Permittivity-Asymmetric <i>Quasi</i>-Bound States in the Continuum

Abstract

Breaking the in-plane geometric symmetry of dielectric metasurfaces allows us to access a set of electromagnetic states termed symmetry-protected <i>quasi</i>-bound states in the continuum (<i>q</i>BICs). Here we demonstrate that <i>q</i>BICs can also be accessed by a symmetry breaking in the permittivity of the comprising materials. While the physical size of atoms imposes a limit on the lowest achievable geometrical asymmetry, weak permittivity modulations due to carrier doping, and electro-optical Pockels and Kerr effects, usually considered insignificant, open the possibility of infinitesimal permittivity asymmetries for on-demand, dynamically tunable resonances of extremely high quality factors. As a proof-of-principle, we probe the excitation of permittivity-asymmetric <i>q</i>BICs (ε-<i>q</i>BICs) using a prototype Si/TiO₂ metasurface, in which the asymmetry in the unit cell is provided by the permittivity contrast of the materials. ε-<i>q</i>BICs are also numerically demonstrated in 1D gratings, where quality-factor enhancement and tailored interference phenomena of <i>q</i>BICs are shown via the interplay of geometrical and permittivity asymmetries.