Position- and Temperature-Dependent Responsivity in a KID-Based Phonon-Mediated Detector

We report on the position- and temperature-dependent response of a kinetic inductance phonon-mediated (KIPM) detector. To study position-dependence, we used a controllable mirror attached to a micro-electromechanical system (MEMS) to steer optical light with a 100 $μ$m spot size over the backside of the cryogenic detector. With both pulsed and continuous operation of the LED, we observe features of the detector housing and find that the signal is strongest when the LED is pointed directly at the KID. The "off-KID" response is measured to be 34% of the "on-KID" response and is largely independent of direction. We model phonon propagation and collection in the KIPM detector with G4CMP and assert that the isotropic angular dependence is unlikely to be due to discrete phonon absorption "sinks", such as the microwave feedline or the device mounting points, but can be explained instead by an isotropic energy loss in the crystal. We furthermore utilize the temperature flexibility of the MEMS and KIPM detector system to investigate temperature-dependent responsivity, both at the sensor level (i.e., frequency shifts in the KID) and at the detector level (i.e., position dependence). We fit the KID's response to a two-component heat capacity and present the model as an alternative way to understand the KID's response to absorbed energy. We show on- and off-KID data at a range of temperatures and show that the phonon dynamics are not temperature dependent. We comment on the resulting implication for KID dynamics and design improvements.