Low-temperature properties of low-loss macroscopic lithium niobate bulk acoustic wave resonators

We investigated gram-scale macroscopic bulk acoustic wave (BAW) resonators manufactured from plates of piezoelectric lithium niobate. The intrinsic competing loss mechanisms were studied at cryogenic temperature through precision measurements of various BAW modes. Exceptional quality factors were measured for the longitudinal BAW modes in the 1--100 MHz range, with a maximum quality factor of 8.9 million, corresponding to a quality factor $\ifmmode\times\else\texttimes\fi{}$ frequency product of $3.8\ifmmode\times\else\texttimes\fi{}{10}^{14}$ Hz. Through measurements of the acoustic response to a strong drive tone, anomalous self-induced absorption and transparency effects are observed. We show that such observations can be explained by microscopic impurities and defect sites in the crystal bulk by the use of a nonlinear model of acoustic dissipation. The losses associated with these defects provide the ultimate limit of resonator performance, which could be improved in the future if more pure samples were available.