Bidirectional Vectorial Holography Using Bi-Layer Metasurfaces and Its Application to Optical Encryption

Abstract: The study of optical systems with asymmetric responses has grown significantly due to their broad application potential in various fields. In particular, Janus metasurfaces are notable for their ability to control light asymmetrically at the pixel level within thin films. However, previous demonstrations were restricted to the partial control of asymmetric transmission for a limited set of input polarizations, focusing primarily on scalar functionalities. Here, we introduce optical metasurfaces consisting of bi-layer silicon nanostructures that can achieve a fully generalized form of asymmetric transmission for any possible input polarization. Their designs owe much to our theoretical model of asymmetric optical transmission in reciprocal systems that explicates the relationship between front- and back-side Jones matrices for general cases, revealing a fundamental correlation between the polarization-direction channels of opposing sides of incidence. To practically circumvent this constraint, we propose a method to partition transmission space, enabling the realization of four distinct vector functionalities within the target volume. As a proof of concept, we experimentally demonstrate polarization-direction-multiplexed Janus vector holograms generating four vector holograms. When integrated with computational vector polarizer arrays, this approach facilitates optical encryption with a high level of obscurity. We anticipate that our mathematical framework and novel material systems for generalized asymmetric transmission may pave the way for applications such as optical computations, sensing, and imaging.