Characterizing and improving the phonon collection efficiency of KID-based phonon-mediated (KIPM) dark matter detectors

Probing dark matter candidates with sub-GeV masses requires quantum sensors with sub-eV energy resolutions (σ_E). The kinetic-inductance-detector-based (KID-based) phonon-mediated (KIPM) detectors have the potential to deliver sub-eV energy resolutions. They also offer possible advantages in readout and sensitivity over the current leading phonon-sensing technology, which uses quasiparticle-trap-assisted transition-edge sensors (TESs). Three prototype KIPM devices have been fabricated and studied. The current detector energy resolution achieved is 318 eV, with the absorbed energy resolution (σ_E^abs) at 2.1 eV. The resolution is limited by a sub-percent phonon collection efficiency (η_ph), while TES-based detectors have shown η_ph ~ 30%. Understanding the phonon losses in the KIPM detectors and increasing the phonon collection efficiency are critical to improving their energy resolution. This contribution presents an empirical model characterizing the phonon losses to different components in the KIPM detectors and a road map to enhance the KIPM phonon collection efficiency and improve the energy resolution down to sub-eV.