Polarization-independent hollow nanocuboid metasurfaces with robust quasi-bound states in the continuum

All-dielectric metasurfaces supporting resonant quasi-bound states in the continuum (qBIC) offer an ideal platform for various applications relying on intense light–matter interaction in highly localized and enhanced fields. Here, we propose a dielectric metasurface composed of hollow GaP nanocuboid quadrumers periodically arranged on a silica substrate. The metasurface supports a qBIC resonant mode with an antiferroelectric field configuration, which is very robust to large perturbations of the cuboid structure thanks to its perpendicular electric dipole field profile. The perturbed quadrumer arrangement retains C4v symmetry, thus allowing for polarization-independent optical response for normally incident planewaves. In addition, the resonant mode dispersion is investigated, revealing interesting features, such as low birefringence along the ΓM contour of the Brillouin zone and very low dispersion for the TM polarization along the ΓX direction. The metasurface is designed to resonate close to 785nm, which is highly relevant for Raman spectroscopy, leveraging the strong field enhancement at the interface with the overlayer material. Moreover, its performance as a bulk refractometric sensor is discussed. The proposed dielectric metasurface is promising for emerging photonic phenomena where strong light–matter interaction plays a key role.