Fig. 1. Schematic and parameters of THz LiNbO3 metasurface. (a) Structure schematic. (b) Unit cell with geometric parameters including lattice constants Pz=60  μm and Py=45  μm, cylinder radius r=11  μm, and cylinder height h=15  μm. (c) Top view (y-z plane) of the unit cell with d=2  μm.

Fig. 2. Sample fabrication process and robust assessment of LiNbO3 metasurface. (a) Process steps for CCP-RIE. (b) Process steps for ICP-RIE. (c) Q factors and FOMs for conical dimer metasurface with varying slope angle of the sidewall θ.

Fig. 3. Transmission spectra and bandgap structure analysis of the LiNbO3 metasurface. (a) d=0  μm and (b) d=2  μm. (c) Theoretical spectra using the Fano formula with the geometric parameter d=2  μm (shown as red dashed line). (d) Dispersion curves for modes supported by the LiNbO3 metasurface.

Fig. 4. Multipole decomposition and field distributions of LiNbO3 metasurface. (a) Scattering powers of electric dipole (ED), magnetic dipole (MD), toroidal dipole (TD), electric quadrupole (EQ), and magnetic quadrupole (MQ) with larger versions of MQ shown in the inset. (b) Distribution of electric field in the unit cell at resonance in the y-z plane. (c) Distribution of magnetic field in the unit cell at resonance in the y-z plane. The arrows, colored to indicate the magnitude of electromagnetic field intensity, demonstrate their respective directions.

Fig. 5. Transmission spectra and Q factors of the proposed metasurface structure. (a) Transmission spectra with varying geometric parameters d while other parameters remain unchanged as shown in Fig. 1. (b) Log–log plot of Q factors for MQ resonances as a function of the absolute value of asymmetric parameter α. Red dots represent calculated data points. (c) Transmission spectra with varying external voltage V (d=0  μm). (d) Log–log plot of Q factors for quasi-BIC resonances as a function of the absolute value of asymmetric parameter β. Blue dots represent calculated data points.

Fig. 6. Transmission spectra for LiNbO3 asymmetric metasurface structure with varying structural parameters (d=2  μm). (a) Period along the z-axis, Pz; (b) period along the y-axis, Py; (c) radius r; (d) height h.

Fig. 7. Transmission spectra and Q factors for LiNbO3 asymmetric metasurface structure with varying external voltage. (a) Transmission spectra with varying external voltage V while other parameters remain unchanged as shown in Fig. 1. (b) Q factors and refractive index ne of LiNbO3 for quasi-BIC resonances with varying external voltage V.

Fig. 8. Frequency shifts and FOMs of MQ resonances on the LiNbO3 metasurface. (a) Frequency shift of MQ resonance dips versus analyte thickness (n=1.2) with red dots connected by smooth red curves. (b) Frequency shifts of MQ resonance dips versus analyte refractive index with 40 μm analyte on the LiNbO3 metasurface, with dashed lines representing linear fittings. (c) FOMs of MQ resonance dips (red dots) versus analyte refractive index with 40 μm analyte on the LiNbO3 metasurface.