Optically Anisotropic Porous Silicon Microlenses with Tunable Refractive Index and Birefringence Profiles

The effect of spatially varying birefringence on the focusing behavior of porous silicon (PSi) and porous silicon dioxide (PSiO2) gradient refractive index (GRIN) lenses is investigated. Both materials attain broad, tunable refractive indexes and birefringence profiles, with PSi having a maximum birefringence of similar to 0.26 and PSiO2 a reduced maximum birefringence of similar to 0.03 at 633 nm. These GRIN lenses exhibit polarization-dependent split focusing behavior, wherein the divergence angle between the twin foci increases with the birefringence gradient. PSi's large birefringence allows the divergence angle to be tuned such that light focuses away from the center of the lens. These GRIN elements demonstrate how tunable birefringent materials can be used to engineer polarization-selective optical responses. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement