Metasurfaces are becoming a vital platform to precisely control light fields at the subwavelength scale by tailoring the electromagnetic properties (amplitude, phase, structural birefringence, etc.) of constituent meta-atoms by appropriately choosing their material, morphology, and spatial arrangement.1^–6 Similar to the majority of photonic devices relying on resonant tailoring, metasurfaces usually work at a specific wavelength or a narrow spectral band within the visible and infrared spectrum.7^–9 However, broadband working wavelengths of metasurfaces can directly lead to enhanced functionalities in a wider application scope, such as higher information density, better image display quality, and multifunctional devices like achromatic metalenses,10^–12 vivid structural color displays,13^–18 or information encryption devices.19^–22 Therefore, it becomes a significant objective to expand the working spectral range of metasurfaces to enhance their multiplexing capability.23^–25

Straightforwardly, the operation spectral band can be extended by spatially arranging meta-atoms with different operating bands on one metasurface.26^,27 This approach is widely utilized and has led to numerous breakthroughs and applications, including full-color structural color display, color holograms, and information encryption.9^,28^–32 Usually, numbers of meta-atoms with varied resonant wavelengths (usually also of different sizes) are combined in a spatially even manner. Inevitably, adjacent meta-atoms will spectrally interact with each other, which will result in two critical issues, i.e., spectral cross talk and an efficiency imbalance between different wavelength channels. These two issues significantly impact the performance of wavelength-multiplexing metasurfaces, and the negative influence gets worsened with the increase of meta-atom types. As a result, multifunctional metasurfaces devices with multiplexed working wavelengths over three are seldom reported.

Here, we propose a concept of an unevenly distributed pixel (UEDP)-based metasurface consisting of meta-atoms with varying sizes and spatial arrangements to address the above-mentioned issues. As a proof of concept, a UEDP-based silicon metasurface is designed and fabricated for the application of metasurface imaging, specifically, in the integration of hologram and printed images. Four independent holographic images are integrated within one colored printed image to simultaneously exhibit balanced image intensities with negligible spectral cross talk among different color channels. Moreover, such integration enables the camouflage function that the color printing under white light illumination serves as the interference information with the ciphertext of four holograms to be read out at different observation planes and viewing angles away from the printed image. Our research showcases UEDP as an efficient and practical way to enhance the wavelength multiplexing of metasurfaces, as well as to simultaneously provide highly restricted access to manifold information, which is very promising for optical cryptography-related applications, such as information multiplexing, encryption, and anticounterfeit packaging.

Our study begins by identifying the origins of the two issues of spectral cross talk and intensity imbalance for different wavelength channels. When a light is incident on a wavelength-multiplexing metasurface consisting of meta-atoms of various sizes, the polarization, phase, and amplitude of the output signal are modulated, but the modulation efficiency usually varies with the wavelength of the incident light [Fig. 1(a)]. In order to increase the whole efficiency of the metasurface, all the constituent meta-atoms should resonate at given working wavelengths. The resonant optical spectrum of certain-sized meta-atom has its specific spectral line shape, i.e., spectral peak and bandwidth [Fig. 1(a)]. When various meta-atoms are evenly and closely arranged on one metasurface, meta-atoms will interact with their adjacent neighbors. As a result, the original resonant properties of each meta-atom are inevitably changed, which causes spectral cross talk and intensity imbalance of different wavelength channels, in particular, at the short wavelength range [Fig. 1(b)].

To be specific, the spectral overlap between different meta-atoms indicates that the metasurface will exhibit the same functions at both λ1 and its adjacent wavelength λ2. Although the function is designed for the wavelength λ1 with an optimized efficiency, the output at λ2 still influences the quality of the final output at λ1. For example, if the metasurface is designed to present two different images at λ1 and λ2, the channel cross talk means that two images will be simultaneously observed under the incident light of either λ1 or λ2, greatly lowering the functional performance of the metasurface [Fig. 1(b)]. In an extreme case, if the peak efficiency at λ3 is much lower than that of λ2, the channel of λ3 cannot work properly, which will greatly limit the channel number of the metasurface.

To address the above two issues, we propose a new solution termed UEDP, which is a novel type of superpixel consisting of meta-atoms with varied sizes and spatial densities [Fig. 1(c)]. In our approach, the meta-atoms with similar sizes (i.e., similar resonance wavelengths) are elaborately separated in space based on preanalysis of near-field interactions, which maximally reduces the channel cross talk. Furthermore, it is found that the total transmission intensity of each UEDP Ic is proportion to T·D·S, where T is the transmittance of one meta-atom, D is its density, and S is the area of a meta-atom of the same size. Although T is determined by the intrinsic properties of the meta-atom and can hardly be changed, we can balance intensities Ic of different UEDPs by carefully designing D and S. In order to demonstrate the advantage of UEDPs, we demonstrate a UEDP-based camouflage metasurface by hiding four wavelength-dependent holographic images [Fig. 1(c)]. Our proposed UEDP-based metasurface achieves the integration of four holograms in a single printed image, thereby demonstrating the potential of UEDP in information encryption applications.

The proposed metasurface is expected to work within a broadband range in the visible. So, three basic wavelength channels are red (633 nm), green (532 nm), and blue (473 nm), corresponding to the three primary colors. Furthermore, in order to increase the information capacity and enhance the color display quality, a yellow light channel (594 nm) will also be inserted between red and green. The sizes of meta-atoms should be judiciously optimized to improve the total efficiency at the four operating wavelengths, which serves as the criterion to select subwavelength structures with appropriate localized modes at the desired working wavelengths. It is worth noting that our metasurface platform can also be designed to work at the infrared spectrum, thereby further increasing the number of signal channels.

As shown in Fig. 2(a), the working performance of metasurfaces is determined by four geometry parameters, i.e., height (H), width (W), length (L), and period (P). All the simulations are performed under an illumination of a circularly polarized light using the commercial software Lumerical FDTD. Perfectly matched layers are applied along the z axis and periodic boundary conditions are applied along the x and y axes. In general, larger H can provide higher polarization conversion efficiency which is significant for manipulating the Pancharatnam–Berry (PB) phase.33^–35 However, larger H is not friendly for fabrication and device integration. Therefore, after comprehensive considerations, the height of the meta-atoms is set to be 600 nm [Fig. S2 in the Supplementary Material]. With a fixed of H, the spectral line shape and peak positions of the meta-atom will vary for different combinations of L and W. For instance, the combination of larger L and smaller W will lead to a higher transmission when λ=633nm [Fig. 2(b) and Fig. S1 in the Supplementary Material]. However, the minimal value of W is restricted by the practical fabrication capability, so W is set to be 40 nm, which is a relatively small size with high reproducibility. For a fixed L, one can see that the resonant peaks of meta-atoms vary slightly as W increases [Fig. 2(c)]. In contrast, the resonant peak is mainly determined by L. When W=40nm, the resonant wavelength of the meta-atom varies from 473 to 633 nm as L increases from 80 to 160 nm, nearly covering the entire visible band [Fig. 2(d)]. We also simulated the scenario of substituting the nanopillar material with amorphous silicon (Fig. S3 in the Supplementary Material). However, the results indicate that amorphous silicon is not an appropriate material for wavelength-multiplexed metasurfaces. Thus, we achieved high-efficiency manipulation at the corresponding wavelengths by optimizing the geometric parameters of the nanopillars.

As to period P, a small value is preferable from the point view of device integration. However, a small period P will result in spectral cross talk among neighboring meta-atoms due to the electric field overlap. Thus, it is crucial to achieve a trade-off between minimizing P and preventing near-field coupling. When 160-nm-long meta-atoms are horizontally aligned with a period P of 200 nm, their end-to-end separation is merely 40 nm. The electric field is highly localized at the gap between two meta-atoms at the wavelength of 633 nm [the middle inset in Fig. 2(e)]. The strong interaction will bring about a negative influence on the generation of the PB phase and manipulation efficiency. In contrast, when P increases to 300 nm, the electric field distributions at the two ends of the meta-atoms hardly overlap, and the edges of meta-atoms are clearly visible [the right inset of Fig. 2(e)], indicating that the near-field interaction is negligible. Therefore, period P is set to be 300 nm for an operating wavelength of 633 nm. As to the shortest wavelength of 473 nm, the period will be set to be 200 nm after near-field analysis (more details regarding the selection of the period are also discussed in Fig. S4 in the Supplementary Material).

It takes three steps to design a UEDP-based metasurface. The first step is to choose the basic meta-atoms according to the operating wavelengths. In fact, according to the discussion about Fig. 2, one can find that such a step can be readily completed by choosing meta-atoms with different L. Figure 3(a) shows the transmission of light with a cross-polarization state using different meta-atoms for four colors. The transmission of blue color (i.e., 473 nm) is relatively lower than the counterpart of the other three colors. Therefore, in order to compensate for the intensity of the blue channel, the density of blue meta-atoms should be specially addressed.

The second step involves identifying the fundamental UEDP type and adjusting the density of meta-atoms presented in UEDPs [Figs. 3(b) and 3(c)]. To achieve the images in Fig. 1(c), three types of UEDPs are required. The first type of UEDP is for the mixed color, consisting of meta-atoms resonating at red, green, and blue [Fig. 3(b)]. This type of UEDP for mixed color exists for two reasons. On the one hand, there is always a large gray background part in a printed image, which can be produced by adjusting the proportions of the three primary colors. On the other hand, the UEDP for mixed color at least involves three types of meta-atoms, leading to three distinct color channels for generating holographic images. Therefore, the UEDP is a crucial pixel type not only for presenting the background color of a printed image but also for contributing to multiple holograms.

The second or third type of UEDP works for monochromatic color. Each type of UEDP is composed of the same sized meta-atoms but with different densities. Here, two types of monochromatic UEDPs are preferred for blue and yellow colors [Fig. 3(c)]. The meta-atoms for yellow are chosen to avoid the channel cross talk between the yellow channel and its adjacent red and green channels (detailed discussions are presented in Table S1 in the Supplementary Material). Using the monochromatic UEDP for yellow, both the printed and holographic images in the yellow channel can be easily separated from their counterparts in the red and green channels, greatly helping to reduce the channel cross talk. In the proposed UEDP-based metasurface design strategy, the density of meta-atoms in each UEDP is regarded as another key parameter to offset the negative influence caused by differences in pixel transmittance, loss, etc.16 Therefore, the density will be carefully optimized by combining theoretical calculations and experimental measurements, ensuring the uniform intensity of holographic images. Since silicon has a higher absorption of blue color, the meta-atoms in both the mixed color (i.e., gray background area) and the blue monochromic UEDP areas contribute to the blue holographic image for a comparable intensity with other colors. Furthermore, the density of meta-atoms for the blue color is also higher than the counterparts for the other two colors. It is worth noting that the problems of spectral cross talk and efficiency imbalance are tackled through spatial separation and density adjustment, respectively. These two methods are compatible. The transmission efficiency of these types of UEDP is shown in Fig. S6 in the Supplementary Material, and the measured results are in good agreement with the simulated ones. Thus, by optimizing the density of the nanopillars, we achieved the intensity balance. The scanning electron microscope (SEM) images vividly display that the densities of the meta-atom in three types of UEDPs are decidedly different [Figs. 3(b) and 3(c)].

In the design, three types of UEDPs are defined from the perspective of the printed image, including one type of mixed-color UEDP (gray color) and two types of monochromatic UEDPs (yellow and blue colors). The mixed-color UEDP consists of three sized meta-atoms, which support red (A1), green (A2), and blue (part of A3) colors, respectively. In addition, there is also an area solely presenting the blue image (part of A3). In contrast, the meta-atoms supporting yellow only exist in the yellow image area. Since there are four sized meta-atoms, four holographic images can be generated in four colors. Specifically, a red (H1) or green (H2) holographic image will be generated by the corresponding meta-atoms in the mixed-color UEDP area, while a blue (H3) holographic image will be produced by meta-atoms in both mixed-color UEDP and monochromatic UEDP areas [Fig. 3(d)]. Correspondingly, the yellow (H4) holographic image is generated by the meta-atoms in the yellow area. To generate the holographic image, the phase delay of meta-atoms will be provided by the PB phase, which is determined by the orientation of each meta-atom. Then the final step is to decide the specific orientation of each meta-atom in the metasurface. The PB phases for each colored channel are synthetically calculated using a modified Gerchberg–Saxton (GS) algorithm [Fig. 3(e)].36^,37

A UEDP-based camouflage metasurface is experimentally demonstrated to verify the proposed strategy. The performance of the UEDP-based metasurface is characterized using a home-built optical setup [Fig. 4(a)]. The polarization state of incident light is converted into circular polarization after passing through a linear polarizer (LP) and a quarter-wave plate (QWP), and then the light is incident on the designed metasurface. The output signal is then collected by a 10× objective (NA = 0.25), and the signal with the copolarization state to the incident light will be filtered by another pair of QWPs and LPs. Finally, the image with the cross-polarization state at the focal plane of the objective is projected by a lens to the CCD camera.

The nanofabrication process involves four primary steps to transfer the pattern onto the crystalline silicon layer: transfer and polish of the crystalline silicon layer, electron beam lithography, resist development, and inductively coupled plasma etching (Fig. S5 in the Supplementary Material). Top and 30 deg inclined side views of the fabricated metasurface are shown in Fig. 4(b). The measured height of the meta-atom is about 600 nm, which is consistent with our design. Additionally, the UEDPs are closely arranged, which can help to maximize the metasurface efficiency and quality of the image display.

The measured printed image is shown in Fig. 4(c). When the metasurface is illuminated under white light, one can observe a printed image with a gray background and the monochromatic patterns in blue and yellow colors, which are vividly presented as designed. Furthermore, when the incident light is changed to monochromatic laser sources, four colored holographic images are observed at different spatial positions away from the metasurfaces [Fig. 4(d)]. The holograms show clear boundaries without cross talk. Also the output image intensity is balanced under the incident lights with similar intensity for various wavelengths. Therefore, it is evident that the cross talk and intensity imbalance problems in wavelength multiplexing have been optimized effectively.

In particular, the holograms of the four channels can be designed with varying focal lengths and spatial angles [Fig. 4(d)] to avoid spatial cross talk. The white cross indicates the center of the viewing screen of the holographic images. In our design, the focal lengths of the four holographic images are intentionally set to be 800, 1200, 1600, and 2000μm with spatial angles (azimuthal angle and elevation angle) at (−90deg, 70 deg), (0 deg, 70 deg), (90 deg, 70 deg), and (180 deg, 70 deg), for blue (473 nm), green (532 nm), yellow (594 nm), and red (633 nm) light illumination [Fig. S7 in the Supplementary Material], respectively. In this way, multiple holographic images loading on different wavelength channels do not interfere with each other, ensuring their color purity. Additionally, the polarization state can also be set as a degree of freedom to separate different images, for example, the polarization state set for left-handed circular polarization (LCP) for yellow and red incident lights and right-handed circular polarization (RCP) for blue and green incident lights [Fig. 4(d)]. Herein, our metasurfaces are designed to respond to the circularly polarized lights, but the idea of UEDP can also be applied for controlling the linearly polarized or partially polarized lights.

Obviously, the target holographic image can only be observed under a proper experimental condition, i.e., a correct combination of incident wavelength, polarization state, and viewing distance and angle, which can thus serve as a high level of encryption method for information security. For example, camouflaged information is required to be intentionally created to increase the security of encrypted true information. Therefore, it is crucial to store camouflaged information and true information with different levels of decryption difficulties.37^–40 In general, the substrate of the metasurface is transparent and the sample area is only a few hundred micrometers, so that it is easy to conceal it in everyday items such as cell phones and computers. Initially observed under white light, we can only see the printed image, which can be the plaintext information. For the actual encrypted information, the retrieval of holographic images necessitates the simultaneous satisfaction of specific criteria related to wavelength, polarization, and viewing condition. Thus this device can exhibit a high level of information security.

We propose a metasurface design strategy using UEDPs. As a proof of concept, a camouflage metasurface is experimentally demonstrated to encrypt multiwavelength holographic images in a multicolor printed image with minimized spectral cross talk and color intensity imbalance. The printed image can be directly observed under an illumination of an incoherent white light, while each holographic image can only be observed under a specific combination of wavelength, polarization, viewing angle, and focal length. The designed combination of illumination and observation conditions serves as an effective encryption mechanism for hiding specified information. The experimental results agree well with our design, validating the feasibility of integrating high-capacity and high-information-security compact data into a metasurface microchip. The UEDP-based metasurface design strategy holds great potential in the applications of information encryption, anticounterfeiting marks, and other related fields.

Yaqin Zheng received her PhD in physics from Sun Yat-sen University in 2024. Her main research focus is on nanophotonics, concentrating on the multi-dimensional control of the light field by designing the nano-structures of metasurface, to achieve applications such as image display and information integration.

Zhang-Kai Zhou received his PhD in physics from Wuhan University in 2011. He is currently a professor in the School of Physics, as well as the Director for the Office of Faculty of Science, Sun Yat-sen University. His research interests mainly focus on the field of meta-optics, including quantum plasmonics, optical field manipulations, functional nanodevices based on metasurfaces, etc.

Dangyuan Lei is professor of materials science and engineering in The City University of Hong Kong. He received his BSc, MPhil and PhD degrees in physics from Northwest University, The Chinese University of Hong Kong and Imperial College London in 2005, 2007 and 2011, respectively. His research interest centers on plasmonic nanophotonics and low-dimensional quantum materials, with particular interest in the nanoscale cavity-matter interaction and applications in miniaturized photonic and optoelectronic devices for on-chip optical sensing and imaging as well as energy harvesting, conversion, storage and saving.

Biographies of the other authors are not available.