Fig. 2: Sample layout and linear optical characterization.

a Schematic cross-section of a single unit cell of the metasurface. All the samples have the same periodicity p = 800 nm and LiNbO₃ height before (h_LN = 600 nm) and after patterning (h’_LN = 450 nm) with a substrate thickness, h_sub = 2 μm. The angle θ = 65° accounts for the sidewall slopes introduced by the physical etching. The variable parameters from sample to sample are the filling factor \(\left.{FF}=1-({D}_{1}\,+{D}_{2})/p\right)\) and the asymmetry parameter \(\alpha=({D}_{2}-{D}_{1})/({D}_{2}+{D}_{1})\). Red and blue arrows indicate the polarization of the incident wave that excites the TM (parallel to the LN ordinary axis) and TE (parallel to the LN extraordinary axis) mode, respectively. b COMSOL simulation of the optical field z-component of the optical field E_opt, excited at the quasi-BIC resonance for FF = 0.3 and α = 0. Incident optical field E_opt is TE-polarized and propagates with k || (–x). c COMSOL plane-wave simulation of the metasurface reflectance depending on the asymmetry parameter \(\alpha\), with fixed FF = 0.3. d Dark field and bright-field images in real colours of the LiNbO₃ chip. The LiNbO₃ metasurfaces are located in the areas with strong scattering (bright areas in dark field). The white scale bar on the bottom right is equal to 100 μm. The orange lines in the bright field image identify the edges of the interdigitated electrodes. e Scanning-electron micrograph showing the metasurface obtained by patterning the LiNbO₃ thin film. The scale bar on the bottom right is equal to 500 nm. The white, dashed line indicates the LiNbO₃ extraordinary axis. f Normalized reflection spectra of the sample with FF = 0.3 and α = 0.19. The solid lines show experimental data obtained by sweeping the laser wavelength in steps of 10 p.m., with electric field exciting the TE (blue solid line) and TM (red solid line) mode. The dashed lines are the corresponding numerical simulations. g, h Zoom on spectra shown in panel f (thin lines) fitted with Fano profiles (thick lines) for TM and TE polarization, respectively.