UV Mie-void reflective metasurfaces in silicon

Reflective and folded optical architectures are attracting growing interest for compact photonic systems, whereas ultraviolet (UV) nanophotonics offer unique opportunities enabled by higher photon energies and the intrinsically reduced diffraction limit at shorter wavelengths. However, extending resonant metasurfaces into the UV regime remains fundamentally challenging due to material absorption in conventional dielectric platforms. Here, we realize reflective ultraviolet metasurfaces in silicon based on elliptical Mie-void resonators, which confine light predominantly inside air inclusions within the absorbing, high-refractive-index medium. Using a wavelength of 325 nm, the metasurfaces enable reflective off-axis focusing with numerical apertures up to 0.2 under 45 deg illumination, achieving diffraction-limited focal spots up to a numerical aperture of 0.15. In addition, we demonstrate this phase freedom by superimposing an azimuthal phase term up to l=3 onto the focusing profile as a standard route toward orbital angular momentum generation. Our results establish Mie voids as a promising platform for ultraviolet wavefront shaping and reflective flat optics in absorbing material systems, fully compatible with industry-standard silicon fabrication.