Permittivity-Asymmetric Quasi-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 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.

Active control of surface plasmon polaritons with phase change materials

“…The proposed structure can be fabricated by using the two-step lithography method [37] as two different dielectric layers exist simultaneously. First the gold film is to be evaporation-deposited on the SiO 2 substrate.…”

Enhancing the Goos–Hänchen shift based on quasi-bound states in the continuum through material asymmetric dielectric compound gratings

“…Two common methods to break the plane symmetry of a grating or metasurface are to change the geometry of the grating or metasurface unit [17][18][19] and to change the refractive index of the grating or metasurface unit material. [20][21][22][23] Most of the current studies are based on geometric asymmetry to achieve symmetry-protected QBIC resonance, but this approach suffers from the problem of imposing fundamental limitations on achieving the lowest geometric asymmetry, especially in the near-infrared and visible spectral bands. [23] The Q factor is directly proportional to the negative quadratic of the asymmetry of the structure.…”

“…There are few studies based on material asymmetry to achieve QBIC resonance and even fewer studies based on material asymmetry to achieve GH shift enhancement. Although Berte, [20] Liu, [21] Yu, [22] Chen [23] and some others realized symmetry-protected QBIC resonance based on material asymmetry, none of them applied QBIC in GH shift enhancement.…”

High-quality-factor space–time metasurface for free-space power isolation at near-infrared regime

“…This is because the index contrast between the neighboring elements of the phase gradient metasurfaces leads to the emergence of epsilon-QBIC mode in systems with a broken symmetry in their permittivity. 47 An alternative promising technique to boost the Q-factor of a resonator is engineering its dimension to support higher-order Mie resonances. 48 Similar to their dipolar counterparts, the higher-order Mie modes leverage the confined field distribution that enables local wavefront manipulation while featuring a sufficiently high Q-factor that is obtained thanks to the reduction of radiative losses.…”