Traditional lenses and mirrors focus light using curved surfaces. Metasurfaces can achieve similar functions with a flat layer of nanoscale structures that locally control the phase of light. Extending these flat optics into the ultraviolet could support compact systems for high-resolution imaging, spectroscopy, and sensing, but it introduces a fundamental materials problem: many of the materials used to make metasurfaces strongly absorb UV light.

Researchers at the University of Stuttgart have developed an alternative approach that turns this limitation on its head. Rather than trying to make an absorbing material transparent, their metasurface confines much of the light inside tiny air-filled cavities etched directly into silicon.

The device focuses reflected ultraviolet light at a wavelength of 325 nm and demonstrates a new approach to controlling light in wavelength ranges where suitable transparent materials can be difficult to find.