Nanogap experiments for laser cooling - A progress report

One of the challenges of laser cooling a semiconductor is the typically high index of refraction (greater than 3), which limits efficient light output of the upconverted photon. This challenge is proposed to be met with a novel concept of coupling the photon out via a thin, thermally insulating vacuum gap that allows light to pass efficiently by frustrated total internal reflection. This study has the goal of producing a test structure that allows investigation of heat transport across a "nanogap" consisting of a thin film supported over a substrate by an array of nanometer-sized posts. The nanogap is fabricated monolithically by first creating a film of SiO₂ on a silicon substrate, lithographically defining holes in the SiO₂, and covering this structure including the holes with silicon. Selective lateral etching will then remove the SiO₂, leaving behind a thin gap between two Si layers spaced apart by nanometer-scale Si posts. Demonstration of this final step by successfully undercutting the a-Si upper layer due to the hydrophobic nature of silicon and the slow etch rate of buffered oxide etch in the small gap has proved to be problematic. Arriving at a feasible solution to this conundrum is the current objective of this project in order to begin investigating the thermal conductivity properties of the structure.