Air holes turn silicon wafers into flat ultraviolet focusing mirrors

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.

As reported in Advanced Photonics, 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.

Many conventional dielectric metasurfaces rely on tiny pillars made from materials with a high refractive index. Light becomes concentrated inside the pillars, and variations in their geometry allow the metasurface to control the emerging wavefront. Silicon works well for this purpose at visible and longer wavelengths, but becomes strongly absorbing in the ultraviolet. Light confined within a silicon resonator would therefore be lost rather than efficiently redirected.

Structures called Mie voids invert this concept. Instead of forming resonators from solid pillars surrounded by air, the researchers etched small air cavities into bulk silicon. Light entering a void reflects from the surrounding silicon boundaries, creating a resonant mode concentrated predominantly in the air cavity, where absorption is negligible. This reduces the optical field's overlap with the absorbing silicon.

Counterintuitively, silicon's strong absorption at ultraviolet wavelengths can contribute to this confinement. The complex optical properties that accompany absorption increase the reflectivity of the air-silicon interface, helping light remain confined within the void. The strategy therefore does not eliminate silicon's UV absorption. Instead, it uses the optical properties of the absorbing material while keeping much of the resonant field away from it.

To turn these resonators into a focusing element, the voids are shaped as ellipses approximately 470 nanometers long and 100 nanometers deep. When circularly polarized light illuminates an elliptical void, the reflected light acquires a so-called geometric phase equal to twice the void’s rotation angle. By varying this rotation across the half-millimeter-sized metasurface, a focusing profile is encoded directly into the silicon. Under a standard optical microscope, this arrangement appears as colored ring patterns, making the encoded focusing design directly visible.

When illuminated at 45 degrees incidence angle with 325 nanometer UV light, these metasurfaces act as flat equivalents of off-axis parabolic mirrors, redirecting the beam by 90 degrees while focusing it. Metasurfaces with focal lengths from 5 millimeters down to 1.25 millimeters were fabricated and produced circular focal spots as small as about one micrometer. Up to a numerical aperture of 0.15, the spots approached the diffraction limit, which is the fundamental resolution bound set by the wave nature of light. The versatility of possible wavefront shaping is further demonstrated by adding a spiral phase to the layout, generating so-called orbital angular momentum beams. They can be understood as light twisting its phase around its propagation axis and display doughnut-shaped intensity distributions in the beam's cross-section.

The core advantage of the concept is its simplicity. The metasurfaces are fabricated 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. Moreover, because the mechanism tolerates and even benefits from material absorption, it can be extended into the deep and vacuum UV spectral range and can be applied to a multitude of other material platforms. Furthermore, because many biomolecules have characteristic UV absorption signatures, such compact UV metasurfaces could eventually find applications in integrated sensing.

For details, see the original Gold Open Access article by S. Arslan et al., “UV Mie-void reflective metasurfaces in silicon," Adv. Photon. 9(1), 016002 (2026), doi: 10.1117/1.AP.9.1.016002