Fig. 1. Schematic diagram for the evolution from bulk to monolayer q-BIC metasurfaces, where the symbols p, t, w, d1, d2, d3, and θ denote the period, thickness of waveguide layer, width and heights of various asymmetrical rectangular pairs, and the incident angle, respectively.

Fig. 2. (a) Transmittance of Si metasurface with in-plane asymmetric q-BIC as a function of α, where d1=100  nm. (b) Transmittance of Si metasurface with in-plane asymmetric q-BIC as a function of d2, where α=0.9. (c) Transmittance of metasurface as a function of MoS2 thickness d3, where α=0.9. (d) Electric field distributions of metasurfaces for different MoS2 thicknesses at the resonance wavelengths of q-BIC1. For all the figures, p=560  nm.

Fig. 3. (a), (b) Transmittance spectra of monolayer MoS2 metasurface as α changes from 0 to 0.9 for (a) q-BIC1 and (b) q-BIC2. (c), (d) Q-factor and FWHM as functions of α for (c) q-BIC1 and (d) q-BIC2.

Fig. 4. (a) Resonance behavior of q-BIC1 and q-BIC2 as incident angle θ changes from 4° to 17°. (b) Q-factors of q-BIC1 and q-BIC2 with the change of θ. (c) Transmittance spectra and the corresponding electric field distributions for four incident angles of 6°, 10.4°, 10.5°, and 15°, respectively.

Fig. 5. SHG response and enhancement factor for monolayer MoS2 metasurface and sheet Si (10 nm thick) metasurface with monolayer MoS2. Inset (I) denotes the average electric field enhancement factors for various modes. Inset (II) represents the power dependence of SHG in logarithmic scale. Here α is fixed at 0.9.

Fig. 6. Fabrication process flow of asymmetry pair of monolayer MoS2 metasurface.

Fig. 7. Real and imaginary parts of the refractive index for (a) Si, (b) bulk MoS2, and (c) monolayer MoS2.

Fig. 8. Dependence of the Q-factor for the two q-BIC modes on the asymmetry parameter α and the asymmetry pair thickness d2. (a), (c) q-BIC1. (b), (d) q-BIC2.