Techniques are provided to optomechanically couple light to a crystal structure, thereby producing stable, coherent bulk acoustic modes within the structure. In some embodiments, a resonator may comprise a plano-convex crystal structure to which pump light may be applied. The pump light may transfer energy to acoustic phonon modes of the crystal structure so as to create acoustic phonon modes with a coherence length greater than a length of the crystal structure. High frequency and high quality factor resonators may thereby be produced and operated.