emissivity at 8–13 μm and reducing atmospheric absorption, and the unwanted solar radiation is strongly reflected, leading to sub-ambient surface temperatures, T< 300 K. Note that the object is insulated in (b) to minimize non-radiative heat gains from conduction and convention (e.g., wind), i.e., besides avoiding the radiative heat gains. (c) Illustration of obstruction effect of high-rise buildings on the access of coolers to the sky. The black and purple arrows indicate top and sidewalls emission, respectively. In (b) and (c), sun and outer space are omitted for clarity. (d) Ideal reflectivity (R, green), emissivity (ε, blue), and transmissivity (T, red) in ultraviolet (UV), visible (VIS), near-infrared (NIR), short-wave-infrared (SWIR), and mid-infrared (MIR) spectra, together with the normalized AM 1.5G solar irradiance spectra (gray, <4 μm) and the infrared transmission of the atmosphere (gray, > 4 μm).

FIG. 9.

(a) Solar absorption and thermal radiation properties of conventional surfaces (without metasurfaces cooler). Sub-optimal thermal emission (for λ > 4 μm) and strong parasitic solar absorption are observed resulting to object's temperature larger than the ambient (300 K). (b) With a metasurface cooler on top (yellow layer), the radiative cooling is enhanced, by enhancing emissivity at 8–13 μm and reducing atmospheric absorption, and the unwanted solar radiation is strongly reflected, leading to sub-ambient surface temperatures, T< 300 K. Note that the object is insulated in (b) to minimize non-radiative heat gains from conduction and convention (e.g., wind), i.e., besides avoiding the radiative heat gains. (c) Illustration of obstruction effect of high-rise buildings on the access of coolers to the sky. The black and purple arrows indicate top and sidewalls emission, respectively. In (b) and (c), sun and outer space are omitted for clarity. (d) Ideal reflectivity (R, green), emissivity (ε, blue), and transmissivity (T, red) in ultraviolet (UV), visible (VIS), near-infrared (NIR), short-wave-infrared (SWIR), and mid-infrared (MIR) spectra, together with the normalized AM 1.5G solar irradiance spectra (gray, <4 μm) and the infrared transmission of the atmosphere (gray, > 4 μm).

Close modal

Conventional approaches to meet these stringent requirements, however, come with large space demands, often requiring complex shelter and shading systems, and therefore limit applicability, often reducing also the emission power.293 For real objects in local thermal equilibrium, Kirchhoff's law of thermal radiation still holds at every combinatorial specific set of wavelength (λ), angle (θ), and polarization (p).300 The field of photonics though, which explores the use of light molding techniques in the visible and infrared, assisted by the recent advances in nano-fabrication, is capable of engineering ε at selected λ, θ, and p, i.e., using the concepts of photonic crystals (PCs), metasurfaces, and metamaterials. Metasurfaces and metamaterials, for instance, involve structuring the material at a subwavelength scale. Relying on local and non-local resonant phenomena (as a result of proper structuring, i.e., of suitably designed building-blocks, known as meta-atoms), metamaterials and metasurfaces provide unique optical properties and enable novel ways to tailor light. The careful engineering of shape, size, and arrangement of meta-atoms can imprint the desired response, such as enhanced directionality,300 or tune the spectral bandwidth of operation,301 also offering extra advantages, such as ultra-low thickness and weight,305 and flexibility. Therefore, metamaterials/metasurfaces have been employed as coatings for PRC applications in recent years [see Fig. 9(b)].305–307 Additionally, to date, considerable attention has been paid to utilizing metasurfaces and metamaterials also to relax the constraints coming from space-reversal symmetry or by reciprocity (connected to time-reversal symmetry) via (i) breaking the time-reversal symmetry with non-reciprocal materials and gratings, e.g., magneto-optic materials, time-modulated systems, or Weyl semimetals,308 or (ii) breaking the space-reversal symmetry of transmission with reciprocal but anisotropic or bi-anisotropic systems, such as metallic gratings with subwavelength slits, gradient metasurfaces, hyperbolic metamaterials, and chiral structures.309,310 Consequently, it would be of paramount importance to fabricate non-reciprocal thermal emitters (coolers) or asymmetric configurations that would emit or allow heat in certain directions without absorbing or allowing incoming heat.293,294 Currently and in recent years, there is significant research on examining different photonic strategies in terms of applicability, feasibility, and potential of enhancing and stabilizing PRC. In this perspective, we provide an overview of the recent and ongoing research on metasurfaces, metamaterials, and PCs as means to control the fundamental aspects of thermal radiation for cooling purposes. Such aspects are related to (i) spectral selectivity, (ii) tunability, (iii) directionality, and (iv) asymmetric propagation. Finally, we discuss the challenges and potential future directions.

Spectral selectivity. As mentioned already, spectral selectivity is a quite important feature of a radiative cooling system, as it can allow emission/absorption only in the beneficial for cooling 8 13 μm spectral window (atmospheric transparency window) and reject the non-beneficial parts of the EM spectrum, Figs. 9(b) and 9(d). In 2015, Hossain et al.301 fabricated an anisotropic, conical-shaped waveguide-metamaterial emitter composed of 14 alternating layers of aluminum (Al) and germanium (Ge) (size 2 μm, i.e., thickness and bottom diameter) to realize polarization-insensitive, highly selective PRC, Fig. 10(a). They demonstrated ultra-high emissivity (>0.85) and broadband operation at 8 13 μm, owing to the slow-light modes at different wavelengths (with peaks' emissivity >0.9) and the tapered shape. They measured a remarkably high cooling power, of 116.6 W/m2, at ambient air temperature and a temperature of 12.2  °C below ambient.

FIG. 10.

Examples of PRC metasurfaces of different geometries and configurations. (a) Array of fabricated conical-shaped metamaterial pillars.301 (b) Polymer-based hybrid metamaterial with randomly distributed SiO2 microspheres for large-scale PRC.302 (c) Janus emitter applied to an automobile under direct sunlight, where heat is trapped by the greenhouse effect, allowing broadband absorption of IR waves from the enclosure and selective emission to the ultracold space.299 (d) Structure of the temperature-adaptive radiative coating and principle of operation.297 (e) Phase change metasurface consisting of periodic VO2/SiO2/VO2 cavities.303 (f) Spectra of the thermochromic smart window with and without radiative cooling regulation in VIS, NIR, and MIR at 20 °C (blue) and 90 °C (red).304 (g) Asymmetric transmission periodic micro-structure of inverted silver trapezoids and relevant forward/backward transmission, reflection and absorption over the atmospheric transparency window range and beyond.294 (h) Schematic of the asymmetric transmission pyramidal micro-structure and principle of operation.295 (i) Array of fabricated SiO2/AlOx double-shell hollow microcavities for directional thermal emission.300 All images are reprinted (adapted) with permission from the respective Journal and copyright remains with the original publisher.

FIG. 10.

Examples of PRC metasurfaces of different geometries and configurations. (a) Array of fabricated conical-shaped metamaterial pillars.301 (b) Polymer-based hybrid metamaterial with randomly distributed SiO2 microspheres for large-scale PRC.302 (c) Janus emitter applied to an automobile under direct sunlight, where heat is trapped by the greenhouse effect, allowing broadband absorption of IR waves from the enclosure and selective emission to the ultracold space.299 (d) Structure of the temperature-adaptive radiative coating and principle of operation.297 (e) Phase change metasurface consisting of periodic VO2/SiO2/VO2 cavities.303 (f) Spectra of the thermochromic smart window with and without radiative cooling regulation in VIS, NIR, and MIR at 20 °C (blue) and 90 °C (red).304 (g) Asymmetric transmission periodic micro-structure of inverted silver trapezoids and relevant forward/backward transmission, reflection and absorption over the atmospheric transparency window range and beyond.294 (h) Schematic of the asymmetric transmission pyramidal micro-structure and principle of operation.295 (i) Array of fabricated SiO2/AlOx double-shell hollow microcavities for directional thermal emission.300 All images are reprinted (adapted) with permission from the respective Journal and copyright remains with the original publisher.

Close modal

Apparently, fabrication feasibility, scalability, and cost-effectiveness are also vital for promoting PRC as a viable energy technology. With this in mind, Zhai et al.,302 two years later, demonstrated a highly transparent hybrid metamaterial cooler composed of resonant polar dielectric (silica) microspheres ( 4 μm diameter) randomly distributed in a polymeric matrix (50-μm-thick), Fig. 10(b). By accessing the high-order Fröhlich resonances of the polar silica spheres, the metamaterial provided larger than 0.93 and broadband infrared emissivity across the atmospheric window. When backed with a silver coating to reflect solar radiation, the metamaterial showed a noon-time radiative cooling power of 93 W/m2 under direct sunshine. The high significance of this work is related to the high-throughput and economical roll-to-roll manufacturing of the metamaterial combined with the high PRC performance.

Although the impact of PRC on emitters' surface temperature was early highlighted, its impact on cooling inner (enclosed) spaces (that trap heat by the greenhouse effect) remained unclear until 2020. In 2020, Heo et al.299 presented a Janus thermal emitter for cooling enclosed spaces, consisting of an Ag-coated quartz grating and a polydimethylsiloxane (PDMS) layer on quartz substrate's front and rear side, Fig. 10(c). The Janus emitter acted as selective emitter on the top side, to dissipate heat to outer space, and as broadband emitter on the bottom side, to draw heat from the inner space, Fig. 10(c). The selective emitter showed a sub-ambient cooling of 6  °C, obtained by exploiting spoof surface plasmon polaritons (sSPPs) to achieve near-ideal selectivity. Notably, the temperature of an object inside the enclosure was 4  °C lower than that achieved when using a conventional radiative cooler with reflective bottom surface. These results demonstrate that metasurfaces and metamaterials can provide high-efficiency PRC ( 4 12  °C) even paired with high-throughput, economical, and scalable manufacturing, promoting PRC as a viable energy technology.

Tunability. As mentioned also earlier, the ability to tune a cooler's thermal emission depending on the environmental conditions allows for an adaptive PRC system whose cooling power switches off when not needed, e.g., in cold nights or winter times, avoiding overcooling that increases the heating cost. In 2018, Ono et al.311 proposed an one-dimensional (1D)-PC consisting of two components, a spectrally reflective filter and a switchable radiative cooler. The on-top spectrally selective filter was a 11-layer stack of alternating Ge/MgF2 layers, which served as a reflector of solar radiation and a passband filter with high transmissivity at 8 13 μm. The radiative cooler at the bottom consisted of a VO2/MgF2/W tri-layer, where VO2 (material with phase transition hysteresis) was the switching component. The angle and polarization averaged emissivity ( 0 ° 90 °) in the wavelength range of 8 13 μm was 0.64 in the metallic state of VO2. On the contrary, the averaged emissivity dropped to 0.05 when VO2 was in insulating phase, leading to a 0.59 modulation performance. At “on” state, the radiative cooler reached an equilibrium temperature around 10 °C below ambient. At low ambient temperatures ( 25  °C, when cooling is not needed) the system switched off, accompanied by a sudden reduction of the cooling power, maintaining cooler's temperature near the critical temperature ( 25  °C). The same year (2018), Wu et al.303 proposed a phase-change metasurface consisting of periodic VO2/SiO2/VO2 cavities supporting a thermally switchable Fabry–Pérot (FP)-like fundamental resonance mode at 8 13 μm, Fig. 10(e). The thermal emissivity there switched from 0.2 to 0.65 ( 0.45) with a critical (VO2 phase-change) temperature 68  °C. The cooling power was calculated 118 and 528 W/m2 at device temperatures 67 and 69  °C, respectively, compared to 1.3 and 187 W/m2 for a simple 300-nm-thick VO2 film. In 2020, Zhang et al.312 proposed a trapezoidal hyperbolic metamaterial (HMM) emitter composed of Ge and VO2 for self-adaptive PRC and a MgF2/Ge multilayer filter placed on top for reflecting solar radiation and allowing selective transparency (at 8 13 μm). When VO2 was in metallic phase, close-to-unity, broadband (>4 μm), and angle-insensitive emission was achieved due to the slow-light waveguide mode in the trapezoidal HMM, while with the presence of the filter on the top, system's emissivity peaked (>0.95) only at three wavelengths (8.4, 9.7, and 11.8 μm), creating spectral-selective emission (at 8 13 μm). The thermal emissivity modulation performance was 0.67 (from 0.20 to 0.87), leading to a cooling power >100 W/m2 (up to 127.8 W/m2) when cooler's temperature was 69  °C, and 27.9 W/m2 at 68  °C. Considering scalability, flexibility, and low cost, Tang et al.,297 in 2021, fabricated a phase-change metamaterial consisting of a 2D array of thin WxV1−xO2 blocks embedded in a BaF2 dielectric layer on an Ag film, in various design configurations, for production in a roll-to-roll fashion, Fig. 10(d). When WxV1−xO2 was in the insulating phase, the metamaterial was mainly transparent to the IR radiation at 8 13 μm and highly emissive when it switched to the metallic state. The thermal emissivity was further amplified by a designed photonic resonance involving adjacent WxV1−xO2 blocks and by the 1 4-wavelength cavity formed in BaF2 due to the bottom ultra-thin Ag layer, Fig. 10(d). This flexible, temperature-adaptive metamaterial switched its thermal emissivity from 0.2 to 0.9 (0.7) at 8 13 μm when the surface temperature raised above 22  °C. The metamaterial was 2  °C warmer than two reference (non-adaptive) roof coatings when the ambient temperature was below 22  °C.

In addition to modulation of thermal emissivity, the simultaneous modulation also of solar heating (i.e., absorbing/reflecting near- and short-wave-IR radiation and remaining transparent or semi-transparent in the visible spectrum), e.g., in smart and/or thermochromic windows, may further improve the regulation of heat flow to maintain a system's temperature near a desired set point, increasing energy-savings. In 2020, Wang et al.313 proposed a compound metasurface of large and small cross-shape resonators for realizing self-adaptive PRC and solar heating in one system. The core of the self-adaptive response is the utilization of a PVP (pNIPAM) spacer below crosses, which can expand (contract) with heat and contract (expand) with cold, increasing or decreasing crosses' spacing. The absorptivity/emissivity modulation performance in both solar and 8 13 μm spectral regimes was 0.8 (from 0.1 to 0.9) within a narrow bandwidth. The results showed that the compound metasurface can remain cool at 35  °C and warm at 25  °C. In 2021, Wang et al.304 fabricated a scalable smart window based on W-doped VO2 nanoparticles dispersed in a PMMA solution on top of a PMMA spacer for simultaneous thermal emissivity and solar reflectivity modulation, Fig. 10(f). Interestingly, the critical temperature was tuned through W-doping around 27.5  °C (i.e., much lower than that of pure VO2, 68  °C). The stacking formed a FP resonator with weak resonance (at 8 13 μm) at low temperatures and strong FP resonation effect at higher temperature (due to the insulator-to-metal transition of VO2), enhancing emissivity. The emissivity at low temperatures was 0.21 and at high temperatures 0.61 (0.4 modulation performance), combined with near-IR solar modulation of 9.3%, due to the change of the transmittance of VO2 with temperature, Fig. 10(f). The proposed smart window yielded higher energy-saving performance than a commercial low-ε window, up to 324.6 MJ/m2, revealing the importance of integrating PRC modulation in smart and/or thermochromic windows.

Stabilizing PRC. In 2018, Wong et al.294 proposed an asymmetric EM transmission structure (ATS) placed on top of a thermal emitter/cooler to reduce its dependence on humidity and clouds that lower its radiative cooling power, Fig. 10(g). The proposed ATS, consisting of a tapered metallic grating, exhibited high-contrast asymmetric transmission at 8 13 μm ( 0.8 forward and 0.4 backward, ratio 2), Fig. 10(g). Therefore, the ATS, on one hand, permits outgoing transmission of thermal radiation emitted by the radiative cooler, and, on the other hand, it reflects incoming radiation from the sky that lies within the same wavelengths [see also Fig. 10(h)]. Theoretical predictions under humid, semi-transparent sky revealed 57% (50 W/m2) recovery of cooling power, translating to a radiative cooler's temperature 8  °C lower than the standalone radiative cooler (without ATS). In this case, however, the metallic grating is made of silver, Fig. 10(g), which is a reciprocal material, meaning the directional absorption must equal the directional emission. Therefore, the amount of recovered cooling power should be reconsidered.314 Two years later, Wei et al.295 proposed an ATS placed on top of a thermal emitter/cooler to mitigate the combined effects of weather (humidity and clouds) and terrain [e.g., obstruction effect of high-rise buildings, Fig. 9(c)] on its radiative cooling power, Fig. 10(h). Using a glass imprinted with properly designed micro-structures [Fig. 10(h)], a high forward-backward transmissivity contrast ratio at 8 13 μm was achieved ( 0.7 forward and 0.1 backward, ratio 7). Theoretical predictions revealed that the radiative cooler integrated with the ATS has an insensitive (no matter the conditions and terrain) equilibrium temperature 10  °C below ambient, compared to 2  °C of the standalone thermal emitter/cooler. As in the previous case, though, the equilibrium temperature should be reconsidered.314 

In 2023, Cho et al.300 reported directional, polarization-insensitive thermal emission, which facilitates PRC from the sidewalls of buildings [Fig. 10(i)], see also purple arrows in Fig. 9(c). They fabricated hexagonally arrayed holes (12-μm pitch, 5.5-μm depth, and 10-μm diameter) realized by sub micrometer-thick SiO2/AlOx double shells (100/100 nm), resulting to a hollow cavity film, Fig. 10(i). The cavity film exhibited average emissivity values of 0.51–0.62 at angles (θ) 60 ° 75 ° and 0.29 0.32 at 5 ° 20 °, yielding a parabolic antenna-shaped distribution. The angular selectivity peaked at four different wavelengths at 8 13 μm (8, 9.1, 10.9, and 12 μm), identified as Berreman modes (only in p-polarization at epsilon-near-zero (ENZ) wavelengths of SiO2) and photon-tunneling modes (at maximum negative permittivity wavelengths of AlOx), creating broadband (at 8 13μm), angle-selective emission bands, resulting to superior directional PRC compared to an isotropic black body-like thermal emitter (up to 4  °C). We note that Wang et al.315 proposed the same year that directional thermal emission can also be realized in a broader spectrum beyond the previously considered ENZ and Berreman mode region. A two-phase metamaterial emitter composed of only two materials (material 1 containing subwavelength particles of material 2 on top of a perfect electrical conductor) was proposed showing numerically strong (ε > 0.8) directional (80  ° ± 5°) broadband thermal emission, where the bandwidth of directional thermal emission can be controlled through the methods of gradient ENZ and effective medium theory. These results suggest that by utilizing metasurfaces and metamaterials, PRC can be less affected by the environmental conditions and the surroundings, which promise all-weather and all-terrain applications.

Regarding non-reciprocal emitters, where Kirchhoff's law of thermal radiation does not hold, i.e., emissivity is not equal to absorptivity, their impact on PRC is not extensively examined in the literature. Two recent studies in 2023, by Liu et al.316 and Shayegan et al.,317 demonstrated direct observation of Kirchhoff's law violation under a moderate external magnetic field (<1.5 T). They reported inequality between emissivity and absorptivity over a broad MIR band, matching also 8 13 μm.317 In Liu et al. case, they used a photonic design that supports a guided-mode resonance coupled to a magneto-optic material316 while in the Shayegan et al. case, they employed magnetized gradient epsilon-near-zero thin films.317 

Using concepts coming from metasurfaces, metamaterials, and PCs research, recent studies have demonstrated tunable (>0.4) and high-efficiency PRC ( 4 12  °C below ambient) during daytime under direct sunlight, even combined with high-throughput, economical, and scalable manufacturing, Fig. 10. Here, we emphasize on flexible, roll-to-roll processing technologies that earned sufficient attention as a future contender on the PRC industry manufacturing roadmap [see Figs. 10(b) and 10(d)].297,302 In this case, radiative coolers' reliability should be further examined in relation to integrated polymers' degradation (thermally- or light-induced due to ultraviolet (UV) radiation) or to metal reflectors' oxidation by oxygen and moisture. Employing chemical additives, coatings, or hybrid organic-inorganic ultra-thin PRC multilayers306 may improve polymers' outdoor performance, although thin polymer films with extended outdoor lifetimes are already available.302 Moreover, a way to avoid the metal layers or more intricate 1D multilayers (commonly used to reflect sunlight) and still strongly reflect sunlight (including UV radiation) is by integrating layers with light-scattering pores on the nano- and micro-scale.318 Porous structures' durability, though, affected by dust or moisture, should be further examined. Alternatively, to avoid porosity and efficiently reflect solar light, paints based on a binder (e.g., polymeric) with various fillers and high-index pigments to scatter light (such as TiO2, SiO2, Al2O3, and BaSO4 nano- and micro-particles) could be utilized,318 i.e., “complementary” to porous structures. Finally, minimizing non-radiative heat gains from conduction and convention [wind, Fig. 9(b)] should also be addressed for efficient PRC below ambient air temperature (i.e., besides avoiding the radiative heat gains).296 This could be achieved by tuning metamaterial's thermal conductivity, which can be achieved by changing its chemistry or micro-structure.319,320

In addition to the above, great effort has been conducted to stabilize PRC subject to unideal conditions such as clouds, humid subtropical climate, and high-rise buildings [Fig. 9(c)].294,295,300,315 Despite the demonstrated superior directional PRC (up to 4 o C compared to an isotropic black body-like thermal emitter), enhanced selectivity and directionality come with reduced sky access and emission power [Figs. 9(b)–9(d)],296 limiting heat dissipation to outer space and, therefore, cooling power. Moreover, integrating ATSs composed of reciprocal materials to break the symmetry of transmission as means to stabilize PRC [Figs. 10(g) and 10(h)]294,295 is still under consideration.314 Theoretical studies proved that asymmetric transmission is achievable using reciprocal materials but without increasing the overall radiative cooling power.314 Therefore, it would be interesting to experimentally examine the applicability and potential of this photonic strategy on stabilizing PRC compared to conventional approaches for enhancing directionality and selectivity, given also the plethora of photonic designs for asymmetric transmission proposed in the literature (e.g., gratings, arrays, and chiral metamaterials) for various applications, even at 8 13 μm.321 We note that, according to recent studies,293–295 asymmetric transmission and PRC cannot be enabled in the same component or material (e.g., one grating or metamaterial), which may increase PRC system's complexity, Fig. 10(h).

A promising route to stabilize PRC, maintaining high cooling power enabled by a single component, is to avoid the reciprocity constraints with non-reciprocal materials.316,317 Maximal violation of detailed balance in thermal radiation (absorbed equals emitted) currently faces several challenges, as it requires a temporal or magnetically induced modulation.322 The design, simulation, and optimization of such approaches, though, are inherently complex while the involved phenomena are typically resonant.322 Consequently, novel solutions to achieve high-contrast asymmetric propagation with broadband operation in MIR are required.316 Moreover, the impact of non-reciprocal thermal emission on PRC, to our knowledge, has not been experimentally demonstrated yet, also due to limited fabrication feasibility, scalability, and increased cost in standard lithographic techniques. Toward this direction, emerging low-cost, robust, and scalable approaches could be employed, such as two-photon polymerization (2PP), 3D/4D-printing, and laser-induced periodic surface structures (LIPSS) with ultrashort pulsed lasers, for nano-structuring a wide range of materials (metals, semiconductors, dielectrics, and polymers). Decoupling α and ε can lead to novel functions, ranging from reducing losses from solar radiation re-emission in solar energy harvesting systems (occurring due to reciprocity) to radiative camouflage and optimum PRC.292,317

G.P., A.C.T., and M.K. acknowledge support by the Hellenic Foundation for Research and Innovation (HFRI) under “Sub-action 2 for Funding Projects in Leading-Edge Sectors—RRFQ: Basic Research Financing (Horizontal support for all Sciences),” Project ID 15117 (MultiCool).

Anastasiia Zaleska, Wayne Dickson, David Richards, and Anatoly V. Zayats

a.zayats@kcl.ac.uk

Chemical reactions facilitated by light are a cornerstone of clean energy conversion, environmental remediation and material synthesis. While photochemical transformations can take place when reactants are directly illuminated with light, the use of various material systems, often nanostructured, as photocatalysts, can enable a broader class of chemical reactions and can offer enhanced efficiency and selectivity in light-driven processes. Nanomaterials, particularly nanoparticles, play a crucial role in photocatalytic applications by providing large surface areas for molecular adsorption and unique electronic properties, enabling efficient light absorption and facilitating controlled photochemical transformations. The size, shape, and composition of nanomaterials and their surface chemistry can be engineered to optimize photocatalytic activity and selectivity, thereby enhancing the overall efficiency of light-driven reactions. Both semiconductor and metallic nanostructures can be used for photocatalysis, exploiting strong light absorption in the designed spectral range, resulting in electron–hole pair excitation through interband transitions in semiconductors and metals, or the generation of energetic hot electrons using surface plasmon resonances in metallic nanostructures. An additional side-effect of such electronic excitations is the rise of the local temperature of the material which influences the reactivity through thermal effects, as in conventional thermal catalysis. Overall, the integration of nanostructures with photochemistry and photocatalysis holds significant promise for advancing sustainable technological solutions.

In this respect, metamaterials and metasurfaces provide a highly flexible platform for engineering enhanced light–matter interactions which control electric field localization and its polarization as well as local temperature, while providing large surface areas important for chemistry.323,324 The planar geometry of metasurfaces also facilitates their integration into photo-electrochemical systems, while the use of metaparticles—nanoparticles engineered using metamaterial design principles325—provides the combination of increased surface area and engineered optical response with the advantage of scaleable fabrication afforded by nanoparticle synthesis. Furthermore, the opportunity to engineer these nanoparticles as a coating on powders is preferred for some photocatalytic applications.

Many realizations of metallic (plasmonic) and semiconductor metasurfaces and metamaterials have been recently developed and investigated for enhancing and controlling photocatalytic and photo-electrocatalytic transformations. While plasmon resonances have been widely employed in metallic catalysts, which provide enhanced light absorption and hot-carrier excitation not limited by a bandgap, semiconductor, and dielectric metastructures are also actively considered for enhancing light absorption at sub-bandgap energies and engineering strong absorption in catalytically inert dielectrics decorated with metal or semiconductor catalysts. These often exploit archetypical metasurface designs based on nonradiative high-index nanophotonics, such as multipolar Mie resonances and bound states in the continuum.

Photocatalysts based on semiconductor materials, such as titanium dioxide (TiO2) and gallium phosphide (GaP) are traditionally attracting significant attention because of the possibility to utilize solar energy directly.326,327 They generally benefit from high chemical stability and reactivity, are nontoxic and inexpensive. Holes in the valence band and electrons in the conduction band, excited upon interband light excitation, can then interact with molecules on the surface promoting chemical transformations. However, optical absorption in semiconductor materials is limited by their bandgap. TiO2 has a relatively large bandgap of about 3.2 eV, which makes this catalyst predominantly active under UV light, in the wavelength range shorter than 400 nm, compared with a bandgap of around 2.2 eV for GaP. This means TiO2 photocatalysts can harvest only a few percent of the solar spectrum energy (about 3%–5%). Additionally, the photocatalytic activity of semiconductor materials is limited by the rapid recombination of photogenerated electron–hole pairs and insufficient charge carrier transfer. Extensive research efforts have been dedicated toward the improvement of their photocatalytic performance through material modification and incorporation or doping with other functional materials for visible light utilization.

Metasurfaces patterned on TiO2 and GaP have been designed to increase the absorption of light and broaden their working spectral range.328–331 For TiO2 metasurfaces, doping can be first used to introduce defect states near the bandgap edges, and the associated absorption is then increased at selected wavelengths of visible light by engineering nonradiative modes via the interplay between electric and magnetic dipoles of the individual nanostructures forming the metasurface (meta-atoms), lattice modes, or the effects of bound states in the continuum. While the defect related excitation of electron–hole pairs may have its drawbacks, primarily in lower carrier mobility, the enhanced absorption at the resonances in the visible spectral range can be sufficient to demonstrate photocatalytic transformations at wavelengths where reactivity would be negligible without the metasurface. The impact of patterning also increases the surface area available for photocatalytic activity, which may be the dominant effect, as was demonstrated for NO degradation, when a 20-nm-thick TiO2 film was conformally coated onto a photonic crystal, with some contribution also from the waveguided mode excitation enabled by the photonic crystal.332 This approach can be applied to various types of semiconductor material systems by varying the active materials, leading to substantial improvements in air/water contamination, water splitting or artificial photosynthetic processes (Fig. 11).

FIG. 11.

(a) SEM images of the TiO2 metasurfaces for the photoreduction of Ag nanoparticles and the dependence of the obtained mean particle size as a function of illuminating time for the black TiO2 metasurface (red square line), the black TiO2 film (blue triangle line), and the conventional TiO2 metasurface (green dot line). Scale bar: 200 nm. The insets show the corresponding structural colors under a bright-field microscope. A schematic of the electron transfer process is shown on the left.328 (b) An active metasurface based on a periodically patterned SiNx coated with TiO2 photocatalyst. The spectra of the photochemical efficiency for the flat sample and the two different structured samples.332 (c) Enhancement of hydrogen evolution reaction (HER) under simulated solar illumination from a dielectric metasurface made of amorphous a-GaP on an ITO-covered glass slide, compared to that with a 100 nm thick continuous a-GaP film.331 (d) Wavelength-dependent reactivity for photocatalytic H2 dissociation using a quasi-BIC metasurface with and without Ni nanoparticle loading.334 Adapted with permission from.334 Copyright 2024 American Chemical Society. (e) Photo-electrocatalytic HER performances of core–shell Cu/Pt lattices combining individual LSPs and collective plasmonic SLRs under different illumination wavelengths.335 Adapted with permission from Deng et al., Nano Lett. 21, 1523 (2021).335 Copyright 2021 American Chemical Society. (f) Plasmon-enhanced visible-light-driven hydrogen production from water using a TiN plasmonic metasurfaces (blue line) and compared with a TiN film (black line).336 Adapted with permission from Yu et al., ACS Photonics 8, 3125 (2021).336 Copyright 2021 American Chemical Society.

FIG. 11.

(a) SEM images of the TiO2 metasurfaces for the photoreduction of Ag nanoparticles and the dependence of the obtained mean particle size as a function of illuminating time for the black TiO2 metasurface (red square line), the black TiO2 film (blue triangle line), and the conventional TiO2 metasurface (green dot line). Scale bar: 200 nm. The insets show the corresponding structural colors under a bright-field microscope. A schematic of the electron transfer process is shown on the left.328 (b) An active metasurface based on a periodically patterned SiNx coated with TiO2 photocatalyst. The spectra of the photochemical efficiency for the flat sample and the two different structured samples.332 (c) Enhancement of hydrogen evolution reaction (HER) under simulated solar illumination from a dielectric metasurface made of amorphous a-GaP on an ITO-covered glass slide, compared to that with a 100 nm thick continuous a-GaP film.331 (d) Wavelength-dependent reactivity for photocatalytic H2 dissociation using a quasi-BIC metasurface with and without Ni nanoparticle loading.334 Adapted with permission from.334 Copyright 2024 American Chemical Society. (e) Photo-electrocatalytic HER performances of core–shell Cu/Pt lattices combining individual LSPs and collective plasmonic SLRs under different illumination wavelengths.335 Adapted with permission from Deng et al., Nano Lett. 21, 1523 (2021).335 Copyright 2021 American Chemical Society. (f) Plasmon-enhanced visible-light-driven hydrogen production from water using a TiN plasmonic metasurfaces (blue line) and compared with a TiN film (black line).336 Adapted with permission from Yu et al., ACS Photonics 8, 3125 (2021).336 Copyright 2021 American Chemical Society.

Close modal

The engineered enhanced light absorption and planar geometries of metasurfaces enable their use as a photo-electrode material,333 demonstrating, in the case of a GaP-nanodisk-based metasurface, an overall photocurrent enhancement by more than fivefold compared to a planar GaP film under hydrogen evolution reaction conditions [Fig. 11(c)].331 Nanoimprint lithography techniques and electrochemical approaches enable the rapid development of large-area nanopatterned surfaces of various materials necessary for industrial applications as a new generation of highly efficient solar meta-electrodes.

Dielectric (catalytically inactive) metasurfaces decorated with metal catalytic nanoparticles can be employed as an antenna-reactor photocatalyst where the virtually lossless metasurface funnels light to efficiently drive a chemical reaction.334 By combining a Si3N4 metasurface exhibiting strong quasi-bound states in the continuum (quasi-BIC) resonances and large electric field enhancements, with Ni nanoparticles as reactors, this hybrid metasurface-based catalytic system was demonstrated to efficiently drive H2 dissociation under the resonant illumination [Fig. 11(d)]. The experimental and theoretical analysis suggest that both photothermal heating and electronic transitions at the quasi-BIC-Ni surface facilitate catalytic chemistry (the thermal effect also cannot be ruled out in the above-discussed semiconductor metasurfaces when defect-related transitions are considered). This approach, based on engineered dielectric metasurfaces combined with otherwise weak absorbing metal catalysts, provides a universal platform which can be exploited for various combinations of dielectrics and catalytic metals.

In contrast to dielectric and semiconductor materials, plasmonic nanostructures provide the unique possibility to engineer light absorption throughout visible and near-infrared spectral ranges with spectral selectivity as well as strong electromagnetic field enhancement. Surface plasmons excited in metal nanostructures by light absorption decay either radiatively through re-emission of photons or non-radiatively via, e.g., Landau damping on a femtosecond timescale, resulting in hot carriers with energies comparable to the energy of the exciting photons.337 These highly energetic hot carriers generated from the non-radiative decay of surface plasmons can either directly interact with molecular adsorbates on the metal surface or be transferred into adjacent semiconductor or metal co-catalysts efficiently inducing chemical transformations.

Plasmonic hot-electrons can be harvested in metal-semiconductor Schottky barriers, by combining the semiconductor catalyst with plasmonic nanostructures to achieve efficient electron–hole pair separation to prevent their direct recombination, thus increasing the lifetime of the hot-carriers and prolonging their opportunity to interact with adsorbed molecules. One drawback of metal–semiconductor heterostructure catalysts is the low charge mobility from the Schottky contacts. Many designs use a combination of plasmonic metasurfaces on or in a catalytic semiconductor layer simply to increase the local temperature and enhance thermal catalysis.

Alternatively, by incorporating strong plasmonic effects from metallic (Au, Cu, Al, etc.) nanostructures with highly catalytic active materials, core-shell or decorated nanostructures can be employed for various types of reactions. Plasmonic metasurfaces decorated with transition metals (Pd, Pt) can enable efficient coupling of light energy into a catalyst through strong electromagnetic field enhancement, hot-carrier generation and photothermal effects. Such metaparticles (or hetero-nanoparticles) have demonstrated a significant increase in photocatalytic activity over monometallic particles by efficient transfer of hot carriers to the catalytic surface for various reactions.338–341 

As an alternative to conventional plasmonic metals, titanium nitride (TiN) has started attracting significant attention for photocatalysis. TiN metasurfaces can be designed to exhibit broadband optical absorption in the visible range (i.e., an average of more than 92% in the 400–750 nm spectral range).336 When coated with a polymeric photocatalyst, plasmon-enhanced hydrogen production from water under visible-light illumination was achieved, with the hydrogen evolution rate increased by 300% compared to a smooth TiN film [Fig. 11(f)]. The increased efficiency is attributed to a combination of enhanced light absorption, carrier separation, hot carrier transfer and thermal effects induced by the plasmonic metasurface. Refractory plasmonic TiN metasurfaces were also used for driving heterogeneous photothermal catalytic reactions. Self-assembled TiN cylindrical nanocavities act as plasmonic “nanofurnaces” capable of reaching temperatures above 600 °C under moderately concentrated solar irradiation (∼20 Suns). Upon decoration with catalytic Rh nanoparticles, the obtained photothermal metasurface achieved a high rate of CO2 production under solar-spectrum-simulated light intensities342 as well as in the reverse water gas shift reaction.343 

Metasurfaces based on arrays of strongly coupled bimetallic core–shell nanoparticles were also exploited to enhance photo-electrocatalytic activity for hydrogen evolution reactions.335 Large-area Cu–Pt nanoparticle lattices fabricated by combining top-down lithography and solution-based chemistry support two different types of plasmon modes, localized surface plasmons from individual particles and surface lattice resonances (SLRs) from the 2D lattice, that increased the catalytic activity under white-light illumination up to 60% [Fig. 11(e)].

In addition to optical excitation by light illumination, hot electrons can also be generated electrically by excitation with tunneling electrons in tunnel junctions.344 Recent realization of an electrically driven plasmonic nanorod metamaterial provides the opportunity to use an electron tunneling effect for the simultaneous excitation of hot electrons and surface plasmons, providing the means to realize a new kind of hot-electron-activated nanoreactor. By constructing a high-density array of plasmonic tunnel junctions at the top surface of a plasmonic metamaterial composed of vertically oriented gold nanorods, the efficient electrical excitation of the plasmonic modes of the metamaterial by inelastic electron tunneling can be realized. During the tunneling process, the majority of electrons (∼99.9%) tunnel elastically in the electrically driven nanorod metamaterials, appearing as energetic hot electrons in the nanorod tips. The highly efficient and confined hot-electron generation makes the tunnel junctions highly reactive and opens up opportunities for the precise activation of chemical reactions in the junctions, which can be further detected with high sensitivity by observing the light emission from the tunnel junction or by observing changes in the tunneling current due to the extreme sensitivity of this highly confined tunneling process to any changes in the junction.

Photonic metasurfaces have emerged as a promising platform for driving photocatalytic reactions. To develop a highly efficient photocatalyst with a photonic metasurface, several key design concepts need to be considered (Fig. 12). First of all, scalability of the fabrication method. For commercial exploitation, high durability, low unit cost, and efficient use of materials are undoubtedly the most appealing criteria for economic viability and sustainable chemical/energy production. Most of the aforementioned metasurfaces were fabricated using EBL or lithographic techniques which are relatively expensive fabrication processes and do not offer a large catalytic surface area. Nanoimprint lithography and self-assembled techniques can be used instead to reduce the fabrication cost.

FIG. 12.