Owing to its remarkable spin stability and robustness during propagation, circularly polarized light detection technology holds profound implications for numerous application domains. However, the human eye and conventional detection methods exhibit significant limitations in identifying polarization information, thereby underscoring the critical importance of specialized photonic devices designed for the detection and discrimination of circularly polarized light. As polarization detection technology continues to advance, the performance requirements for circularly polarized devices are correspondingly elevated. These devices are not only expected to possess high extinction ratios, broad operating wavelength ranges, and high transmittance, but must also meet the demands of miniaturization and integration. Nevertheless, due to material constraints, operational principles, and size-related issues, existing circularly polarized devices still face numerous challenges and are in urgent need of performance enhancement. Two-dimensional metasurfaces, periodic artificial microstructures at the subwavelength scale, exhibit unique properties not found in natural materials. These properties overcome the size and functionality constraints of traditional optical components, offering promising avenues for developing ultrathin, mid-infrared, circularly polarized detectors. The incorporation of phase-change materials offers new opportunities for creating integrated photonic devices with tunable optical properties. In this context, we concentrate on designing and implementing ultra-thin circular polarization detectors using phase-change material metasurfaces.

We concentrate on designing metasurfaces utilizing phase-change materials, and presents a broadband and high circular dichroism metasurface featuring an adjustable working wavelength. We offer a comprehensive elaboration of the design principles and theoretical underpinnings of this metasurface through the application of Jones matrices, and illustrates its asymmetric transmission characteristics, circular dichroism, and extinction ratio performance via simulation results. Moreover, by examining the near-electric field distribution of the metasurface and conducting multipole decomposition of the reflected electric field, we delve into the physical mechanisms that enable its performance. Full-wave numerical simulations are performed using Lumerical FDTD Solutions, a commercial software based on the finite-difference time-domain (FDTD) method. Periodic boundary conditions are applied in the x- and y-directions of the unit cell, while perfectly matched layers (PML) are employed in the z-direction. The excitation source is a circularly polarized plane wave, generated by superimposing x- and y-polarized plane waves with equal amplitudes and a phase difference of 90°, propagating along the z-axis. The frequency range of the excitation source spans the entire band of interest, allowing for a comprehensive analysis of the structure’s electromagnetic response. A transmission monitor is placed on the transmitted side of the metasurface to measure the intensity distribution and spectral characteristics of the transmitted light.

When the Sb₂S₃ phase-change material is in its amorphous state, the designed broadband circular dichroism metasurface exhibits asymmetric transmission parameters Δ_LCP (0.9997) and Δ_RCP (-0.9867), with absolute values approaching unity under left-handed circularly polarized light (LCP) and right-handed circularly polarized light (RCP) incidence, respectively, thereby achieving nearly perfect selective transmission for circularly polarized light (Fig.2). In this state, the circular dichroism (CD) of the left-handed (LH) chiral metasurface reaches an exceptionally high value of 0.9997, while the circular polarization extinction ratio (CPER) attains an impressive 77 dB (Fig. 3). Under different crystalline states of the phase-change material, the metasurface maintains a CD value above 0.9914 and a CPER value higher than 23 dB at the corresponding working wavelengths, with a tunable wavelength range of 1.3 μm (Fig. 4). Analysis of the near-electric field distribution across various sections of the metasurface reveals that, under LCP light incidence, the electric field is localized within the air gaps between the nanostructures, facilitating smooth propagation of the light through the metasurface; in contrast, under RCP excitation, the mismatch between the incident light and the chirality of the structure confines the electric field to the edges of the nanostructures, inducing high reflectivity and thus imparting the metasurface with pronounced circular dichroism. The working wavelengths of the metasurface in the amorphous and crystalline states of Sb?S? are determined to be 4.19 μm and 5.44 μm, respectively, thereby demonstrating the tunability of the working wavelength and highlighting the versatility of the metasurface design (Figs. 5, 6). Multipole decomposition of the electric field further elucidates the significant contrast in intensity between RCP and LCP light, which serves as the fundamental physical mechanism underlying the high CD value of the nanostructure (Figs. 7, 8). Additionally, the cutoff wavelength of the chiral response is predicted based on magnetic dipole (MD) resonance, thereby corroborating the accuracy of the theoretical analysis (Figs. 9, 10).

In summary, we propose and design an all-dielectric chiral metasurface with twofold rotational symmetry, achieved by breaking mirror symmetry. By controlling the crystallization rate of the phase-change material, the metasurface’s working wavelength can be tuned within the 4.2?5.6 μm range, offering extensive application prospects in this spectral band. The structure exhibits LH and RH chirality, enabling selective transmission of LCP and RCP. Notably, the metasurface achieves optimal values for CD, CPER, and asymmetric transmission (AT). Furthermore, through an in-depth analysis of the metasurface’s near-electric field distribution and scattering cross-section multipole moments, we elucidate the underlying physical mechanisms of its superior performance. Additionally, the cutoff wavelength of the chiral response is predicted based on magnetic dipole resonance, which aligns with the simulation results and thereby validates the design’s accuracy and practicality. This study advances the application of chiral metasurfaces in various fields, such as circularly polarized light detection, atmospheric sensing, and environmental monitoring in the mid-infrared band. It also offers new possibilities for designing miniaturized tunable photonic devices and their applications in related fields.