Strong optomechanical coupling at room temperature with a centimeter-scale quartz crystal

Brillouin-based optomechanical systems with high-frequency acoustic modes provide a promising platform for implementing quantum-information processing and wavelength conversion applications, and for probing macroscopic quantum effects. Achieving strong coupling through electrostrictive Brillouin interaction is essential for coupling the massive mechanical mode to an optical field, thereby controlling and characterizing the mechanical state. However, achieving strong coupling at room temperature has proven challenging due to fast mechanical decay rates. While Brillouin strong coupling has been demonstrated, a comprehensive characterization of the system in this regime is still lacking. Here, we report an optomechanical system with independent control over pumping power and frequency detuning to achieve and characterize the strong-coupling regime of a bulk acoustic-wave resonator. Through spectral analysis of the cavity reflectivity, we identify clear signatures of strong coupling, namely, normal-mode splitting and an avoided crossing in the detuned spectra, while estimating the mechanical linewidth ${\mathrm{\ensuremath{\Gamma}}}_{m}/2\ensuremath{\pi}=7.13\phantom{\rule{0.1em}{0ex}}\mathrm{M}\mathrm{Hz}$ and the single-photon coupling rate ${g}_{0}/2\ensuremath{\pi}=7.76\phantom{\rule{0.1em}{0ex}}\mathrm{Hz}$ of our system. Our results provide valuable insights into the performance of room-temperature macroscopic mechanical systems and their applications in hybrid quantum devices.