Permittivity-Asymmetric Quasi -Bound States in the Continuum.

Breaking the in-plane geometric symmetry of dielectric metasurfaces allows us to access a set of electromagnetic states termed symmetry-protected quasi -bound states in the continuum ( q BICs). Here we demonstrate that q 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 q BICs (ε- q BICs) using a prototype Si/TiO 2 metasurface, in which the asymmetry in the unit cell is provided by the permittivity contrast of the materials. ε- q BICs are also numerically demonstrated in 1D gratings, where quality-factor enhancement and tailored interference phenomena of q BICs are shown via the interplay of geometrical and permittivity asymmetries.