Multiplexing Optical Images for Steganography by Single Metasurfaces

Switchable Two-Dimensional AND and Exclusive OR Operation Based on Dual-Wavelength Metasurfaces

Logic operation serves as the foundation and core element of computing networks; it will bring huge vitality to advanced information processing with its adaptation in the optical domain. As fundamental logic operations, AND and exclusive OR (XOR) operations serve a multitude of purposes, such as their ability to cooperate in enabling image processing and interpretation. Here, we propose and experimentally demonstrate a wavelength multiplexed AND and XOR function based on metasurfaces. By combining two cosine gratings with distinct frequencies and an initial phase difference of π/2, we extract the similarities and differences between two input images simultaneously by illuminating them with 445 and 633 nm wavelengths. Additionally, we explore its potential in information encryption, where overall security is enhanced by distributing distinct parts of initial information and encoded keys to different receivers. This design possesses the benefits of convenient mode switching and high-quality imaging, facilitating advanced applications in pattern recognition, machine vision, medical diagnosis, etc.

Longitudinally variable 3D optical polarization structures

Generation and manipulation of three-dimensional (3D) optical polarization structures have received considerable interest because of their distinctive optical features and potential applications. However, the realization of multiple 3D polarization structures in a queue along the light propagation direction has not yet been reported. We propose and experimentally demonstrate a metalens to create longitudinally variable 3D polarization knots. A single metalens can simultaneously generate three distinct 3D polarization knots, which are indirectly validated with a rotating polarizer. The 3D polarization profiles are dynamically modulated by manipulating the linear polarization direction of the incident light. We further showcase the 3D image steganography with the generated 3D polarization structures. The ultrathin nature of metasurfaces and unique properties of the developed metalenses hold promise for lightweight polarization systems applicable to areas such as 3D image steganography and virtual reality.

Electrically Tunable Steganographic Nano-Optical Coatings

Thin film coatings with tunable colors have a broad range of applications, from solid-state reflective displays to steganography. Here, we propose a novel approach to chalcogenide phase change material (PCM)-incorporated steganographic nano-optical coatings (SNOC) as thin film color reflectors for optical steganography. The proposed SNOC design combines a broad-band and a narrow-band absorber made up of PCMs to achieve tunable optical Fano resonance in the visible wavelength, which is a scalable platform for accessing the full-color range. We demonstrate that the line width of the Fano resonance can be dynamically tuned by switching the structural phase of PCM from amorphous to crystalline, which is crucial for obtaining high-purity colors. For steganography applications, the cavity layer of SNOC is divided into an ultralow loss PCM and a high index dielectric material with identical optical thickness. We show that electrically tunable color pixels can be fabricated using the SNOC on a microheater device.