A strongly resonant silicon square-slot metasurface hosting bound states in the continuum for lasing and nonlinear applications: theoretical studies and experimental verification

We theoretically and experimentally study a dielectric metasurface supporting a variety of bound states in the continuum (BICs), including the intricate dark toroidal dipole and quadrupole modes, in the wavelength range from 1300-2000 nm. The metasurface consists of square slots etched in a thin silicon layer and periodically arrayed in a subdiffractive lattice residing on a sapphire substrate. The symmetry of the metasurface is appropriately reduced enabling the coupling of the symmetry-protected modes to the radiation fields. A set of appropriate theoretical tools, including eigenfrequency simulations with an augmented-fields formulation and multipolar expansion analysis, are employed to comprehensively assess the modal properties of the supported BICs, such as their quality factor and far-field characteristics. The results are corroborated by experimental studies verifying the existence of the quasi-BICs. The investigated metasurface constitutes an promising platform for nonlinear applications when leveraging the Kerr effect of silicon, as well as for lasing when the slot is filled by a low-index material hosting organic dye molecules.