Air voids turn silicon into flat mirrors that focus 325 nm ultraviolet light

Flat optics for a hostile wavelength

A team at the University of Stuttgart has built flat optical elements out of ordinary silicon that reflect and focus ultraviolet light at a wavelength of 325 nanometers, according to Phys.org. The work, published in the journal Advanced Photonics, tackles a longstanding obstacle in UV optics: silicon, like many materials used for nanoscale optics, absorbs UV light strongly. Rather than fighting that absorption, the researchers designed structures that keep most of the light inside air.

A materials problem turned inside out

Metasurfaces are flat layers of nanoscale structures that control the phase of light locally, doing the job of curved lenses and mirrors in a much thinner package. Conventional dielectric metasurfaces rely on high-refractive-index pillars that concentrate light inside themselves; variations in pillar geometry shape the outgoing wavefront. Silicon is an excellent pillar material at visible wavelengths, but in the UV it becomes strongly absorbing, so light confined to a silicon resonator would be lost rather than redirected.

The Stuttgart group inverted this design. As Phys.org explains, they etched small air cavities — so-called Mie voids — into bulk silicon. Light entering a void bounces off the surrounding silicon walls and forms a resonant mode that lives mostly inside the air cavity, where absorption is negligible and the optical field barely overlaps the absorbing material.

Counterintuitively, silicon's UV absorption actually helps. The complex optical properties that accompany absorption raise the reflectivity of the air-silicon boundary, which helps keep the light confined. The approach does not make silicon transparent; it uses the optical properties of the absorbing material while parking the resonant field largely outside it.

Encoding a focus through rotation

To turn the voids into a focusing element, the researchers shaped them as ellipses roughly 470 nanometers long and 100 nanometers deep. When circularly polarized light strikes an elliptical void, the reflected light picks up a geometric phase equal to twice the void's rotation angle. Rotating the ellipses by different amounts across a half-millimeter-sized surface therefore writes a focusing profile directly into the chip — a pattern visible under a standard optical microscope as colored rings.

Illuminated at an incidence angle of 45 degrees with 325-nanometer light, the devices work as flat equivalents of off-axis parabolic mirrors, redirecting the beam by 90 degrees while focusing it. The team fabricated focal lengths from 5 millimeters down to 1.25 millimeters and produced focal spots as small as about 1 micrometer; up to a numerical aperture of 0.15, the spots approached the diffraction limit, the fundamental resolution bound set by the wave nature of light. Adding a spiral phase to the layout also generated orbital angular momentum beams, whose cross sections show the characteristic doughnut-shaped intensity profile.

One etch step on standard silicon

The core appeal of the concept is its simplicity. According to Phys.org, the metasurfaces are made in a single lithography and etching step on standard silicon substrates, compatible with established semiconductor processing. The bulk substrate remains mechanically robust and dissipates heat effectively.

Because the mechanism tolerates — and even benefits from — material absorption, the authors suggest it can be extended into the deep and vacuum UV spectral ranges and applied to other material platforms. They also note that many biomolecules have characteristic UV absorption signatures, so compact UV metasurfaces of this kind could eventually find use in integrated sensing.

Why it matters

Compact ultraviolet optics are in demand for high-resolution imaging, spectroscopy and sensing, but the toolbox of transparent UV materials is small and often exotic. This result shows that the semiconductor industry's default material, processed with standard fab steps, can focus UV light as long as the design keeps the light in air rather than in the silicon. That opens a practical route to flatter, cheaper UV optical systems — and, because absorption is an asset rather than a liability here, a plausible path to even shorter wavelengths where almost no conventional material is transparent.