Circularly polarized light can present more optical properties of chiral materials and is widely used to analyze and detect biomolecules. In this paper, a dual-frequency terahertz circular polarization detection structure, which is based on multilayer metamaterials, is proposed. The proposed structure consists of a dual-frequency quarter-wave plate, a polyimide spacer, and a filter. The simulation results show that the structure can distinguish the handedness of circularly polarized light by filtering. The extinction ratios are 4 dB and 5.26 dB at 0.952 THz and 1.03 THz, respectively, and the maximum transmittance efficiency reaches 40%. Given the advantages of easy integration and dual-frequency operation, our design is bound to facilitate the development of multi-frequency detection in biomedical imaging devices.

Metalenses have recently attracted increased attention due to their remarkable characteristics. The fabrication technology of metalenses has also become an important research direction. In this study, we propose a metalens structure based on Au–MgF₂–Au in infrared waveband. The preparation process of the metalens included magnetron sputtering, electron beam evaporation, and electron beam exposure. A dose test was performed during the exposure process, adjusting the exposure dose to minimize the proximity effect after exposure. Then, SEM was used to measure the processed metalens structure, and FDTD software was used to build a model based on the metalens, simulating and analyzing its focusing characteristics. The results show that the size deviation produced during the processing has little effect on the functionality of the metalens. The processed metalens can also focus different polarized light incidences at different spatial positions: The metalens can focus at 4.97 μm for x-polarized light and focus at 13.5 μm for y-polarized light. Additionally, the metalens has good focusing effects with different working wavelengths. We believe that the processing method of metalens proposed in this paper provides guidance for the preparation of subwavelength metasurface structures, and our findings are beneficial in developing new methods of near-infrared regime manipulation.

Terahertz (THz) chiral metasurfaces have attracted considerable attention for manipulating circular polarization. However, most reported designs offer either narrowband or broadband circular dichroism (CD) with fixed spectral characteristics, limiting their adaptability to multifunctional scenarios. This work proposes a switchable terahertz chiral metasurface integrating CD response and refractive-index sensing based on the thermally induced insulator-to-metal transition of vanadium dioxide (VO₂) at approximately 68 °C. Without altering the geometric structure, the device can dynamically switch between narrowband and broadband CD responses via the VO₂ phase transition. In the insulating state, the structure produces two sharp CD resonant peaks at 1.88 THz and 3.25 THz with CD values of 0.93 and 0.88, respectively. These narrowband resonances exhibit refractive-index sensing capability: as the ambient refractive index increases across an extended dynamic range from 1.00 to 1.60, the resonant frequencies undergo a continuous redshift, achieving sensitivities of 299.3 GHz/RIU and 258.0 GHz/RIU with figures of merit (FOMs) of 4.6 RIU⁻¹ and 5.3 RIU⁻¹, respectively. Upon the phase transition to the metallic state, the drastically enhanced VO₂ conductivity redistributes the local current density and strengthens inter-resonator coupling, which broadens the localized narrowband resonances and triggers inter-mode coupling. Consequently, the discrete dual-narrowband CD response evolves into a broadband feature. In the metallic state, the maximum CD reaches 0.75 and remains above 0.5 across 1.56–3.48 THz, corresponding to an absolute bandwidth of 1.92 THz and a relative bandwidth of 76%—a substantial broadening compared with the insulating state. These results suggest that the proposed metasurface holds promise for reconfigurable terahertz chiral sensing, broadband polarization filtering, and multifunctional terahertz devices.