High-dimensional Poincaré beams generated through cascaded metasurfaces for high-security optical encryption.pdf
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PhotoniX            (2024) 5:13   https://doi.org/10.1186/s43074-024-00125-8 PhotoniX High-dimensional Poincaré beams  generated through cascaded metasurfaces  for high-security optical encryption Jitao Ji1, Chen Chen1*, Jiacheng Sun1, Xin Ye1, Zhizhang Wang1, Jian Li1, Junyi Wang1, Wange Song1,  Chunyu Huang1, Kai Qiu1, Shining Zhu1 and Tao Li1*  Abstract  Optical encryption plays an increasingly important role in the field of information  security owing to its parallel processing capability and low power consumption.  Employing the ultrathin metasurfaces in optical encryption has promoted the min- iaturization and multifunctionality of encryption systems. Nevertheless, with the few  number of degrees of freedom (DoFs) multiplexed by single metasurface, both key  space and encoding space are limited. To address this issue, we propose a high-security  and large-capacity optical encryption scheme based on perfect high-dimensional  Poincaré beams with expanded DoFs. By cascading two arrayed metasurfaces, more  beam properties can be independently engineered, which gives rise to the extensively  expanded key and encoding spaces. Our work provides a promising strategy for opti- cal encryption with high security level and large information capacity and might  facilitate the applications of Poincaré beams in optical communications and quantum  information. Keywords:  Poincaré beams, Cascaded metasurfaces, Optical encryption Introduction With the rapid development of information technology, information security is becom- ing of vital significance and so do the derived encryption techniques. Optical encryption  possesses the advantages of parallel processing, large capacity and low power consump- tion, and offers an ideal platform for information protection owing to abundant degrees  of freedom (DoFs) of the photon [1–5]. However, conventional optical encryption sys- tem commonly involves in a series of complicated optical elements and results in a bulky  volume. In addition, due to the limited optical field manipulation capability of traditional  optical elements, the multi-dimensional DoFs of light have not been fully exploited. As an ultra-thin planar optical element composed of subwavelength nanostructures,  metasurface has shown powerful capability of multi-dimensional optical field manipu- lation and emerges as a promising candidate for optical encryption in a compact form  [6–13]. For metasurface-empowered optical encryption system, the key space could be  defined as the total number of keys and attributed to both incident and output states.  *Correspondence:    chenchen2021@nju.edu.cn;  taoli@nju.edu.cn 1 National Laboratory of Solid  State Microstructures, Key  Laboratory of Intelligent Optical  Sensing and Manipulation,  Jiangsu Key Laboratory  of Artificial Functional Materials,  College of Engineering  and Applied Sciences, Nanjing  University, Nanjing 210093, China Page 2 of 13 Ji et al. PhotoniX            (2024) 5:13  While the encoding space refers to the information capacity of the output images or  beams through metasurfaces. So far, various DoFs of light, including incident angle  [14–16], wavelength [17], polarization [18, 19], orbital angular momentum (OAM)  mode [20–23] and so on [24, 25], have been explored to construct key space for opti- cal information security. The combination of multiple DoFs has been demonstrated the  capability to enhance the complexity of keys [26–30]. Besides, through encoding infor- mation into optical images in the form of nano-printing or holography, encoding space  has also been established and depends on the number of multiplexing channels of meta- surface [31, 32]. To further expand the key and encoding spaces, encryption algorithms  and dynamic encryption schemes were aroused for high security and large capacity  performance [33–38]. Nevertheless, the above two strategies usually suffer from time- consuming decryption process and the lack of pixelated real-time modulation. Featured  with spatial variant polarization states and phase distributions, Poincaré beams gain a  great deal of attentions in many applications [39–44], especially in optical encryption  and encoding owing to versatile independent DoFs [45–49]. In the recent works, optical  information was encoded into the polarization order and ellipticity of perfect hybrid- order Poincaré beams (HyOPBs) [45]. It presents a new scheme of optical encryption  to enlarge the encoding space, which is distinguished from conventional nanoprinting  or holography encryption. However, such Poincaré beams encryption strategy has con- straints due to limited key space (with only one incident state in essence) and stays a  low security level. With the advantages of sub-wavelength modulation capability and  inherent ultra-thin size, metasurface also allows spatial multiplexing frameworks such as  arrayed metasurface [45] and cascaded metasurfaces [25, 50–52]. Therefore, the combi- nation of cascaded and arrayed metasurfaces boasts great potentials for enhancement of  key and encoding spaces towards high-security and large-capacity features. In this work, we propose a high-security and large-capacity optical encryption strategy  based on multi-channel perfect high-dimensional Poincaré beams (HDPBs) generated  by cascaded metasurfaces. Benefiting from the infinite modes of HDPBs and the scal- ability of arrayed metasurface, both key space and encoding space have been considera- bly expanded. Accordingly, by cascading two arrayed metasurfaces composed of 16 × 10  cells, an optical encryption platform is implemented by encoding information into five  independent parameters of HDPBs. Our work provides a promising solution to exten- sively enlarge the key and encoding spaces and may open an avenue towards advanced  optical encryption systems. Results Concept and principle of high‑dimensional Poincaré beams encryption Figure 1 schematically illustrates the HDPBs optical encryption scheme using cas- caded metasurfaces, which is capable of generating multi-channel HDPBs with  diverse DoFs to encode optical information. In principle, HyOPBs can be described  as a linear superposition of right-handed circular polarized (RCP) and left-handed  circular polarized (LCP) bases with different topological charges [39]. Here, we gen- eralize a more complex vector vortex beam superposed by two HyOPBs as HDPBs,  which can be represented by high-dimensional Poincaré sphere indicated by the  magenta or blue sphere in Fig.  1. Taking the blue sphere for instance, the north  Page 11 of 13 Ji et al. PhotoniX            (2024) 5:13  	 Then, an electron beam resist (PMMA-A4) with the thickness of 200 nm was spin- coated on top of the ­SiNx layer. The patterns of chiral meta-atoms were subsequently  defined on PMMA resist by Electron-beam lithography (EBL) system (ELS-F125, Eli- onix). After development, the generated pattern was transferring to an electron beam  evaporated chromium layer, which served as a hard mask for the following dry etch- ing process of ­SiNx layer in a mixture of ­CHF3 and ­SF6 plasma (Oxford Instruments,  PlasmaPro100 Cobra300). At last, the remaining chromium mask was removed by a  solution of ammonium cerium nitrate. Optical characterization A 470 nm laser was utilized for optical measurement and focused onto the fabricated  metasurfaces through an objective (20×, NA = 0.40). The polarization states of the inci- dent light were controlled by the combination of linear polarizer and quarter-wave plate.  After illuminating the metasurface/cascaded metasurface, the transmitted light was sub- sequently collected by an objective (20×, NA = 0.40) and analyzed by polarizers. Sub- sequently, the intensity profiles of the generated Poincaré beams were recorded using a  CMOS camera. The alignment of two cascaded metasurfaces is detailed in Supplemen- tary Note 5. Abbreviations DOFs	 Degrees of freedom HDPBs	 High-dimensional Poincaré beams OAM	 Orbital angular momentum HyOPBs	 Hybrid-order Poincaré beams RCP	 Right-handed circular polarized LCP	 Left-handed circular polarized H-polarized	 Horizontal linearly polarized V-polarized	 Vertical linearly polarized FDTD	 Finite-difference time domain PML	 Perfectly matched layer PECVD	 Plasma-enhanced chemical vapor deposition EBL	 Electron-beam lithography Supplementary Information The online version contains supplementary material available at https://​doi.​org/​10.​1186/​s43074-​024-​00125-8. Supplementary Material 1.  Acknowledgements Not applicable. Authors’ contributions C.C., J.J. and T.L. conceived the concept. C.C. and J.J. proposed the metasurface design and performed the numeri- cal simulations; J.S. and J.L. fabricated the samples with the help of C.H. and K.Q.; J.J. and C.C. performed the optical  measurements with the help of X.Y. and J.W.; J.J., C.C., Z.W., W.S., T.L. and S.Z. discussed the results; J.J., and C.C. wrote the  manuscript with the help of Z.W. and W.S.; T.L. supervised the project. Funding The authors acknowledge the funding provided by National Key Research and Development Program of China  (2022YFA1404301), National Natural Science Foundation of China (Nos. 62325504, 62305149, 92250304, 62288101), and  Dengfeng Project B of Nanjing University. The authors acknowledge the micro-fabrication center of the National Labora- tory of Solid State Microstructures (NLSSM) for technique support. Availability of data and materials The source data are available from the corresponding author upon reasonable request. All data needed to evaluate the  conclusion are present in the manuscript and/or the Supplementary Information. Page 12 of 13 Ji et al. PhotoniX            (2024) 5:13  Declarations Ethics approval and consent to participate Not applicable. Consent for publication All authors agreed to publish this paper. Competing interests The authors declare no conflicts of interest. Received: 17 January 2024   Revised: 6 March 2024   Accepted: 11 March 2024 References 	1.	 Matoba O, Nomura T, Perez-Cabre E, Millan MS, Javidi B. Optical techniques for information security. Proc IEEE.  2009;97(6):1128–48. 	2.	 Chen W, Javidi B, Chen X. Advances in optical security systems. Adv Opt Photonics. 2014;6(2):120–55. 	3.	 Liu S, Guo C, Sheridan JT. A review of optical image encryption techniques. Opt Laser Technol. 2014;57:327–42. 	4.	 Jiao S, Zhou C, Shi Y, Zou W, Li X. Review on optical image hiding and watermarking techniques. Opt Laser Technol.  2019;109:370–80. 	5.	 Liu S, Liu X, Yuan J, Bao J. 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Title: High-dimensional Poincaré beams generated through cascaded metasurfaces for high-security optical encryption ​
Authors: Jitao Ji, Chen Chen, Jiacheng Sun, Xin Ye, Zhizhang Wang, Jian Li, Junyi Wang, Wange Song, Chunyu Huang, Kai Qiu, Shining Zhu, Tao Li ​
Publisher: Springer Nature ​
Date: 17 April 2024 ​
Abstract: Optical encryption plays an increasingly important role in the field of information security owing to its parallel processing capability and low power consumption. ​ Employing the ultrathin metasurfaces in optical encryption has promoted the miniaturization and multifunctionality of encryption systems. ​ Nevertheless, with the few number of degrees of freedom (DoFs) multiplexed by single metasurface, both key space and encoding space are limited. ​ To address this issue, we propose a high-security and large-capacity optical encryption scheme based on perfect high-dimensional Poincaré beams with expanded DoFs. ​ By cascading two arrayed metasurfaces, more beam properties can be independently engineered, which gives rise to the extensively expanded key and encoding spaces. ​ Our work provides a promising strategy for optical encryption with high security level and large information capacity and might facilitate the applications of Poincaré beams in optical communications and quantum information. ​