Securing optical information to avoid counterfeiting and manipulation by unauthorized persons and agencies requires innovation and enhancement of security beyond basic intensity encryption. In this paper, we present a new method for polarization-dependent optical encryption that relies on extremely high-resolution near-field phase encoding at metasurfaces, down to the diffraction limit. Unlike previous intensity or color printing methods, which are detectable by the human eye, analog phase decoding requires specific decryption setup to achieve a higher security level. In this work, quadriwave lateral shearing interferometry is used as a phase decryption method, decrypting binary quick response (QR) phase codes and thus forming phase-contrast images, with phase values as low as 15°. Combining near-field phase imaging and far-field holographic imaging under orthogonal polarization illumination, we enhanced the security level for potential applications in the area of biometric recognition, secure ID cards, secure optical data storage, steganography, and communications.
Information security is an important concern in daily life from civil to military applications , . Optical encryption for information security has gained enormous attention owing to its ability to provide many degrees of freedom to encode the information relying on various optical channels such as frequency, amplitude, phase, and polarization , . These efforts led to various encryption methods, including Lippmann plate , spatial correlators , and holograms . Recently, an ultrathin artificial material with subwavelength structure called metasurface , ,  has been developed to control the electromagnetic wave across the entire frequency range for various optical applications such as lenses , , , , , cloaking , , holograms , , , , polarizers , , , perfect absorbers , , retroreflectors , etc. The compactness and versatile functionality make metasurfaces perfect candidates for optical encryption. The basic idea is to leverage on different optical channels of the metasurface to encode different optical information. Proof-of-concept demonstration of metasurface spectral encoding, displaying sharp and hyper-resolved images observable under a microscope, has been demonstrated , . Metasurfaces have also been designed to perform far-field holographic encoding using the concept of meta-holograms. The latter is the most common approach in the area of metasurface encryption using different light sources to decrypt the holographic images. A great deal of attention has been paid on the design of light source–dependent meta-hologram, such as polarization selectivity , , , , orbital angular momentum selectivity , , incoming direction of the incident light , , , etc. Other attempts have also been made for metasurface encryption, such as the combination of color printing and the holographic image , , image postprocessing based on spatial frequency , and tunable meta-hologram . All of the proposed metasurface encryptions, relying either on the intensity of the color print or on the holographic image projection, are directly perceived by human eyes, which could limit their applicability for information security.
Here, we introduce a new class of optical encryption that combines the near- and far-field information. On the one hand, the near-field encryption is based on phase imaging at the metasurface plane. The information is encoded in the phase of the transmitted light without any modulation of its intensity, making the information inaccessible with a conventional microscope. In order to resolve the encoded information, one has to obtain the phase map information, which we measured in this article using quadriwave lateral shearing interferometry (QLSI) technique , , ,  (see more details in Section 5). The phase-addressing capability of metasurfaces, i.e., the spatial phase distribution of the phase elements, can be scaled down to the single pixel with a pitch of 300 nm, which is beyond Abbe’s classical diffraction limit with an incident wavelength higher than 600 nm. Obviously, as to retrieve the phase information, one is using traditional optical characterization tools generally limited to diffraction limits, and it is not necessary to encode with such offensive resolution. As a proof of concept, we have considered phase pixels of about 750 nm. The proposed phase encryption could complement the class of color printing at the optical diffraction limit ,  in high-density spectrally encoded optical data storage and so on. On the other hand, far-field encryption relying on the holographic image requires a full 2π phase range, which according to our results, appears to be not necessary for near-field encoding mentioned above, allowing more flexibility for realistic applications. The combination of phase and projected intensity, respectively encoded in the near field and far field, improves optical information security.
In the following section, we describe the theoretical design as well as the experimental fabrication and characterization methods. Two different examples of phase encryption are demonstrated in Section 3. The first example shows several functional phase-encoded binary quick response (QR) codes with decreasing phase-encoding values ranging from 180° down to 15° phase difference, below which the experimental realization fails to deliver readable codes. In the second example, near-field encryption, i.e., phase imaging at the metasurface plane, is multiplexed with far-field encryption, i.e., holographic image projection, to display two different “Bat” and “Batman” images.
The principle of the dual-mode encryption is shown in Figure 1. The metasurface consists of a birefringent dielectric meta-molecule array with Jones matrix of (assuming full transmission of the incident wave). When a plane wave with x-polarization (Ex) illuminates the metasurface, as shown in Figure 1B (left panel), the output near field of the metasurface is , which is encoded by a phase information of . By locally designing different metastructures, we can get a specific phase pattern in the near field (Figure 1B, middle) and a random intensity profile in the far field (Figure 1B, right). However, the y-polarized light works in the opposite way, creating a disordered near-field phase image but encrypting a holographic image in the far field, as shown in Figure 1C.
The design of the metasurfaces is realized using full-wave finite-difference time-domain simulations at the wavelength of 617 nm. The unit cell of the metasurface consists of semiconductor-based GaN nanopillars with a thickness of h = 1 μm and period of P = 300 nm on sapphire substrate, as shown in Figure 2A and 2B. GaN has the merits of (1) low loss in the entire visible range to design the metasurfaces with high efficiency and (2) high refractive index to manipulate the phase in the full 2π range using relatively short nanostructures. Other low loss and high refractive index like SiN and TiO2 could be alternative choices , . The GaN nanopillars support waveguide-like modes for which the accumulated phase of the transmitted light is strongly dependent on the dimensions Lx and Ly . The simulated results of the transmitted amplitude and phase, as shown in Figure 2C–2E, demonstrate that the transmitted amplitude of the metasurface is almost near unity, while the phase is widely covering a full 2π range.
In this study, we propose two sets of encrypting metasurfaces, (1) a single-channel metasurface (SC-MS) encryption working only for x-polarized light and (2) a dual-channel metasurface (DC-MS) encryption that works differently for x- and y-polarized light. SC-MS keeps Ly = 120 nm, and Lx is controlled to achieve different phase information for x-polarized light, as shown by the white stars in Figure 2C and 2D. DC-MS controls both phase information of x- and y-polarized light, as shown in the purple and red stars in Figure 2C–2F. By controlling the size of Lx and Ly of the DC-MS, the transmitted phase of x-polarized light φxp and φxr (subscript “p” indicating purple stars and “r” indicating red stars) is constant along the purple and red stars, respectively, with a phase difference φxp − φxr of 90° for binary phase imaging in the near field. Meanwhile, the phase of the y-polarized light φyp and φyr varies in the 2π range for the phase encoding of the holographic image in the far field. For simplicity, the holographic phase profile has been encoded in only four phase levels.
As a proof of concept, we demonstrate a binary QR code in the SC-MS with the binary encoded phase of φh and φl (subscripts “h” and “l” indicating high- and low-level phase, respectively) for phase imaging. The QR code consists of 29 × 29 pixels with the binary phase of and , which redirects to the CNRS-CRHEA website when it is scanned using a portable QR code scanner. Each pixel of the QR code contains 20 × 20 metastructures. Thus, the total size of the SC-MS is 174 × 174 μm. We fabricate seven samples of SC-MS with different phase contrast ranging from 5°, 15°, 30°, 60°, 90°, 135°, to 180°. The measurement setup for phase imaging is shown in Figure 3 (see more details in Section 5.2). The measured amplitude and phase profile using QLSI are shown in Figure 4A and 4B, respectively. One can see that the QR codes are almost invisible on the amplitude images and are thus impossible to read while they are clearly visible on the phase images. When the phase contrast , the measured QR code phase images can be scanned and directed to the CNRS-CRHEA website, but it starts to fail when . In order to estimate the actual values of of the measured QR code phase images, we plot the pixel-by-pixel histograms for each image, as shown in Figure 5A. The histogram is fitted with Gaussian functions with two peaks, one corresponding to the low-level phase and the other corresponding to the high-level phase . The difference between the two peaks gives the phase contrast , while the full width at half maximum estimates the error of the extracted phase. The results for different phase contrast from 15° to 180° are shown in Figure 5A. The expected measured phase and standard deviation of and are shown in Figure 5B.
It can be seen that when is too small, , the width of each Gaussian profile is larger than , which makes the two peaks indistinguishable. The discrepancy of the measured phase mainly comes from the fabrication error because the length difference of the GaN nanopillars corresponding to the binary phase of 5° is only 2.5 nm, which is lower than our electron-beam lithography (EBL) precision. Increasing the designed phase contrast, i.e., such as , the two peaks become distinguishable, and the extracted phase contrasts agree well with the expected ones. However, when the phase contrast is close to 180°, such as and , the standard deviation is increasing owing to the phase wrapping issues from the phase retrieval algorithm used to extract the pixel phase value.
The near-field phase imaging can be further combined with far-field holographic imaging so that a dual-mode optical encryption can be achieved. Here, the phase contrast of near-field phase imaging is chosen as , according to its good performance shown in Figure 5. The experimental results of DC-MS for near-field and far-field encryption are shown in Figure 6A. When the metasurface is illuminated by x-polarized light, a phase image of “Bat” measured by QLSI is shown in Figure 6B; however, owing to the designed phase in the near field only, the holographic image in the far field has no useful information, as shown in Figure 6C. Instead, considering a y-polarized light illumination, a phase profile without useful information is shown in Figure 6D, displaying a holographic image of “Batman” in the far field, as shown in Figure 6E, to achieve a dual-mode optical encryption able to combine near-field phase imaging and far-field holographic image projection.
In conclusion, we have demonstrated a new class of optical encryption relying on dual-mode metasurfaces. Our method is able to print optical phase information at the nanoscale, which can be extracted using QLSI technique. We encoded several phase QR codes in simple binary phase images, demonstrating significant advantage with respect to conventional full-phase holographic image encryption. We demonstrated that a phase difference as low as 15° is sufficient to successfully scan the QR code and redirect to the desired website page. It is interesting to point out that gradually varying the nanopillar geometry enables analog phase addressing, offering an interesting perspective for multiplexed coding capabilities. Moreover, by combining the holographic display in the orthogonal polarization, a dual-mode optical encryption is demonstrated for near-field and far-field encryption, expanding promising applications in optical information security, data encryption, optical ID tags for authentication and verification, high-density optical data storage, etc.
The fabrication of the metasurface starts from growing a 1-μm GaN thin film on a double-side polished c-plane sapphire substrate using a molecular beam epitaxy RIBER system. Conventional EBL is used to pattern the GaN nanopillars. The GaN thin film is spin-coated by a double layer of 200-nm Polymethyl methacrylate (PMMA) resist (495A4) and baked on a hot plate at a temperature of 125 °C. The resist is exposed to an electron beam of 20 keV (Raith ElphyPlus, Zeiss Supra 40) considering the desired pattern and then developed using IPA:MIBK (3:1) solution. A 50-nm-thick nickel film is deposited on the sample in the e-beam evaporator, and a nickel hard mask is obtained by removing the resist through the lift-off process in acetone solution. The GaN nanopillars are obtained by reactive ion etching (Oxford system) with a plasma composed of Cl2CH4Ar gases. Finally, the nickel hard mask on the GaN nanopillars is dissolved in HCl:HNO3 (1:2) solution, revealing only the GaN nanopillar pattern on the transparent sapphire substrate.
The measurement setup for far-field holographic imaging is shown in Figure 3A. A laser with a wavelength of 617 nm is used as the light source. The latter passes through a linear polarizer followed by a lens to weakly focus on the metasurface and creates a holographic image in the far field, which is captured by a projector placed after the metasurface at a distance of 10 cm. The setup for phase imaging is shown in Figure 3B. It utilizes a quantitative phase microscopy technique based on QLSI , , , . A light-emitting diode with a wavelength centered at 617 nm integrated in a Köhler configuration is used as an illumination source with a controlled optical plane wave (controlled illuminated area and controlled numerical aperture). The light beam passing through the metasurface is collected by a microscope objective and sent to the QLSI wavefront analyzer (SID4 camera from Phasics Company) to measure both the intensity and the optical path difference of the light at the microscope’s imaging plane, coinciding with the metasurface position.
Funding source: European Research Council (ERC)
Funding source: European Union’s Horizon 2020
Award Identifier / Grant number: 639109
The authors acknowledge the support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement no. 639109).
Author contribution: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
Research funding: The study was supported from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement no. 639109).
Conflict of interest statement: The authors declare no conflicts of interest regarding this article.
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