Quasi-bound states in the continuum induced via surface roughness

Regular structural perturbations have been widely introduced into optical metasurfaces to break their symmetry and excite quasi-bound states in the continuum (BICs), which possess a high-quality factor. Here, we investigate how the irregular roughness of 1D metasurfaces influences the disruption of structural symmetry and impacts the excitation of quasi-BIC. First, random roughness is introduced on the sidewalls of a 1D grating constituting a metasurface. Then, the excited resonance of a rough metasurface is compared with the quasi-BIC of a metasurface possessing regular asymmetry with a flat sidewall to verify that the excited resonance is due to the quasi-BIC. Furthermore, the influence of degree of roughness is examined by analyzing the resonance wavelength and the excited field intensity. As the roughness increases, the resonance wavelength distribution broadens toward longer wavelengths, and the largest field intensity increases, although lower intensities dominate overall. Second, the distribution of resonance wavelengths and the range of maximum field intensities are analyzed with an increase in the number of gratings with independent rough profiles in a super-cell. As additional gratings are incorporated into the super-cell, the resonance wavelength distribution converges to the true BIC wavelength. At the same time, the range of maximum field intensity gradually decreases. Last, we investigate rough metasurfaces covered with a homogeneous high-refractive-index layer and confirm the excitation of two quasi-BICs having opposite parities.