Engineering nitrogen-vacancy center electron-phonon coupling with a semi-confocal diamond acoustic resonator*

Diamond-based microelectromechanical systems (MEMS) provide direct coupling between quantum states of nitrogen-vacancy (NV) center and the phonon modes inside the resonator. As a prime example, diamond thin film bulk acoustic resonators (BARs) feature integrated piezoelectric transducer and high-quality factor resonance modes up to the GHz frequency range. The bulk acoustic modes allow mechanical manipulation of deeply imbedded NV centers with long spin/orbital coherence, as recently demonstrated in experiments. Limited by the resonator size, ~100 um, coherent NV electron-phonon interaction is still scarce in current diamond BAR devices. In this talk, we present the design and fabrication of a semi-confocal diamond BAR device with f*Q product >10^{13}. The semi-confocal geometry confines the phonon mode laterally below 10 um. This drastic modal size reduction offers a boost in the NV center electron-phonon coupling with potential applications in spin-mediated resonator cooling and quantum resonator state control.