Tunable Visibly Transparent Optics Derived from Porous Silicon

Visibly transparent porous silicon dioxide (PSiO₂) and PSiO₂/titanium dioxide (TiO₂) optical elements were fabricated by thermal oxidation, or a combination of thermal oxidation and atomic layer deposition infilling, of an electrochemically etched porous silicon (PSi) structure containing an electrochemically defined porosity profile. The thermally oxidized PSiO₂ structures are transparent at visible wavelengths and can be designed to have refractive indices ranging from 1.1 to 1.4. The refractive index can be increased above 2.0 through TiO₂ infilling of the pores. Applying this oxidation and TiO₂ infilling methodology enabled tuning of a distributed Bragg reflector (DBR) formed from PSi across the visible spectrum. At the maximum filling, the DBR exhibited a transmission of 2% at 620 nm. Simulations match well with measured spectra. In addition to forming DBR filters, phase-shaping gradient refractive index (GRIN) elements were formed. As a demonstration, a 4 mm diameter radial GRIN PSiO₂ element with a parabolic, lens-like phase profile with a calculated focal length of 1.48 m was formed. The calculated focal length was reduced to 0.80 m upon the addition of TiO₂. All the structures showed broad transparency in the visible and were stable to the materials conversion process.