Spatial Frequency Multiplexed Meta-Holography and Meta

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Spatial Frequency Multiplexed Meta-Holography and Meta-Nanoprinting Juan Deng, Yan Yang, Jin Tao, Liangui Deng, Daoqun Liu, Zhiqiang Guan, Gongfa Li, Zile Li, Shaohua Yu, Guoxing Zheng, Zhongyang Li, and Shuang Zhang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b03738 • Publication Date (Web): 01 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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Spatial Frequency Multiplexed Meta-Holography and Meta-Nanoprinting Juan Deng,†,‡,♫ Yan Yang,§,♫ Jin Tao,‡,♫ Liangui Deng,†,♫ Daoqun Liu,§ Zhiqiang Guan,|| Gongfa Li,┴ Zile Li,*,† Shaohua Yu,*,‡ Guoxing Zheng,*,†,‡ Zhongyang Li,† and Shuang Zhang# †Electronic

‡NOEIC,

Information School, Wuhan University, Wuhan 430072, China

State Key Laboratory of Optical Communication Technologies and Networks, Wuhan

Research Institute of Posts & Telecommunications, Wuhan 430074, China §Integrated

Circuit Advanced Process Center, Institute of Microelectronics, Chinese Academy of

Sciences, Beijing 100029, China ||School

┴Key

of Physics and Technology, Wuhan University, Wuhan 430072, China

Laboratory of Metallurgical Equipment and Control Technology of Ministry of Education,

Wuhan University of Science and Technology, Wuhan 430081, China #School

of Physics & Astronomy, University of Birmingham, Birmingham B15 2TT, UK

ABSTRACT: Metasurfaces are flat structured surfaces which are designed to control the twodimensional distributions of phase, polarization and intensity profiles of optical waves. Usually, the optical response of metasurfaces is dispersive and polarization-dependent, which indicates the capability of metasurfaces for information multiplexing using wavelength and polarization state.

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However, most multiplexing techniques based on metasurfaces reported so far are carried only in the spatial domain. Here we experimentally demonstrate metasurface multiplexing exploiting the degree of freedom of the spatial frequency domain. Specifically, we overlap two independent holographic images at high and low spatial frequencies respectively, and record them onto a single piece of metasurface-hologram (meta-hologram). These two holographic images can be successfully separated from the reconstructed overlapped images by using two digital Gaussian filters. In addition, we demonstrate spatial frequency multiplexing by meta-nanoprinting, in which a complex multiplexed image (the combination of Einstein and Monroe’s portraits) is recorded and de-multiplexed with high fidelity. The presented spatial frequency multiplexing with metasurfaces suggests a route to increase the information channel and may contribute to the researches and applications in optical information encoding, optical storage, optical information hiding, information security and compact display, etc.

KEYWORDS: metasurfaces, spatial frequency multiplexing, meta-holography, meta-nanoprinting Metasurfaces have innovated the implementation of various advanced optical elements through their superior manipulation of light propagation, which is unavailable in conventional optical elements.1–12 The enormous freedom in metasurface designs makes them suitable for compact image display, such as holography13–16 and nanoprinting.17–20 With the judicious design of metasurfaces, it is possible to generate either holographic images in the near/far field (Fresnel or Fraunhofer regime),16,21 or high-resolution nanoprinting images right at the sample surface.22,23 Since most metasurfaces are sensitive to the operating wavelength and polarization state of the incident light, various multiplexing techniques have been proposed to encode multiple information channels onto a metasurface.24 Specifically, recent progresses in metasurface-based multiplexing


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are focused on wavelength multiplexing,25,26 polarization multiplexing,27–30 hybrid multiplexing31– 34

and reconfigurable metasurface multiplexing,35–37 etc. For instance, silicon metasurfaces formed

by combining three types of nanobricks are capable of independent manipulation of light of three different colors: red, green, and blue.25 Different combinations of incident/output polarization can produce twelve information channels for image encoding.29 In addition, addressable metasurfaces combining controlled chemical reactions and helicity of light to encrypt information at visible frequencies has been reported.37 Generally, the above-mentioned multiplexing techniques are carried out in the spatial domain. The multiscale perceptual mechanisms of human vision indicate that the eyes recognize images through different spatial frequency channels.38 Specifically, at a closer observation distance, human eyes are sensitive to image details. As the observation distance increases, visual perception mainly captures global luminance variations and broad contours.39,40 Inspired by this mechanism, we implement the multiplexed metasurfaces in the spatial frequency domain. By using image processing techniques in the spatial frequency domain, we can multiplex two independent optical images overlapped in the spatial domain and de-multiplex them by employing two Gaussian filters. Our approach can provide two independent information channels recorded by metasurfaces and it can be further combined with other multiplexing techniques without increasing the complexity of meta-device design and fabrication. We implement our idea to two compact image display platforms – meta-holography and meta-nanoprinting. The reconstructed optical image containing different spatial frequency components from two independent images can be readily separated by high-pass and low-pass filters, so that the extracted images can represent different information. In addition, we explored the application of optical information encoding by combining spatial frequency multiplexing with helicity multiplexing and experimentally achieved six reconstructed


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holographic images from an identical meta-hologram. Our approach may have promising applications in anti-counterfeiting, information encoding, optical storage, compact display and many other related fields. RESULTS AND DISCUSSION Principle of spatial frequency multiplexing. The spatial frequency multiplexing aims at combining two images (𝐼1 and 𝐼2) at different spatial frequency domains into a hybrid image 𝐼, which is a static picture that can produce two different interpretations.39 As illustrated in Figure 1, ℎ𝑖𝑔ℎ the hybrid image 𝐼 is generated by superimposing two spatial frequency components 𝐼𝑙𝑜𝑤 1 and 𝐼2

from two images (Figure 1a). The low spatial frequency component 𝐼𝑙𝑜𝑤 1 is obtained by filtering image 𝐼1 with a low-pass filter 𝐺𝑙, and the high spatial frequency component 𝐼ℎ𝑖𝑔ℎ is obtained by 2 filtering another image 𝐼2 with a high-pass filter 𝐺ℎ. For the low-pass and high-pass filters, we use two two-dimensional (2D) Gaussian filters and the operations are defined in the spatial frequency domain. Low spatial frequencies represent global information, such as general orientation and proportions. High spatial frequencies represent abrupt spatial changes in the image, such as edges, and generally correspond to feature information and fine details. Visually, the image 𝐼𝑙𝑜𝑤 is blurred 1 and the image 𝐼ℎ𝑖𝑔ℎ is sharp. 2 To achieve spatial frequency multiplexing of these two images, the cut-off frequencies of the low-pass and the high-pass filters need to be chosen with care. The principle for choosing the cutoff spatial frequency includes the following aspects. Firstly, the spatial frequency ranges of the ℎ𝑖𝑔ℎ filtered images (𝐼𝑙𝑜𝑤 1 and 𝐼2 ) should have small overlapping. Under this premise, the hybrid

image should contain as much information as possible. To this end, we calculate the spatial frequencies of these two images based on the 2D Fourier transform. Following this, we sum up the magnitudes of the Fourier coefficients |𝑋(𝑓)| of image 𝐼1 from low to high spatial frequency and


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do the same for image 𝐼2 but with the inverse frequency order. The statistical results are shown in Figure 1b. Based on the above principles, the final cut-off spatial frequencies are set as 8 cycle/image (c/i) and 20 c/i for the low-pass filter 𝐺𝑙 and the high-pass filter 𝐺ℎ, respectively.

Figure 1. Schematic of spatial frequency multiplexing. a, Reconstituting spatial frequencies of two images with different numbers (𝐼1 and 𝐼2) to generate a hybrid image 𝐼. 𝐼𝑙𝑜𝑤 1 is obtained by filtering image 𝐼1 with a low-pass filter 𝐺𝑙. 𝐼ℎ𝑖𝑔ℎ is obtained by filtering image 𝐼2 with a high-pass 2 filter 𝐺ℎ. b, Statistic results of the spatial frequencies of the two images. We analyze the spatial frequencies of images by using the Discrete Fourier Transform (DFT), where 𝑋(𝑓) is the Fourier coefficient of spatial frequency𝑓. The horizontal axis represents the magnitudes of spatial


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frequencies. The unit c/i is utilized for the description of the spatial frequencies because of its independence from the image size. The vertical axis represents the cumulative magnitudes of Fourier coefficients. The dotted lines indicate the cut-off spatial frequencies of the two images. Spatial frequency multiplexed meta-holography. The dielectric nanobrick based metasurfaces combining geometric phase and magnetic resonance41, 42 are employed as a phaseonly hologram to reconstruct the hybrid image in the far field. By carefully designing the geometry size, each silicon nanobrick acts as a reflective half-wave plate and the nanobrick arrays with different orientation angles can continuously manipulate the phase of incident circularly polarized (CP) light. The details about the dielectric metasurface design are presented in the Methods. Each unit cell is designed with length L of 200 nm, width W of 100 nm, height H of 220 nm, and cell size C of 300 nm. Based on the above designed metasurface, we chose the hybrid image 𝐼 (shown in Figure 1a) as the target image to design a Fourier meta-hologram. The meta-hologram with dimensions of 480 × 480 μm2 is designed according to the classical Gerchberg-Saxton algorithm41 and it can create projection angles of 46° × 55°. A 2 × 2 periodic array is used to avoid the laser speckles.16 The target image is pre-compensated to avoid pattern distortion. Standard electron beam lithography (EBL) and inductively coupled plasma process were used to fabricate the sample. More details about the fabrication are demonstrated in the Methods. Figure 2a,b shows the scanning electron microscopy (SEM) images of the partial view of the meta-hologram. The experimental setup is shown in Figure 2c. The holographic image is projected on a white screen with high fidelity and readily captured by a commercial camera (Nikon D5100). The measured efficiency of the hologram, which is defined as the ratio between the optical power projected into the image region


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and the input power, reaches 30% (more details about the efficiency measurement are shown in Figure S1).

Figure 2. SEM images and experimental setup of the meta-hologram. a, b, SEM images of the fabricated metasurface (silicon nanobrick arrays). c, Illustration of the experimental setup of the reflection-type meta-hologram. An incident beam from a super continuum light source (YSL SCpro) is converted into CP light by passing through a linearly polarizer (LP) and a quarter-wave plate (QWP). After that, the reflected sub-beams form the holographic image at one plane vertical to the optical axis of the incident beam in the far field. Figure 3d,j shows the simulated and experimental holographic images in the spatial domain, which agree well with each other. To facilitate the image display, the spatial frequency spectrum is calculated as ln (1 + |𝑋(𝑓)|) and normalized to a range of 0 and 1. Figure 3a,g is the corresponding spatial frequency spectrums of Figure 3d,j. The similarity of spatial frequency


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spectrums between simulation and experiment indicates the high fidelity of the experimental results. To extract all information from the holographic image, we numerically apply the high-pass and low-pass filters to separate the overlapped image. The high and low spatial frequency components of Figure 3a are acquired by utilizing the Gaussian filters with cut-off frequencies of 20 c/i and 8 c/i, respectively, as shown in Figure 3b,c. Figure 3e,f shows the images in the spatial domain corresponding to their spatial frequency spectrums shown in Figure 3b,c. With the same image processing, the experimentally measured holographic image is decomposed in the spatial frequency domain (Figure 3h,i) and recovered in the spatial domain. The recovered images (Figure ℎ𝑖𝑔ℎ 3k,l) are in good match with the target images 𝐼𝑙𝑜𝑤 shown in Figure 1a, which proves the 1 and 𝐼2

feasibility of spatial frequency multiplexed meta-hologram. Benefitting from the robustness of geometric metasurfaces (GEMSs), that is, the phase delay is independent of the wavelength and only depends on the orientation angle of the nanostructure, the designed meta-hologram can work under a broad wavelength range. To investigate the spectral response, we utilize a super continuum light source (YSL SC-pro) in the range from 480 nm to 680 nm in steps of 40 nm to illuminate the meta-hologram sample, and all experimental images are clearly observed, as shown in Figure S2.


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Figure 3. Simulated and experimental results of the holographic images in spatial and spatial frequency domains. a, The spatial frequency spectrum of the simulated holographic image. b, c, The high and low spatial frequency components of the simulated holographic image, which are acquired by the high-pass and low-pass filters in spatial frequency domain, respectively. d, The simulated holographic image in the spatial domain. e, f, The corresponding images in the spatial domain of b, c. The experimental results in the spatial frequency domain are shown in g-i, and j-l are their corresponding results in the spatial domain. Our proposed spatial frequency multiplexing technique is easy to operate and does not require special nano-fabrication processing, which endows us with possibilities to combine the technique with other multiplexing methods. The phase delay  of GEMSs linearly depends on the orientation


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angle  of a nanostructure, i.e.,  = 2, with the sign determined by the polarity of the incident light. It indicates that the two holographic images obtained by GEMSs are centrosymmetric to each other, when the incident light is right-circularly polarized (RCP) and left-circularly polarized (LCP), respectively. As an example of practical application, we experimentally combine the spatial frequency and helicity multiplexing30,37 to build six channels for information encoding. Specifically, we chose a hybrid image (Figure 4a) as the target image to design a meta-hologram, and used two Gaussian filters (the cut-off frequencies are 8 c/i and 20 c/i, respectively) to extract the low and high spatial frequency components of the holographic images. The holographic images are captured by a commercial camera under the illumination of RCP, LCP and linearly polarized (LP) light, respectively. The meta-hologram is designed with dimensions of 480 × 480 μm2 and image projection angles of 70° × 33°. The reflected LCP light forming the holographic images is shown in Figure 4b. To reveal all the information in the holographic image, we employ the 2D Gaussian filters to separate the overlapped image in the spatial frequency domain. The result in Figure 4c shows that the image extracted by a high-pass filter contains a circle and a hexagon located at the left and right sides, respectively. When the holographic images undergo the lowpass filter, the positions of two patterns are swapped, as shown in Figure 4d. It is to say, for LCP incident light, the holographic image achieved by spatial frequency multiplexing contains two kinds of information. Figure 4e shows six ideal holographic images that would be reconstructed from only one metahologram. All extracted results from the experimental images are shown in Figure S3, which indicates that our proposed method can provide six channels to encode information. The feasibility and flexibility of spatial frequency and helicity multiplexing allow advanced optical information encryption. As shown in Figure 4f, if different messages are sent to different persons, these


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messages can be all encrypted on one meta-hologram. The receivers can decrypt the information with the customized keys upon receipt of the same sample. For example, if one receiver captures the holographic image illuminated by RCP light and filters the image by a high-pass filter, he can observe one hexagon and one circle located in the third and the fourth quadrants, respectively, which means "3.1415926". If the incident light is LCP, the receiver firstly acquires the high spatial frequency component of the holographic image and then filters the holographic image with a lowpass filter, and the corresponding decrypted message is “Pai”. The last message “” in the example is decrypted with the Key3, as shown in Figure 4f. Similarly, other receivers can decode their desirable message using their personal keys.

Figure 4. Illustration of holographic images with LCP incident light, optical encoding with six channels and the process for optical information encryption and decryption. a, Target image for an encoding hologram. b, Experimentally holographic image with LCP incident light. c, d, The separated high and low spatial frequency components of the holographic image. e, Six ideal holographic images reconstructed from a single metasurface (𝑥 and 𝑦 are coordinates in


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image plane). f, Proof-of-concept experiment for optical information encryption and decryption. The receivers can decrypt the information of a single metasurface with the customized keys. Spatial frequency multiplexed meta-nanoprinting. Usually, a digital hologram with the periodic design is capable of generating an image but with spot-arrays (shown in Figure S4), which makes it unsuitable for reconstructing a complex image with abundant high spatial frequency information. Fortunately, nanobrick-polarizer based nanoprinting can make up for the deficiency because it can generate a high resolution and continuous grayscale image without interference noise that meta-holograms always suffer from. Noted that nanoprinting based on a half-wave plate has recently been proposed for encoding high-resolution images into different polarization states of light.23,43 Here, inspired by our previous work,44 we propose the nanobrick-polarizer based nanoprinting to record complex images. According to the Malus’ law, by elaborately arranging the orientation angle of the nanobrick cell by cell, an arbitrary grayscale and high-resolution image can be recorded in the nanobrick-polarizer based metasurface with LP incident light. More details about the nanobrick-polarizer arrays design are demonstrated in the Methods. When a LP beam illuminates the metasurface at a normal incidence, the nanobricks with different orientation angles reflect the sub-beams with spatially varying intensity. Following this, the high-resolution image can be captured with the help of an optical microscope. As an example, we chose two complex images (the portraits of Einstein and Monroe) with 300 × 300 pixels to form a hybrid image, as shown in Figure 5.


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Figure 5. Schematic of the nanobrick-polarizer based meta-nanoprinting. When the metasurface sample is illuminated by LP light, the reflected light forms the hybrid image ( 300 × 300 pixels, each pixel includes 3 × 3 unit cells) and the image is captured by an optical microscope with a 50 × objective lens. With the same image processing operation shown in Figure 1, we superimposed the high spatial frequency component of the Einstein’s portrait and low spatial frequency component of the Monroe’s portrait with high-pass and low-pass filters (the cut-off spatial frequencies are 45 c/i and 15 c/i), respectively. The nanoprinting metasurface was fabricated by the standard EBL with dimensions of 270 × 270 μm2 (more details are demonstrated in the Methods). The simulated and experimental results are shown in Figure 6. We evaluated the performance of nanbrick-polarizer based nanoprinting from both the spatial frequency and spatial domains. Here, the hybrid image generated by the nanoprinting metasurface (Figure 6j) can be observed clearly, which agrees well with the simulated image (Figure 6d). The spatial frequency spectrums are also in a good match with simulation results (Figure 6a,g). Interestingly, when the high and low spatial frequency components (shown in Figure 6b,c) are separated by two Gaussian filters, we can see two quite different images in the spatial domain (Figure 6e,f). With the same image processing, the experimental image was decomposed in the spatial frequency domain (Figure 6h,i) and recovered in the spatial domain


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(Figure 6k,l). The portraits of Einstein and Monroe with high fidelity confirm the good performance of the spatial frequency multiplexed meta-nanoprinting.

Figure 6. Simulated and experimental results of the nanoprinting metasurface in spatial and spatial frequency domains. a, The normalized spatial frequency spectrum of the simulated image. b, c, The high and low spatial frequency components of simulated results, which are separated with two Gaussian filters in spatial frequency domain. The cut-off spatial frequencies of high-pass and low-pass filters are 45 c/i and 15 c/i, respectively. d, The simulated hybrid image in spatial domains. e, f, The corresponding images in spatial domain of b, c. The experimental results in the


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spatial frequency domain are provided in g-i and their corresponding results in the spatial domain are shown in j-l. The experimental image is captured by a micro-camera (Moticam X) of an optical microscope (Motic BA310MET-T) with quartz-halogen lamp. Our demonstrated spatial frequency multiplexing has been employed in meta-holography and meta-nanoprinting, which may provide a paradigm for optical information encoding related fields. Spatial frequency multiplexing also exhibits its compatibility. That is, in addition to helicity multiplexing, wavelength multiplexing, angle multiplexing, hybrid multiplexing and distance multiplexing, etc., can be used to combine with spatial frequency multiplexing to increase the information encoding channel numbers. More importantly, the combination does not increase the complexity of nano-structural design and fabrication, since spatial frequency multiplexing is accomplished with image processing on a computer rather than adjustment of nano-structure and fabrication. Besides information encoding, the application of spatial frequency multiplexed metasurfaces can be extended to more information related fields, i.e., information hiding (an image can be hidden into a stripe and recovered by a low-pass filter) and visual illusion (observers with different viewing distances capture different images when they focus on the same hybrid image). More details about the principles and designs are presented in Figures S5-7. CONCLUSIONS In summary, we propose spatial frequency multiplexed metasurfaces by combining spatial frequencies of two independent images to form a hybrid image. Each independent image can be decoded through digital filtering in the spatial frequency domain. A significant feature of our approach for information multiplexing is that it is implemented in the spatial frequency domain and we can overlap independent optical images at the same spatial position and de-multiplex them with digital Gaussian filters. The feasibility of our proposed work has been experimentally


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demonstrated by meta-holography and meta-nanoprinting techniques. In addition, by combining the spatial frequency and helicity multiplexing, we experimentally reconstructed six different holographic images from only one single meta-hologram, which enables to offer six combinations to encode information. Overall, we introduce the image processing into the field of metasurface multiplexing and our proposed scheme can be readily combined with many other multiplexing methods, requiring no additional nanostructure design or fabrication. We believe our concept would have great potential in information encoding, optical storage, optical information hiding, information security, compact display and many other related applications. METHODS Design and fabrication of geometric metasurfaces based on silicon on insulator (SOI). Silicon on insulator (SOI), being widely used in integrated circuits, is a promising material to make a reflective-type dielectric GEMSs at visible range. In the design, we employed a SOI consisting of a silicon dioxide (SiO2) layer with a thickness of 2 μm. By carefully designing the geometry size, each nanobrick acts as a reflective-type half-wave plate. The schematic of one unit cell is shown in Figure 7a, whereby the structural parameters have been demonstrated in the main text. The simulated efficiencies of the reflected co-polarized and cross-polarized parts are depicted in Figure 7b. Significantly, the polarization conversion efficiency can reach as high as 65.9%, while the unwanted co-polarized light contributing to zero-order diffraction can be suppressed to below 1%. To investigate the high reflection of the metasurface, we simulated the distribution of electric and magnetic fields at the cross-section of a nanobrick unit cell. As shown in Figure 7c-f, vortexlike electric fields and enhanced magnetic fields inside the nanobrick unit cell indicate the excitation of magnetic dipole resonances along the long and short axes.


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The sample was fabricated on an 8-inch SOI wafer with top Si layer of 220 nm-thick and buried oxide (BOX) of 2 μm-thick. First of all, alignment mark was patterned and dry etched on the wafer, and then the wafer was cleaned using SPM (mixed solution of H2SO4 and H2O2) and APM (mixed solution of H2O2 and NH3·H2O2) methods. Then, the wafer was coated with 100 nm-thick negative photoresist with hard baking, followed by exposure using EBL technology. The nanobrick pattern was transferred to the top Si layer by a single dry etching process using the mixed gas of HBr and CF4. The following photoresist striping and the wafer cleaning was done by the same process as the I-Line process. Finally, photoresist was coated on the wafer to protect the devices from contamination during dicing. After the dicing process, acetone and ethanol were used to remove the protective photoresist before the device characterization and testing.

Figure 7. Illustration of the GEMS unit-cell structure, its polarization conversion efficiency by numerical simulations and normalized electric and magnetic field distributions of a nanobrick unit-cell. a, Unit cell structure of the meta-hologram. b, Simulated reflectivity of the cross-polarized and co-polarized parts under a normal CP light incidence. c, d, Vortex-like electric


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field and enhanced magnetic field distributions at the cross-section of a nanobrick unit-cell and the incident electric field is polarized along the y-axis at a wavelength of 633 nm. e, f, Vortex-like electric field and enhanced magnetic field distributions at the cross-section of a nanobrick unitcell when the incident electric field is polarized along the x-axis at a wavelength of 633 nm. For electromagnetic simulation, the orientation angle of nanobrick φ was designed to be 0º. Design and fabrication of metasurfaces based on silver nanobrick-polarizer arrays. A schematic diagram of the silver nanobrick-polarizer is shown in Figure 8a. The working principle is that an incident beam with the polarization direction aligned with the long-axis of the nanobrick is strongly reflected, whilst that along the short axis is nearly totally transmitted44. To obtain the best polarization separation efficiency determined by the difference between Rx and Ty shown in Figure 8a, all structural parameters of the silver nanobrick were optimized by using the CST software. The nanobrick-polarizer is designed with cell size C of 300 nm, length L of 160 nm, width W of 80 nm and height H of 70 nm. Numerical simulation results (Figure 8b) show that the reflectivity in the x-axis direction reaches 92.6% (nearly totally reflected), whilst the transmissivity in the y-axis direction reaches 95.3% (nearly totally transmitted) at the operation wavelength (633 nm). More interestingly, the polarizer-related nanoprinting metasurface can work in a relatively large wavelength range. In a range from 550 nm and 700 nm, the efficiency of reflected beam with the polarization direction aligned with the x axis of the nanobrick can reach more than 40%, while that of y-axis can be suppressed to below 10%. This forms the basic of high-efficiency nanobrick based polarizer. It is well known that the intensity of an outgoing beam passing through a polarizer satisfies I=I0cos2φ according to the Malus’ law, where I0 is the intensity of an incident beam and φ is the orientation angle. Therefore, by arranging orientation angle of the nanobrick cell by cell,


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an arbitrary grayscale and high-resolution image can be recorded in the nanobrick-polarizer based metasurface with LP incident light. The sample was fabricated on an indium tin oxide-coated float glass substrate with standard EBL. Firstly, a standard electron beam process was used for patterning a poly (methyl methacrylate) (PMMA) mask on the substrate, then Ti (2 nm) and Ag (70 nm) were deposited by electron-beam evaporator (PVD 75, Kurt J. Lesker Company), followed by lift-off process in hot acetone.

Figure 8. Design of the unit-cell structure of silver polarizer and its polarization separation efficiency by numerical simulations. a, Illustration of the unit-cell structure of a nanobrickpolarizer. A silver nanobrick sits on a glass substrate. Incident light with polarization direction along the long-axis of the nanobrick is reflected and that with polarization direction along the short-axis is transmitted. b, Simulated reflectivity (Rx, Ry) and transmissivity (Tx, Ty) versus wavelengths. The orientation angle of nanobrick φ was designed to be 0º and incident LP light is incident along the long and short axes of the nanobrick, respectively. ASSOCIATED CONTENT


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Supporting Information. The Supporting Information is available free of charge on the http://pubs.acs.org at DOI:XXXXX. Efficiency measurement of the Fourier meta-holograms, experiment on the spectrum response of meta-holograms, optical information encoding with six channels by combining spatial frequency and helicity multiplexing, holographic image consisting of spot-arrays, image hiding based on spatial frequency multiplexing, visual illusion based on spatial frequency multiplexing The authors declare no competing financial interest. AUTHOR INFORMATION Corresponding Authors *Email: [email protected]. *Email: [email protected]. *Email: [email protected]. ORCID Guoxing Zheng: 0000-0002-3226-4735 Shuang Zhang: 0000-0003-4556-2333 Zhiqiang Guan: 0000-0001-5087-6439 Author Contributions G.X.Z., S.H.Y., S.Z. and Z.L.L. conceived the ideal. J.D. and Z.L.L. performed the design and simulation on the metasurfaces. Y.Y., D.Q.L., L.G.D., Z.Q.G. and J.T. designed and fabricated the samples. L.G.D., J.D., G.F.L. and Z.Y.L. performed the measurements. Z.L.L. J.D. and G.X.Z. analyzed the data. J.D., G.X.Z., Z.L.L., J.T. and S.Z. co-wrote the paper. G.X.Z. and S.H.Y. supervised the project. All authors discussed the results and commented on the manuscript.


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Author Contributions ♫

J.D., Y.Y., J.T. and L.G.D. contributed equally.

ACKNOWLEDGMENTS G.X.Z. and J.T. acknowledge the financial supports from the National Natural Science Foundation of China (Nos. 11774273, 11574240, 61805184), the Outstanding Youth Funds of Hubei Province (No. 2016CFA034) and the Open Foundation of State Key Laboratory of Optical Communication Technologies and Networks, Wuhan Research Institute of Posts & Telecommunications (No. OCTN-201605). Z.Q.G. acknowledges the financial supports from the MOST 2017YFA0205800 and the National Natural Science Foundation of China (Nos. 11404247 and 11674256). Z.L.L. acknowledges the financial supports from Postdoctoral Innovation Talent Support Program of China (BX20180221) and the China Postdoctoral Science Foundation (2019M652688). REFERENCES 1. Semmlinger, M.; Tseng, M. L.; Yang, J.; Zhang, M.; Zhang, C.; Tsai, W.-Y.; Tsai, D. P.; Nordlander, P.; Halas, N. J. Vacuum Ultraviolet Light-Generating Metasurface. Nano Lett. 2018, 18, 5738–5743. 2. Yin, Z.; Chen, F.; Zhu, L.; Guo, K.; Shen, F.; Zhou, Q.; Guo, Z. High-Efficiency Dielectric Metasurfaces for Simultaneously Engineering Polarization and Wavefront. J. Mater. Chem. C. 2018, 6, 6354–6359. 3. Zhang, H.; Zhang, X.; Xu, Q.; Tian, C.; Wang, Q.; Xu, Y.; Li, Y.; Gu, J.; Tian, Z.; Ouyang, C.; Zhang, X.; Hu, C.; Han, J.; Zhang, W. High-Efficiency Dielectric Metasurfaces for PolarizationDependent Terahertz Wavefront Manipulation. Adv. Opt. Mater. 2018, 6, 1700773.


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Spatial Frequency Multiplexed Meta-Holography and Meta

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