Roadmap on photonic metasurfaces

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

^*[email protected]

^**[email protected]

^***[email protected]

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

[email protected]

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 components^7,8 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.

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, metagratings^27,28 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.

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 problems^44–46 [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.

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^*

^*[email protected]

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.