Fang-Zhou Shu

Metasurfaces supporting bound states in the continuum (BICs) offer ultrahigh-Q resonances, yet their integration with active tuning mechanisms and mode coupling phenomena remains largely unexplored. In particular, a tunable quasi-BIC-mediated electromagnetically induced transparency (EIT)-like effect using phase-change materials is still an open challenge. Here, we present both numerical and experimental demonstrations of tunable quasi-BIC and EIT-like resonances by integrating asymmetric metasurfaces with the phase-change material Ge₂Sb₂Te₅ (GST). By breaking the in-plane inversion symmetry, a BIC mode is converted to a quasi-BIC mode. Furthermore, EIT-like resonances arise from the coupling between localized surface plasmons and either an electric dipole or quasi-BIC, leading to a dual EIT-like effect with distinct physical origins. The incorporation of GST enables thermal modulation of both quasi-BIC and EIT-like resonances through a phase transition from the amorphous to the crystalline phases, resulting in pronounced changes in transmission amplitude and electromagnetic fields. The hybrid phase-change metasurfaces hold potential for applications in terahertz modulators, filters, and slow-light devices.

Breaking the diffraction limit in optical imaging is crucial for resolving subwavelength details in a wide range of applications, where super-oscillatory imaging and subtraction imaging are two common strategies for surpassing conventional resolution limits. We propose an end-to-end deep learning framework that integrates super-oscillatory focusing and subtraction imaging into a single jointly optimized vectorial Debye integral neural network pipeline, eliminating the traditional two-step acquisition and manual weighting process. With this end-to-end neural network, we further improve the focusing capability of the system to the sub-100-nm regime, enabling deep-subwavelength imaging resolution.

Bound states in the continuum (BICs) have garnered significant attention in recent years due to their ability to enhance light–matter interaction. The high quality ( Q ) factors of quasi-BICs make them particularly promising for applications such as lasing, sensing, and nonlinear harmonic enhancement. Here, we numerically and experimentally demonstrate dynamically tunable multiple quasi-BICs by integrating aluminum rod arrays with GST. By tailoring the geometric parameters of the aluminum rod arrays, multiple quasi-BICs are achieved, manifesting as distinct Fano resonances in the terahertz transmission spectrum. Upon coating the aluminum rod arrays with GST, both the resonant frequencies and linewidths of the quasi-BICs can be dynamically tuned through the phase transition of GST from the amorphous to the crystalline phase. Furthermore, these quasi-BICs exhibit strong electromagnetic field confinement, which can be significantly modulated via the GST phase transition. The dynamically tunable multiple quasi-BICs offer promising potential for applications in terahertz modulators, filters, and imaging.

Metasurfaces operating under oblique incidence conditions have attracted considerable interest in modern photonic engineering, owing to their capability to enhance angular coverage for next-generation devices including wide-field detectors, multi-angle communication arrays, and advanced radar architectures. However, conventional metasurfaces frequently suffer from limitations in preserving functional stability at high incidence angles, where performance deterioration typically stems from reduced coupling efficiency and phase mismatch. This work proposes flexible terahertz metasurfaces with an ultra-large incident angle, addressing the limitations of conventional metasurfaces that suffer from severe performance degradation under high angles and lack mechanical adaptability. The proposed metasurfaces are designed through the synergistic integration of spoof localized surface plasmons (S-LSPs) and electromagnetically induced transparency (EIT). Experimental results demonstrate that the metasurfaces exhibit polarization-insensitive behavior while maintaining stable transmission spectra under oblique incidence up to 80°. Furthermore, bending tests confirm that the transmission spectrum of the metasurfaces remains stable even under significant mechanical deformation (with a maximum central angle of 111.7°). The proposed flexible metasurfaces provide a novel design paradigm for oblique-incident metasurfaces, holding great potential for emerging applications in conformal terahertz systems and adaptive wearable photonics.

Metasurfaces offer innovative approaches for manipulating electromagnetic waves at subwavelength scales. Recent advancements have focused on toroidal dipole (TD) and quasi-bound state in the continuum (quasi-BIC) modes, which are particularly attractive due to their capacity to enhance light-matter interaction. However, most metasurfaces with TD and quasi-BIC modes exhibit passive electromagnetic responses after fabrication, limiting their practical applications. This study presents both numerical and experimental investigations that demonstrate the active control of TD and quasi-BIC modes through the integration of symmetric and asymmetric aluminum dumbbell aperture arrays with the phase-change material Ge₂Sb₂Te₅ (GST). The symmetric hybrid dumbbell aperture array shows a pronounced TD response within the terahertz frequency range. In contrast, modifying the geometric parameters to disrupt the structural symmetry induces a quasi-BIC mode in the asymmetric hybrid dumbbell aperture array. Furthermore, as GST undergoes a phase transition from its amorphous to crystalline state, both TD and quasi-BIC modes become dynamically tunable, driven by changes in the conductivity of GST. Notably, significant modulation of the transmitted terahertz wave occurs around the frequencies corresponding to the TD and quasi-BIC modes during the GST phase transition. Symmetric and asymmetric hybrid dumbbell aperture arrays provide a versatile platform for generating tunable TD and quasi-BIC modes, with promising applications in terahertz modulators and filters.

Recently, the merging of accidental bound states in the continuum (BICs) has attracted significant attention due to the enhanced light–matter interactions. Here, we theoretically demonstrate the merging of accidental BICs in perturbed all-silicon terahertz photonic crystal (PhC) slabs with C₂ and C₂ broken-symmetry structures. The PhC slabs consist of an array of four cylindrical holes and support a TM symmetry protected (SP) vector BIC at the Γ point. Our results indicate that the merging and band transition of accidental BICs can be achieved by varying the diameter of diagonal holes in a C₂-symmetry structure. Notably, in a C₂ broken-symmetry PhC slab, the SP BIC will first convert to a quasi-BIC, then transit to a new accidental BIC, which are well displayed and interpreted by tracing the accidental BICs in momentum space. We believe that the results presented in this work show potential for the design and application of BICs in both symmetric and asymmetric PhC slabs.

Breaking the diffraction limit has been a key challenge in optical engineering and super-resolution imaging. In this work, we utilize a vectorial Debye integral neural network to design sub-diffraction focusing fields for high-NA objectives. By training the polarization states of incident light, we flexibly achieve transitions from diffraction-limited focusing to superoscillatory regimes. Through parameter adjustments, we optimize focal spot size, energy efficiency, and sidelobe distribution, achieving a focus with a 0.367λ FWHM and enhanced energy utilization. This method significantly simplifies the design process and demonstrates great potential for advanced optical applications, including super-resolution imaging and 3D field engineering.