Researchers at the University of Stuttgart developed flat ultraviolet focusing mirrors by etching air-filled cavities into silicon wafers. The approach solves a materials problem in ultraviolet optics: silicon absorbs UV light strongly, making traditional metasurface designs inefficient at these wavelengths.
Instead of trying to make silicon transparent, the team confined light inside tiny air cavities called Mie voids. Light reflects from the surrounding silicon boundaries and concentrates predominantly in the air, where absorption is negligible. Silicon's strong UV absorption actually helps by increasing reflectivity at the air-silicon interface, keeping light trapped in the void.
The metasurfaces use elliptical voids approximately 470 nanometers long and 100 nanometers deep. By rotating these ellipses across the half-millimeter-sized surface, researchers encoded a focusing profile using geometric phase. When hit by circularly polarized UV light at 325 nanometers, the metasurfaces redirected beams by 90 degrees while focusing them. Focal spots reached about 1 micrometer at focal lengths ranging from 5 millimeters down to 1.25 millimeters. Up to a numerical aperture of 0.15, the spots approached the diffraction limit.
The researchers also generated orbital angular momentum beams by adding spiral phase patterns, creating doughnut-shaped intensity distributions. The metasurfaces fabricate in a single lithography and etching step on standard silicon substrates. Julian Schwab led the work, published in Advanced Photonics in 2026. The approach may extend into deep and vacuum UV ranges and could eventually support compact sensing applications, since many biomolecules have characteristic UV absorption signatures.