Measurements and simulations of athermal phonon transmission from silicon absorbers to aluminium sensors

Understanding the mechanisms of phonon reflection and transmission at interfaces is fundamental to improving the sensitivity of cryogenic particle detectors, but remains an elusive goal. Even the preferred reflection mode (specular or diffuse) is unknown, and measuring transmission coefficients between materials is technically difficult. This study uses aluminum kinetic-inductance detectors (KIDs) deposited on silicon substrates to measure the athermal phonon flux, plus a Monte Carlo phonon simulation to obtain the Si-Al and Si-Teflon transmission coefficients, confirming the specular reflection mode. This could be a big step toward enabling direct detection of dark matter, for example.