A MEMS Mirror Laser-Guidance System for CDMS Experiments

The Cryogenic Dark Matter Search (CDMS) is an experiment to directly detect dark matter, a mysterious substance that makes up the majority of the matter content in the Universe and has so far only been observed due to its gravitational effects. CDMS uses high-purity Ge (HPGe) crystals as detectors for dark matter in the form of weakly interacting massive particles (WIMP). In order to reduce systematic uncertainties from measurements made by the detectors, Monte Carlo simulations are performed that require precise knowledge of semiconductor physics at low temperatures. The purpose of the investigation described in this paper is to study the anisotropic propagation of electrons through Ge and their inter-valley scattering. An apparatus for such experiments was made using a micro-electromechanical system (MEMS) device that controls the 2-dimensional tilt of a mirror via electrostatic attractions, thereby scanning a pulsed laser beam across the HPGe crystals. Measurements of the laser and MEMS mirror setup were made using a checkerboard-like shadow mask in front of a photodetector in the HPGe crystal’s future housing at room temperature and cryogenic temperature. These data demonstrate that the mirror has a very precise and reproducible response that is exceptionally temperature-independent, and that it has a mechanical quality factor close to 106, which is indicative of extremely low mechanical losses. At the time of writing, the inter-valley scattering measurement has not yet been performed, but it will be in the near future, which emphasizes how critical it is to further investigate and perfect this cryogenic laser-scanning system to aid in solving the mystery of dark matter.