Reprogrammable meta-hologram for optical encryption

ARTICLE https://doi.org/10.1038/s41467-020-19312-9 OPEN Reprogrammable meta-hologram for optical encryption 1234567890():,; Geyang Qu 1, Wenhong Yang1, Qinghai Song Din-Ping Tsai 5,6 ✉ & Shumin Xiao 1,2,4,6 ✉ 1,2, Yilin Liu1, Cheng-Wei Qiu3, Jiecai Han4, Meta-holographic encryption is a potentially important technique for information security. Despite rapid progresses in multi-tasked meta-holograms, the number of information channels available in metasurfaces is limited, making meta-holographic encryption vulnerable to some attacking algorithms. Herein, we demonstrate a re-programmable metasurface that can produce arbitrary holographic images for optical encryption. The encrypted information is divided into two matrices. These two matrices are imposed to the incident light and the metasurface, respectively. While the all-dielectric metasurface is static, the phase matrix of incident light provides additional degrees of freedom to precisely control the eventual functions at will. With a single Si metasurface, arbitrary holographic images and videos have been transported and decrypted. We hope that this work paves a more promising way to optical information encryption and authentication. 1 Ministry of Industry and Information Technology Key Lab of Micro-Nano Optoelectronic Information System, Shenzhen Graduate School, Harbin Institute of Technology, 518055 Shenzhen, China. 2 Collaborative Innovation Center of Extreme Optics, Shanxi University, 030006 Taiyuan, China. 3 Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117583, Singapore. 4 National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, Harbin Institute of Technology, 150080 Harbin, China. 5 Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hong Kong, China. 6These authors jointly supervised this work: Din-Ping Tsai, Shumin Xiao. ✉email: ; NATURE COMMUNICATIONS | (2020)11:5484 | https://doi.org/10.1038/s41467-020-19312-9 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-19312-9 I n modern society, almost everything is closely related to the internet currently or in the near future. The recent reports of attacks by hackers on the databases show that the information security and authentication are more challenging, especially for the large commercial and financial companies1. In past decade, many types of optical encryption techniques based on classic and quantum optics have been developed to achieve reliable security of information2,3. Owing to its compact and non-replicable nature, metasurface-based hologram has gradually been a promising approach4–10. Metasurface is a kind of two-dimensional materials that has the capability of precisely and fully controlling the incident light11. By carefully designing the response of each antenna to different polarization, incident angle, wavelength, or orbital angular momentum (OAM), multiple holographic images have been successfully implemented into a single metasurface in the past few years, significantly expanding the capacity and security of information encryption7,12–19. Despite of these progresses, the demonstrated meta-holograms are either static or binary4–20. While different multiplexing techniques based on spatial, polarization, wavelength, and OAM have been combined recently, the number of information channels is still finite and the current record value is only ~26 (refs. 7,15–26). Similar to the double-random-phase encryption technique, the finite channel number makes the meta-holographic encryption vulnerable to certain attacks because the key for decryption has to be used repeatedly without frequent update27. Therefore, from the point view of information security, it is highly desirable and urgent to realize a meta-hologram with infinite channels and higher security. Here, we revisit the light–metasurface interaction and experimentally demonstrate a reprogrammable meta-hologram for optical security and authentication. Results Working principle. In principle, the interaction between light and metasurface can be expressed as28: ZZ Uðx; y; zÞ ¼ Uinc ðx0 ; y0 ÞUmeta ðx0 ; y0 Þhðx  x0 ; y  y0 ; zÞdx0 dy0 ; ð1Þ where Uinc(x0, y0) and Umeta(x0, y0) correspond to the functions of incident light and metasurface, h(x, y, z) is an impulse response. The conventional approaches are mostly focusing on the response of nanoantennas in metasurface4–19. Only few parameters of the incident light have been considered, e.g., polarization and OAM etc.7,15–26. Since the metasurface is typically static, the information channel is thus determined by the degrees of incident light and is strongly limited7,15–26. In this research, we divide the required phase profile of a holographic image into two matrices. One matrix is imposed to the metasurface (Umeta(x0, y0)) as before, the other one is defined on the incident beam (Uinc(x0, y0)). This assumption is not only valid for algorithm but also consistent with the modern optical security system, which consists of light source, lens, detectors, phase only masks, and spatial light modulators (SLMs) etc.29,30. On one hand, the twodimensional matrix of incident beam can significantly expand the number of information channels. Even though the metasurface is static, the large matrix of incident beam still has enough large parameters to alter the output beam (U(x, y, z)) and the corresponding holographic images. On the other hand, the split information in transported light and metasurface can improve the security. The codes for the modulation of incident light is transported through the internet, whereas the metasurface can be implemented into the terminal decrypted devices. In case that the transported codes are cracked, they can only produce a random beam without any useful information (see inset in Fig. 1b). If the 2 metasurface is stolen and illuminated with an incorrect beam, no information can be hacked or a misleading image could be shown up (Fig. 1b). Only when the modulated incident light matches the metasurface, as shown in Fig. 1a, the encrypted image can be displayed. According to Eq. (1), for encrypted information with fixed overall matrix, the matrices for the incident light and the metasurface can be separated arbitrarily. Consequently, the roles of two matrices can be interchanged and the flexibility of the system has been dramatically increased (see Supplementary Fig. 2). Experiments. To test the above analysis, we have numerically designed the reprogrammable meta-hologram31,32. Taking an image of “HIT 100 (100th anniversary of Harbin Institute of Technology)”as an example, the required phase profile for a holographic image has been calculated based on Gerchberg–Saxton algorithm. After considering both of the efficiency and the fabrication difficulty, the continuous phase profile is discretized into eight-level phase step (...truncated)