Local confinement of light is very important to the field of optics, and it is theoretically possible to achieve the perfect confinement for optical modes located in the continuous domain radiation state. Achieving perfect confinement is possible for bound states in the continuum(BIC), which localize waves that lie in the continuous spectrum of the radiative state and remain in a perfect localized state. In this letter, we propose a metasurface that consisted of arrays of silicon nano-ellipsoidal pairs and calculate its transmission spectra and energy bands. Based on an all-dielectric metasurface, a dielectric multilayer film is incorporated to improve the quality factor of the array composed of ellipsoidal table pairs by an order of magnitude. By breaking the symmetry of ellipsoidal table pairs, quasi-BIC resonance modes excited by electric quadrupole(EQ) and magnetic dipoles(MDs) are realized using asymmetric silicon nanoellipsoid surfaces, where the quasi-BIC modes have high Q-factors. In this paper, the relationship between the structural asymmetry of Fano resonance controlled by symmetry-protect BICs and the radiation Q-factor is analyzed and demonstrated theoretically. At the same time, we use quasi-BIC resonances in asymmetric silicon nano-ellipsoidal pairs of metasurfaces to achieve a huge Goos-Hänchen (GH) shift enhancement which exceeds the 3rd-order wavelength and a very large group index to achieve excellent slow-light effects. Our work is centered on the design of simulations for the BIC metasurface. Meanwhile, based on the simulation design results, we propose to advance the application of optical communication devices, BIC sensing, optical switches, and optical modulators.

A new multiple-image optical encryption scheme based on phase retrieval algorithm and fractional Talbot effect is proposed. In encryption process, by using Fresnel domain phase retrieval algorithm, each image is encoded into a pure phase distribution with different spatial constraints. The spatial constraints are chosen as complementary square aperture array. A Talbot illuminator is designed according to the fractional Talbot effect, which is treated as a decoder to obtain the designed square aperture array. Then all obtained phase distributions are combined into the final ciphertext image. In decryption process, the images can be decoded from the ciphertext by using the Talbot illuminator and keys. Benefit from the use of fractional Talbot effect, the demultiplexing setup of the cryptosystem is very simple and straightforward, with no need of lens. The simulation results prove that the multiplexing capability is considerably enhanced. At the same time, the security analysis proves that the system can resist various attacks.

Recent advancements in high-order topological insulators have heralded new opportunities for the innovation and utilization of optical devices. This paper presents a composite asymmetric C4 photonic crystal to achieve multifunctional, robust topological states. Through detailed analysis of the fine changes in the topological bandgap induced by distortion parameters, we facilitate the realization of topological edge states in wavelength division multiplexing applications. We utilize both trivial and nontrivial properties of the topological bandgap to precisely manipulate zero-dimensional angular states, one-dimensional topological boundary states, and two-dimensional body states. Through simulations and experimental results, our advanced asymmetric C4 photonic crystal structure demonstrates superior robustness for the transmission of topological edge states. Our research paves the way for the deployment of more robust topological boundary state transmission systems and advances the application potential of higher-order topological states.

As a special imaging technique, one of the most important advantages of single-pixel imaging (SPI) than conventional imaging method is that it can recover the object image even through turbid medium. In these situations, the noises bring by the turbid medium usually obey special rules in statistics. While in some other applications, SPI is performed in the environment with complicate ambient light, in which no prior information of the environment noise is known. Aiming at this situation, in this work, the frame-by-frame subtraction-based compressive sensing SPI (FFS-CSPI) method with random 0/1 pattern is used to decrease the effect from the unknown ambient light. The noise robustness of the FFS-CSPI method is analyzed and compared with Hadamard CSPI. In simulation and experiment, two kinds of noises, from external light source and from background video, are considered. The results prove that FFS-CSPI with random 0/1 patterns can achieve higher image quality than conventional mean subtraction method and Hadamard CSPI with the same measurement number. Considering the high refresh rate of digital micromirror device when it loads the 0/1 binary patterns, the imaging speed is acceptable. This work will promote the practical applications of SPI in complicated environment.