Researchers at the University of Stuttgart have engineered a flat silicon metasurface capable of focusing ultraviolet (UV) light into circular spots as small as 1 micrometer. This development addresses a significant hurdle in optical engineering: while silicon is an ideal material for many electronics and visible-light optics, it typically absorbs UV radiation rather than reflecting or transmitting it.
Overcoming Silicon's UV Absorption
Traditional dielectric metasurfaces rely on high-refractive-index materials, such as silicon, to control light. However, at ultraviolet wavelengths, silicon becomes strongly absorbing, causing light to be lost as heat rather than being redirected. This makes it difficult to create efficient flat optics for UV applications like high-resolution imaging or spectroscopy.
The Stuttgart team inverted this problem by creating 'Mie voids.' Instead of using solid pillars to concentrate light, they etched small air cavities into bulk silicon. Light entering these voids reflects off the surrounding silicon boundaries, creating a resonant mode concentrated primarily in the air cavity where absorption is minimal.
This image illustrates the physical structure of the silicon metasurface and the 'Mie voids' described in the research.
“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.”
— Air holes turn silicon wafers into flat ultraviolet focusing mirrors for compact optics
Encoding Focus via Geometric Phase
To transform these resonators into a focusing element, the researchers shaped the voids into ellipses approximately 470 nanometers long and 100 nanometers deep. When circularly polarized light hits these elliptical voids, the reflected light acquires a 'geometric phase' based on the void's rotation angle.
By varying the rotation of these ellipses across a half-millimeter-sized surface, the researchers encoded a focusing profile directly into the silicon. Under a standard microscope, this design appears as a series of colored ring patterns, effectively acting as a flat equivalent to an off-axis parabolic mirror.
Performance and Scalability
When tested with 325 nm UV light at a 45-degree incidence angle, the metasurfaces redirected the beam by 90 degrees while focusing it. The team successfully fabricated surfaces with focal lengths ranging from 1.25 mm to 5 mm, producing spots that approached the diffraction limit at a numerical aperture of 0.15.
Because the mechanism actually benefits from the optical properties of absorbing materials, the design can be extended into the deep and vacuum UV ranges. Furthermore, the process is compatible with established semiconductor lithography and etching, making it a viable path for integrated sensing and compact optical systems.