Permittivity-Asymmetric <i>Quasi</i>-Bound States in the Continuum
Rodrigo Berté, Thomas Weber, Leonardo de S. Menezes, Lucca Kühner, Andreas Aigner, Martin Barkey, Fedja J. Wendisch, Yuri S. Kivshar, Andreas Tittl, Stefan A. Maier
Abstract
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.