Electro-Mechanical Tuning of High-Q Bulk Acoustic Phonon Modes at Cryogenic Temperatures

We investigate the electromechanical properties of quartz bulk acoustic wave resonators at extreme cryogenic temperatures. By applying a DC bias voltage, we demonstrate broad frequency tuning of high-Q phonon modes in a quartz bulk acoustic wave cavity at cryogenic temperatures of 4 K and 20 mK. More than 100 line-widths of tuning of the resonance peak without any degradation in loaded quality factor, which are as high as $1.73\times 10^9$, is seen for high order overtone modes. For all modes and temperatures the observed coefficient of frequency tuning is $\approx$ 3.5 mHz/V per overtone number $n$ corresponding to a maximum of 255.5 mHz/V for the $n = 73$ overtone mode. No degradation in the quality factor is observed for any value of applied biasing field.