Optical bound states in slotted high-contrast gratings

This paper investigates the optical bound states in the continuum (BIC) supported by a slotted high-contrast grating (sHCG) structure. The sHCG structure consists of a periodic array of silicon ridges with a slot in each ridge. Given that the BICs are perfectly confined, their spectral locations are identified using a finite-element method formulated from a generalized eigenvalue problem. The real eigenvalues represent the wavelengths of BIC modes and the associated eigenvectors correspond to the electric field distributions. In the spectral and angular vicinity of the BICs, we studied the leaky waveguide modes using the rigorous coupled-wave analysis. The combination of the full-wave eigenvalue solver and the coupled-wave analysis provides an ideal setting to investigate the optical BICs of periodic structures for various applications. The simulation results show, for example, that the sHCG structures can support symmetry-protected bound states with a zero in-plane wave vector as well as high-quality-factor (high-Q) resonances for both TE and TM polarizations. By adjusting the slot, we can turn the BIC mode into high-Q modes and determine the linewidth of the mode by the degree of asymmetry.