Silicon Air Holes Turn Wafers Into Flat Ultraviolet Mirrors

Bypassing silicon absorption with Mie voids

Conventional optical metasurfaces rely on tiny solid pillars made from materials with a high refractive index to shape wavefronts. When engineers use silicon for visible or infrared wavelengths, light concentrates inside these pillars without substantial loss. In the ultraviolet spectrum, however, silicon strongly absorbs light. Any optical energy confined inside a solid silicon pillar is absorbed as heat rather than redirected.

To solve this materials barrier, lead author Julian Schwab and the research team introduced an inverted optical design called Mie voids. Instead of raising solid silicon posts above a substrate, the researchers etched small hollow cavities directly into bulk silicon wafers. Light striking the structure reflects from the interior silicon walls, establishing a resonance that remains concentrated in the air pocket. Because air has negligible absorption, the optical field avoids the surrounding silicon. Counterintuitively, the strong absorption of silicon at ultraviolet wavelengths increases the reflectivity at the boundary, which helps lock the light within the open void. This optical innovation turns a known material limitation into a functional mechanism.

Encoding focal profiles through rotated cavities

The researchers shaped each cavity as an ellipse measuring approximately 470 nanometers long and 100 nanometers deep. When circularly polarized light illuminates an elliptical void, the reflected beam picks up a geometric phase shift equal to twice the rotation angle of the cavity.

By varying the orientation of these ellipses across a metasurface measuring half a millimeter wide, the team encoded a complete focusing profile into the silicon. Viewed under a standard optical microscope, this spatial arrangement appears as concentric, colored ring patterns that trace the distribution of the rotated cavities.

Focusing ultraviolet beams at 325 nanometers

During laboratory testing, the team illuminated the flat metasurface with ultraviolet light at a wavelength of 325 nanometers at an incidence angle of 45 degrees. The device redirected the incoming beam by 90 degrees while focusing it, behaving as the flat equivalent of an off-axis parabolic mirror.

The team fabricated devices with focal lengths ranging from 5 millimeters down to 1.25 millimeters. These metasurfaces generated focal spots as small as approximately 1 micrometer in diameter. At numerical apertures up to 0.15, the measured focal spots closely approached the diffraction limit set by optical wave physics. By introducing a spiral phase delay across the array, the team also produced orbital angular momentum beams, which feature a characteristic doughnut-shaped intensity profile.

Single-step etching for integrated sensors

A practical advantage of the design lies in its manufacturing simplicity. The metasurfaces require only a single lithography and etching step on standard silicon substrates, making the process compatible with existing semiconductor foundry lines. The underlying bulk silicon provides mechanical stability and conducts heat away from the optical zone.

Although the published study does not report a filed patent application, the underlying mechanism operates across wavelengths where conventional transparent materials degrade. The authors note that the architecture can scale into the deep ultraviolet and vacuum ultraviolet ranges. Because many organic molecules and biomolecules exhibit distinctive absorption signatures in ultraviolet light, these flat silicon reflectors offer a route toward compact, chip-scale spectroscopy and environmental sensing devices.

Read also on The Patent: A microscopic Archimedes screw spins between gold tips and boosts conductivity