Berté, RodrigoRodrigoBertéWeber, ThomasThomasWeberMenezes, Leonardo de SouzaLeonardo de SouzaMenezesKühner, LuccaLuccaKühnerAigner, AndreasAndreasAignerBarkey, MartinMartinBarkeyWendisch, Fedja JanFedja JanWendischKivshar, Yuri S.Yuri S.KivsharTittl, AndreasAndreasTittlMaier, Stefan A.Stefan A.Maier2026-03-192026-03-192023-03-22Nano Letters 23 (7): 2651-2658 (2023)https://hdl.handle.net/11420/62228Breaking 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 (qBICs). Here we demonstrate that qBICs 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 qBICs (ϵ-qBICs) using a prototype Si/TiO₂> metasurface, in which the asymmetry in the unit cell is provided by the permittivity contrast of the materials. ϵ-qBICs are also numerically demonstrated in 1D gratings, where quality-factor enhancement and tailored interference phenomena of qBICs are shown via the interplay of geometrical and permittivity asymmetries.en1530-69922023726512658ACS Publ.emergencemetasurfacepermittivityquasi-BICsymmetry breakingTechnology::600: TechnologyJournal Article10.1021/acs.nanolett.2c05021Journal Article