Air holes turn silicon wafers into flat ultraviolet focusing mirrors for compact optics

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.