A metasurface for linear-to-circular polarization conversion and sensing based on quasi-BIC

This work presents a multifunctional metastructure (MS) which realizes linear to circular polarization conversion and sensing function based on quasi-bound states in the continuum (quasi-BIC). MS is made of silicon dioxide as substrate, and silicon as surface material, by etching cross holes and square holes on it to form a 2×2 structure, through the transmission of terahertz (THz) band, to form an ultrahigh quality factor (Q-factor), and realize the conversion of linearly polarized waves to circularly polarized ones. At 178.190 THz, it achieves a Q value of 2969, and in the range 178.193 TH to 178.200 THz, the axial ratio (AR) is less than 3 dB and the insertion loss is less than 0.0001. In addition, by changing the permittivity of the surrounding environment, the minimum of the output wave will produce a good linear frequency shift. Using this feature, the given device can also be used as a dielectric constant sensor to detect air quality. The device has a sensing sensitivity (S) of 6.415 THz RIU⁻¹ and a figure of merit (FOM) of 106.9. The parameters (H, w₂, L₂, g₂), incidence angle (θ) and the polarization angle (φ) are discussed. The effects of different parameters on the Q-factor and AR were analyzed, which helped to select the optimal parameters. The design can also be used in communication and biosensing.