A Superconducting Switch for Surface Acoustic Wave Propagation

Surface acoustic waves (SAWs), with their 5 orders-of-magnitude slower propagation velocity, allow for considerably shorter wavelengths at the same frequency compared to electromagnetic waves. The short wavelengths allow for device miniaturization and on-chip integration. The surface confinement of the waves also makes them ideal candidates to couple to magnetic films deposited on the substrate. Here, we report SAW generation in the high-MHz to low-GHz range using niobium nitride (NbN) interdigitated transducers (IDTs) and Bragg reflectors fabricated on both LiNbO₃ and AlN. We report strong SAW transmission at a fundamental frequency of 330 MHz along with a convoluted 3rd harmonic-bulk acoustic wave (BAW) regime and a 5th harmonic. We demonstrate 6.39 times larger SAW transmission in a device with a cavity compared to a device with no cavity. The superconducting behavior of NbN below a critical temperature of 11 K allows for temperature-based modulation of SAW transmission, where there is strong transmission when T < T_c and negligible transmission when T > T_c. We also demonstrate that the change in SAW transmission behavior mirrors the change in resistance of NbN at and around its T_c.