Fig. 1: Bound states in the continuum in bulk WS2 metasurfaces.
a, Rod-type symmetry-protected BIC unit cell showing the working principle of opposing electric dipoles (denoted by p1 and p2). The geometrical unit cell parameters are fixed as: periodicity P0 = 340 nm, base rod length L0 = 266 nm and rod width W0 = 90 nm. The centre points of the rods are placed at the unit cell coordinates (x, y) = (0, P/4) and (0, 3P/4). Tunability of the resonance position is realized by introducing a multiplicative scaling factor S, which scales the in-plane geometrical parameters according to \(P=SP_0\), \(L=S{L}_{0}\), \(\Delta L=S\Delta {L}_{0}\) and \(W=S{W}_{0}\). h, height. b, Tauc–Lorentz material of WS2 showing the real part n and imaginary part k of the in-plane complex refractive index with and without the exciton. c, Simulated transmittance spectra of BIC resonances on the low-energy side of the exciton tuned via scaling of the in-plane geometric parameters. The markers indicate the modulation of the transmittance signal. d, Simulated transmittance spectra of BICs for different asymmetries ΔL0. Smaller asymmetries lead to a spectral redshift and a reduction of the linewidth. For symmetric structures the resonance vanishes as the quasi-BIC turns into a true BIC. e,f, Electric field enhancements and quality factors for different asymmetry parameters ΔL0. The maximum field enhancement is achieved when both intrinsic and radiative damping rates of the BIC mode are matched (e). The radiative quality factor follows the expected inverse square dependence of a BIC. The intrinsic Q factor shows a slight increase for lower asymmetries due to a slightly larger extinction value of the blue-shifted BIC (f). The total Q factor follows the radiative Q factor for large asymmetries and is dominated by the intrinsic Q factor for small asymmetries.
