Sergei A. Khakhomov

Bound states in the continuum (BICs) exhibit enormous potential for advancing optical response due to their ultra-high Q factors. However, their practical applications are hindered by scattering losses caused by fabrication defects. This study proposes a strategy of merging multiple bound states at the Γ point of symmetric dielectric metasurfaces, aiming to enhance their optical response performance. Based on photonic crystal slabs with C4v/C6v symmetry, we designed structures that support symmetry-protected BICs and multiple accidental BICs capable of merging and separating. The Q factor of the C4v structure at the high-symmetry point remains consistently at 108. Then we can enhance the Q factor of the C6v structure up to 1010 by merging multiple accidental BICs. The merged states maintain excellent stability against variations in the lattice period, which significantly improves the optical responses. We experimentally achieved precise determination of the Q factor, and these results are in excellent agreement with the simulation data. This study of merged BICs provides a novel approach for the development of high-performance sensing and high-precision detection technologies.

This study examines the effect of graphene oxide nanoparticle (NGO) doping (0.12–0.24 mol%) on the properties of FTO layers obtained by spray pyrolysis. The results show that nanographene promotes phase separation within the FTO bulk. The resulting inhomogeneities are fluorine-depleted relative to the surrounding film. NGO doping also modifies the position and intensity of peaks in the absorption spectra, indicating a change in the nature of the absorbing centers. The optical bandgap of the FTO layers decreases with the increase in NGO from 4.4 eV (undoped samples) to 3.95 eV (samples with the highest NGO concentration). Furthermore, NGO doping reduces the sheet resistance of the FTO layers. This reduction is attributed to increased charge carrier mobility resulting from passivation of nanocrystallite interfaces by graphene oxide nanoparticles. However, the sheet resistance depends non-monotonically on NGO content, with the lowest value observed at 0.16 mol% NGO. The increase in resistance at higher NGO concentrations is due to enhanced carrier scattering caused by the growing size and number of the phase inhomogeneities within the FTO layer.

We propose a metasurface platform for the generation of grafted perfect vector vortex beams (GPVVBs) and a multi-channel optical encryption scheme. The metallic metasurface generates GPVVBs with controllable topological charges via phase grafting. GPVVBs produced by multi-segment phase-grafting configurations exhibit stable and controllable intensity distributions. Experimental and simulated results verify that the proposed metasurface can generate GPVVBs with two- and three-region phase-grafting configurations. The measured results show excellent agreement with simulations, confirming the effectiveness of the design. The multi-channel encryption scheme encodes plaintext into hexadecimal sequences and maps them onto vortex beam patterns. This dual encryption scheme, using topological charge multiplexing, greatly enhances the capacity and security of optical communications and information.

This study investigates the effect of co-doping with nanographene on the properties of FTO layers produced by spray pyrolysis. The results show that co-doping with nanographene enhances phase separation processes within the bulk of the FTO layer. The inhomogeneities formed during phase separation are depleted of fluorine compared to the film bulk. Co-doping with nanographene also significantly modifies the optical properties of the FTO layers. Specifically, it alters the position and intensity of the peaks in the reflection spectra, indicating a change in the nature of the absorbing centers. Furthermore, the optical bandgap of the FTO layers decreases with an increasing degree of nanographene doping. Finally, co-doping with nanographene reduces the resistance of the FTO layers, which can be attributed to an increase in charge carrier mobility.

Mitigating quantum decoherence and enhancing long-range entanglement are critical to the advancement of quantum information applications. We propose a photonic crystal-air-photonic crystal sandwich-structured waveguide system, which exhibits zero index by utilizing the accidental degeneracy between the waveguide mode and two bound modes. We evaluate transient and steady-state quantum information (including entanglement and coherence), demonstrating the robust generation of quantum resources over arbitrarily long distances. To the best of our knowledge, this work represents the first realization of an ideal long-range quantum-resource generator based on a zero-index waveguide.

We propose an enhanced polarization-multiplexed metasurface imaging scheme integrated with a visual secret-sharing encryption strategy. Information-bearing images are encoded into a multilayer H-shaped metasurface, establishing the first physical layer of security. Subsequently, the image content is converted into a global encryption key and corresponding user-specific subkeys through pixel overlay, forming the second information layer of protection. To validate the proposed scheme, metasurface samples are fabricated, and experimental results demonstrate robust reconstruction of encrypted images with high fidelity and low crosstalk under different polarization states. This method provides a promising solution for millimeter-wave imaging, physical-layer secure communication, and intelligent encryption applications.

The goal of this project is to create a new type of polarizer using printed circuit boards that can convert an incident linearly polarized wave into a reflected circularly polarized wave in the microwave range. This device represents a metamaterial surface consisting of a metal plate array made up of flat copper rectangular Ω-elements on a glass fiber substrate. By optimizing the shape of these elements, we found that they can also be used as absorbers for microwaves in addition to their ability to transform polarization. We showed that this form of Ω-resonators, which make up the metamaterial surface, are universal for use in THz polarizers. Finally, we investigated the polarization-selective properties of a metamaterial based on standard copper-coated fiberglass. This material exhibited polarization-selective properties near the resonant frequency in the microwave range and can be used as an effective polarization converter for microwaves.

Electromagnetic metasurfaces have important applications in vortex beam generation and steering, which enables great potential in secure communication, imaging systems, and quantum technologies. Conventional geometric phase-based metasurfaces lack the ability to control two spins simultaneously. In this paper, we designed a set of meta-atoms with appropriate geometric dimensions and orientation angles to achieve complete spin decoupling. Metasurfaces engineered by geometric phase and propagating phase can produce various vortex beams with distinct topological charges when illuminated by right-handed circular polarization and left-handed circular polarization. The experiments are conducted to assess the near-field and far-field efficacy of vortex beams. The experimental findings align well with the theoretical predictions. This study provides novel techniques for the arbitrary generation of high-performance vortex beams generated by metasurfaces within the microwave domain, which can be widely used in information encryption and 5G/6G wireless communication systems, facilitating advancements in high-performance wireless encrypted communication systems.

This study explores the propagation characteristics of partially coherent radial-phase-modulated vortex (PCRPMV) beams in anisotropic atmospheric turbulence to improve the reliability of wireless optical communication systems. An explicit theoretical derivation of the cross-spectral density (CSD) function for such beams propagating through anisotropic turbulence is presented. Results show that the self-focusing behavior of PCRPMV beams can be precisely controlled through radial-phase-modulation parameters. Compared to fully-coherent vortex beams, partially coherent counterparts demonstrate superior stability in orbital angular momentum (OAM) signal propagation. Additionally, partial coherence effectively mitigates turbulence-induced beam disorder. Our findings demonstrate that the combined effects of radial-phase-modulation and partial coherence significantly enhance both the robustness of OAM signals and transmission efficiency, suggesting PCRPMV beams’ potential for turbulent free-space optical communication systems.