Photonic metasurfaces are typically realized by the periodic distribution of meta-atoms, which incorporate two or more different materials. This requirement introduces constraints in the design and fabrication that are particularly significant for flexible and conformable metasurfaces. Here, we report on the design and fabrication of efficient, polarization-independent, all-polymeric metasurface membranes for holographic applications in the visible range. These results will facilitate the large-scale production of holographic metasurfaces, advancing their adoption in practical, real-life scenarios.

Photonic holographic metasurfaces (HMs) are one of the most successful platforms for creating practical realizations of computer-generated holograms.1 They can be programmed to encode and retrieve information through the careful management of the properties of the scattered light, e.g., controlling its amplitude, phase, polarization, and direction of propagation. HMs have been used for imaging systems,2 beam shaping,3,4 data encryption,5–7 augmented reality,8 and biophotonics applications.9,10

HMs consist of typically periodic, subwavelength arrangements of nanostructured meta-atoms, which provide them with diverse and highly specialized functionalities.11 The meta-atoms exert precise control over the properties of the scattered light and can be broadly classified depending on the physical mechanism that they rely on. In Pancharatnam–Berry metasurfaces, circularly polarized light acquires a local phase that is controlled by the geometrical orientation of the meta-atoms.12 The most popular practical implementation of this approach is an array of plasmonic nanorods with different orientations.13,14 Huygens' metasurfaces use the superposition of electric dipole (ED) and magnetic dipole (MD) resonances. The management of this superposition creates the required dephasing in the scattered light.15,16 High-contrast metasurfaces use engineered high-refractive index materials that act as truncated waveguides that induce a dephasing dependent on their geometry.17,18 Independently on the approach chosen, these methods typically necessitate the incorporation of multiple materials, to create the required response of the meta-atoms, e.g., to make high dielectric nanopillars on a low refractive substrate, or to create plasmonic effects on metallic layers deposited on dielectric features. Having different materials in the structure increases the cost and complexity of the design and fabrication.

This limitation is particularly important for flexible HMs (FHMs), which are traditionally made using flexible supports, hosting (on their surface or embedded in the flexible support) either dielectric or metallic nano-features, to create the meta-atoms.19–22 

FHMs offer additional degrees of freedom for the design of high-quality holograms, with respect to flat and rigid HMs. They can coat conformally chosen targets, providing them with bespoke photonic functionalities, and their fabrication is independent of the material and shape of the target onto which they are applied. The holographic information can either be robust with respect to deformation22 or strongly related to the shape of the support.19–21 

While advanced fabrication techniques permit the inclusion of dielectric and metallic nano-features in polymeric substrates, it would be strongly desirable to obtain FHMs realized with simple low refractive index polymers.

This would allow us to relief several design and fabrication constraints, thus allowing us to benefit in full of the wide range of properties enjoyed by polymers. For example, depending on the application, it will be possible to further optimize flexibility, cost and/or ease of manufacturing, weight, stretchability, etc. Most importantly, given the key relevance of the visible range in holographic applications,23–27 a purely polymeric FHM would offer the natural advantage of removing all constraints imposed by strong scattering and absorption of multi-component meta-atoms.

Here, we present a thin-film all-polymeric FHM membrane operating in the visible range.

From a geometrical and manufacturing point of view, this device resembles closely traditional examples of photonic crystal membranes28 for sensing and filtering applications.29–31 In the context of metasurface technology, transparent membranes have been recently proposed for imaging applications in the near-infrared range.32 In our case, the holes of the membrane are engineered to implement the meta-atoms of computer-generated holographic metasurfaces.

The structure is designed to work in transmission and offers a full 2π phase modulation of the scattered light. In the following, we present and discuss the role of the geometrical parameters of the meta-atoms and also present the fabrication protocol for a specific implementation and the associated experimental result. Finally, we discuss critically how our approach extends to other polymers and materials.

As further discussed later, our approach can be implemented using any suitable material that can be processed to create patterned membranes. Here, we chose to work with SU-8, a very popular epoxy-based, negative tone resist that can be patterned using photolithography and electron-beam lithography (EBL),33 which exhibits long-term stability.34 SU-8 films can be spun at thicknesses ranging from a few tens of nanometers to several hundred micrometers and can produce high aspect ratio features. Additionally, SU-8 has a relatively high average refractive index np  1.58, with a negligible absorption coefficient in the whole visible range. The effect of the geometrical parameters on the response of the meta-atom, sketched in Fig. 1(a), was numerically modeled with commercial finite element software. To minimize the simulation time, we applied symmetrical boundary conditions in the xz and yz planes, whereas we adopted open boundary conditions for the z-axis. The hexagonal unit cell consists of an air cylinder of thickness ta = 1 μm, etched in a polymeric film of thickness tp and arranged in a triangular pattern with periodicity p = 300 nm. The radius of the air pillar determines the phase of the transmitted light. As discussed later, the thickness of the substrate polymer film has minimal influence on the optical response of the unit cell. However, for practical purposes, it is convenient to use tp > ta. Here, we chose tp = 6 μm.

FIG. 1.

(a) Unit cell design; (b) transmission and phase modulation vs radius; (c) transmission and phase modulation vs radius for different air cylinder heights; and (d) transmission and phase modulation vs radius for different SU-8 film thicknesses.

FIG. 1.

(a) Unit cell design; (b) transmission and phase modulation vs radius; (c) transmission and phase modulation vs radius for different air cylinder heights; and (d) transmission and phase modulation vs radius for different SU-8 film thicknesses.

Close modal

Figure 1(b) shows the phase and transmission of light at λ = 532 nm for varying air cylinder radii, for polarization aligned along the x and y axes. This result demonstrates that the response of the HMs does not depend on the polarization of the incident light. Figure 1(c) shows how the transmission and phase of the transmitted light are affected by the air cylinder height, in a range of ±30% around the nominal value. In this range, the phase coverage of the structure spans from 3π/2 to 5π/2, with a marginal effect on the amplitude of the transmitted light. Not surprisingly, the thickness of the membrane that guarantees a 2π coverage of the phase is higher than λ/np since the presence of the air region reduces the effective index of the unit cell. Figure 1(d) shows the optical response of the meta-atoms for varying substrate thickness. Again, the results show that the phase coverage is unaffected, and the amplitude changes are qualitatively not different. Likewise, changing the lattice constant p produces qualitatively similar results (not shown), provided that the subwavelength condition is retained. For larger p, the phase changes less quickly with the radius. This could help manage potential fabrication imperfections, as deviation from the intended meta-atom size would lead to more contained local error in the phase.

To design the (phase-only) holograms, we used a Gerchberg–Saxton (GS) algorithm with a Rayleigh–Sommerfeld light propagator.20 For the practical implementation of the hologram phase, we discretized the phase into 12 levels, as detailed in Table I.

TABLE I.

Mapping of the phase shifts on the radius of the air cylinders.

Phase −175° −145° −115° −85° −55° −25° 35° 65° 95° 125° 155°
Radius  26 nm  51 nm  68 nm  82 nm  94 nm  104 nm  113 nm  120 nm  127 nm  133 nm  139 nm  145 nm 
Phase −175° −145° −115° −85° −55° −25° 35° 65° 95° 125° 155°
Radius  26 nm  51 nm  68 nm  82 nm  94 nm  104 nm  113 nm  120 nm  127 nm  133 nm  139 nm  145 nm 

Figures 2(a)–2(c) depict the fabrication procedure for the samples. A silicon wafer substrate was cleaned in acetone and isopropanol alcohol for 5 min each, in an ultrasonic bath. The sample was then spin-coated with a 100 nm thick sacrificial layer of omnicoat (Microchem) and baked at 230 °C for 1 min. A 7 μm thick SU-8 was spin-coated on the top at 1000 rpm for 60 s and pre-baked at 65 °C for 5 min and 95 °C for another 5 min. Then, a UV exposure process was applied for 5 min, and the cross-linking of the polymer was completed by baking the sample at 65 °C for 5 min, followed by a further 5 min at 95 °C and a hard baking step at 170 °C for 15 min. A layer of 30 nm gold was deposited via electron-beam evaporation to act as a hard mask. A layer of 300 nm thick polymethyl methacrylate (PMMA) was spin-coated at 1000 rpm for 60 s and baked at 180 °C for 5 min. The air pillars of the metasurface were defined by electron-beam lithography at a dose of 150 μC/cm2. The sample was developed in IPA and DI water solvent for 60 s with a 7:3 ratio. The gold was then reactive ion etched with Ar for 8 min and the pattern transferred onto the SU8 etching for 6 min with a mix of O2 and SF6 with a 5:1 ratio.

FIG. 2.

Schematic of the fabrication process for the SU-8 HMs. (a) Silicon carrier was initially coated with Omnicoat, SU-8, and gold, followed by PMMA, which was patterned through a standard EBL process. (b) After development, the pattern was transferred onto the SU-8. (c) The HM was released from the carrier. (d) Image of the fabricated sample. The blue arrow indicates the HM area. (e) SEM image of the fabricated sample.

FIG. 2.

Schematic of the fabrication process for the SU-8 HMs. (a) Silicon carrier was initially coated with Omnicoat, SU-8, and gold, followed by PMMA, which was patterned through a standard EBL process. (b) After development, the pattern was transferred onto the SU-8. (c) The HM was released from the carrier. (d) Image of the fabricated sample. The blue arrow indicates the HM area. (e) SEM image of the fabricated sample.

Close modal

The sample was then wet etched with a KI2 solution for 30 s to remove the leftover gold. Finally, the HM was lifted off from the substrate, dissolving the Omnicoat with MF319 (Microchem). It should be noted that it would also be possible to fabricate the MSs directly exposing the SU-8 with the complementary pattern, using an e-beam system able to define high aspect ratio features. Figure 2(c) shows an image of the fabricated sample. Figure 2(d) shows a scanning electron microscope (SEM) image of the patterned MS, before the RIE steps.

The experimental results were obtained by a broadband supercontinuum laser (SuperK, NKT Photonics) that was linearly polarized, expanded to a beam with a diameter of 1.5 cm with a telescope made by two lenses with focal lengths 50 and 400 mm and then focused on the metasurface with a lens with a focal length of 1000 mm, as shown in Fig. 3(a). The setup produced a spot size of ∼250 μm diameter at the plane of the sample, thus overfilling the square HM, made with a size length of 200 μm. To obtain the efficiency of the HM, we designed a hologram producing a dot-shape image at 30° with respect to the normal to the HM, at λ = 532 nm. The diffraction efficiency was obtained by normalizing the power in the two first diffracted orders, normalized to the power of the (not diffracted) zeroth order. Figure 3(b) shows the efficiency vs wavelength for different polarizations in the visible range. The efficiency at the designed wavelength peaks at 64%. It is instructive to compare this value with the typical efficiency of other flexible metasurfaces, with high-refractive index meta-atoms (efficiency = 56%, operated in transmission19) and with metal–insulator–metal meta-atoms (efficiency = 37%, operated in reflection22). As expected, the reduction of the efficiency as the HM is operated out of the optimal condition is more severe for longer wavelengths, where the phase modulation does not cover the full 2π range. Additionally, we observe a small polarization dependence of the efficiency, most likely due to experimental imperfections.

FIG. 3.

(a) Optical setup to measure the diffraction efficiency and (b) diffraction efficiency of the HM in the visible range. The continuous lines are a guide to the eye.

FIG. 3.

(a) Optical setup to measure the diffraction efficiency and (b) diffraction efficiency of the HM in the visible range. The continuous lines are a guide to the eye.

Close modal

Figure 4 demonstrates the design and measurement of the proposed FHM membrane, displaying a target image 15 cm away from the HM at an angle of 30° with respect to the normal incidence. Panel (a) shows the simulated hologram and target image. Panel (b) shows the experimental results, where the HM is placed on a glass slide for practical purposes. The result would not change for a framed and suspended membrane.

FIG. 4.

(a) Designed holographic image and target image (inset) and (b) mounted sample and experimental holographic image at λ = 532 nm.

FIG. 4.

(a) Designed holographic image and target image (inset) and (b) mounted sample and experimental holographic image at λ = 532 nm.

Close modal

Figure 5 shows the same MSs illuminated with different wavelengths in the 488–632 nm range. As expected, the image size and position change with the operation wavelength.

FIG. 5.

Hologram measurement across the visible spectrum.

FIG. 5.

Hologram measurement across the visible spectrum.

Close modal

As anticipated, our approach extends to a broad range of materials. In Fig. 6, we show the amplitude and phase of the light transmitted through a unit cell, comparing PMMA and crystalline silicon with SU-8. In this example, the periodicity for the PMMA case was kept at 300 nm, whereas we chose a lattice constant of 190 nm for the silicon case. The minimum thicknesses guaranteeing full 2π coverage of the phase were 1.170 μm and 230 nm for the PMMA and silicon, respectively. The higher refractive index of the silicon leads to a stronger phase change with the radius of the air holes, with a much thinner membrane thickness. It is interesting to remark that even in the case of crystalline silicon, it is possible to take advantage of the flexibility and conformability of the nanomembranes.35 

FIG. 6.

Transmission and phase modulation for SU-8, PMMA, and silicon.

FIG. 6.

Transmission and phase modulation for SU-8, PMMA, and silicon.

Close modal

However, the increased impedance mismatch with the surrounding environment and the material absorption leads to a less favorable transmission.

The use of PMMA is appealing as it can be patterned with a simple direct electron-beam exposure, at the cost of a slightly higher thickness. Other polymeric materials could be used, e.g., polydimethylsiloxane (PDMS), which could be used for stretchable and tunable MSs, taking full advantage of the flexible and conformable paradigm. Additionally, the fabrication of this class of MSs can be scaled up using suitable polymers, with hybrid nano-imprinting and etching techniques (see, e.g., Ref. 36).

This type of MSs could be further designed to be sensitive to a particular polarization, if beneficial to the target application. This could be obtained by creating nonsymmetrical air holes.

It should be noted that single materials holographic metasurfaces offer a generally reduced versatility in controlling the scattering properties of the meta-atoms. However, this is abundantly compensated by the simplicity of design and fabrication, which can be employed for applications in sensing7,37 and augmented reality applications,38 which are not intrinsically dependent on strong scattering efficiencies.

In conclusion, we presented a flexible and polarization-independent, holographic metasurface working in the visible range. At variance with most MSs realization, our devices can be made using a single transparent material, like a low refractive index polymer. We systematically studied the effect of the geometrical and material parameters of the meta-atoms and characterized the fabricated samples experimentally. Our flexible and highly transparent MSs provide a unique platform for the development of scalable applications in augmented reality, sensing, and biophotonics.

This project was supported by the European Research Council (ERC) under the European Union Horizon 2020 Research and Innovation Program (Grant Agreement No. 819346).

The authors have no conflicts to disclose.

Mohammad Biabanifard: Conceptualization (equal); Formal analysis (equal); Investigation (lead); Methodology (equal); Validation (lead); Writing – original draft (equal). Jianling Xiao: Investigation (supporting); Methodology (supporting); Writing – original draft (supporting). Andrea Di Falco: Conceptualization (equal); Formal analysis (equal); Methodology (equal); Supervision (lead); Writing – review & editing (lead).

The data that support the findings of this study are openly available in the University of St Andrews Research Portal at https://doi.org/10.17630/4e19ceb4-03c9-4554-9e23-4596a052d4a3, Ref. 39.

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