Chiral nonlocal metasurfaces in gallium nitride with lithographically-defined vertical symmetry breaking

Recent theoretical works have converged on the understanding that deliberately structuring optically-thin metasurfaces along their vertical or out-of-plane direction enables improved control over the fundamental properties of circularly polarized optical resonances. However, such elaborate broken vertical symmetry is challenging to realize experimentally at visible and near-infrared wavelengths as it is rarely compatible with typical nanofabrication practices. Here, we introduce scalable nanofabrication processes that use only one layer of electron beam lithography to simultaneously define the lateral and vertical dimensions of the constituent nanostructures in a metasurface. We experimentally demonstrate gallium nitride metasurfaces with broken vertical symmetry that support chiral quasi-bound states in the continuum at near-visible wavelengths. Our results hold promise for structured and chiral emission from GaN metasurfaces with monolithically integrated quantum wells.