Researchers at the University of Stuttgart have turned silicon’s ultraviolet absorption into an asset, etching microscopic air holes into standard silicon wafers to build flat, reflective focusing mirrors that target a wavelength of 325 nanometers.
Many standard dielectric metasurfaces use solid pillars with high refractive indexes. These solid pillars concentrate light internally and cause UV beams to be lost to strong absorption. A team at the University of Stuttgart inverted this concept by etching tiny air cavities directly into bulk silicon, as described in Advanced Photonics.
Etched Mie Voids Trap Ultraviolet Light
When an ultraviolet beam enters one of these cavities, it bounces off the surrounding silicon boundaries. This creates a resonant mode concentrated predominantly inside the air pocket where absorption remains negligible.
Counterintuitively, the material’s strong absorption at ultraviolet wavelengths actually increases the reflectivity of the air-silicon interface. This helps the light stay trapped within the void rather than escaping into the substrate.
When an ultraviolet beam enters one of these cavities, it bounces off the surrounding silicon boundaries, creating a resonant mode concentrated predominantly inside the air pocket where absorption remains negligible. Counterintuitively, the material’s strong absorption at ultraviolet wavelengths actually increases the reflectivity of the air-silicon interface, helping the light stay trapped within the void rather than escaping into the substrate. The method keeps the optical field away from the absorbing material while still utilizing its optical properties.
To convert these resonators into functional focusing elements, researchers shaped the air cavities as ellipses measuring approximately 470 nanometers long and 100 nanometers deep. When circularly polarized light strikes an elliptical void, the reflected beam acquires a geometric phase equal to twice the rotation angle of the cavity. Adjusting this rotation across a half-millimeter-sized metasurface encodes a specific focusing profile directly onto the silicon.
Focusing Beams at 325 Nanometers
Tested under an incidence angle of 45 degrees using 325 nanometer UV light, the flat metasurfaces act as equivalents of off-axis parabolic mirrors, simultaneously redirecting a beam by 90 degrees and focusing it. The team fabricated components with focal lengths ranging from 5 millimeters down to 1.25 millimeters, which produced circular focal spots as small as about 1 micrometer. Up to a numerical aperture of 0.15, these spots approached the diffraction limit set by the wave nature of light. Adding a spiral phase to the layout further allowed the generation of orbital angular momentum beams that display doughnut-shaped intensity distributions.
The manufacturing process requires a single lithography and etching step on standard silicon substrates, ensuring compatibility with established semiconductor processing techniques. Because the bulk substrate remains mechanically sound, it dissipates heat effectively while offering potential for integrated sensing applications tied to the characteristic UV absorption signatures of biomolecules.