Fig. 1. Schematic of LN nano-grating metasurfaces fabricated on an LNOI wafer. The grating ridges are oriented parallelly to the optical axis of the LN [y(e)]. Geometries of the metasurfaces are defined by period P_d, ridge width w, and height h.

Fig. 2. Illustration of fabrication process. (a) Flow chart: LNOI substrate was cleaned by organic solvent, and a 40-nm-thick Cr layer was deposited on top of the LNOI by electron beam evaporation. Both the Cr and LN layers were milled by FIB followed by an ICP-RIE process. The fabricated structures were treated afterwards by SC-1 solution to remove LiF. Finally, the Cr mask was removed by wet etching. (b) Representative scanning electron microscopy (SEM) images of the fabricated metasurface with P_d = 400 nm. The cross sections are shown on the right, in which Pt is used as a protection layer for cross-section cutting.

Fig. 3. Spectra of LN nano-grating metasurfaces under y-pol incidence. (a) Simulated angle-resolved transmission (T) spectra of LN metasurface with P_d = 400 nm, where the point associated with the SP-BIC mode is highlighted by a white circle. (b) Evolution of the experimental (red solid line) and simulated (blue dotted line) transmission spectra versus wavelength for incident angles of 0°, 5°, 10°, 15°, and 20°, where the right panels present the E_y component of the corresponding near-field distributions of the SP-BIC mode at 0° and the QBIC modes under other non-zero incident angles. (c) The peak wavelength positions and transmission values corresponding to the observed resonance dips excited by the y-pol light as a function of the incident angles, while the numbers given within parentheses are the Q-factor values obtained at the corresponding angle.