Optimizing Lumped-Element Geometries for Phonon-sensing MKIDs

One class of dark matter candidates consists of extremely light particles—some as small as, or even lighter than, electrons—that deposit only meV amounts of energy when interacting with matter. To detect such faint signals, we use superconducting phonon-sensing Microwave Kinetic Inductance Detectors (MKIDs), which are exceptionally sensitive to small energy depositions. Improving the performance of these detectors is essential for extending the sensitivity of next-generation dark matter experiments.

Each MKID features a resonator inductively coupled to a feedline. The resonator itself is an LC circuit composed of interdigitated capacitive fingers and an inductive meander. We use analytical modeling and numerical simulations to study how the meander’s geometry affects both the internal quality factor and the energy resolution. A major challenge arises from magnetic flux trapped in the superconducting film, which degrades the quality factor by forming vortices. While adding flux traps or narrowing the meander can mitigate this issue, these approaches also limit the maximum readout power we can operate at. Our preliminary results indicate that micron scale meander width improves detector sensitivity and quality factor over existing designs, paving the way for meV energy resolution in phonon-mediated MKIDs.