Kerry J. Vahala

Phonon Lasers in Cavity Optomechanics

The possibility of a phonon laser, the analog of an optical laser except for vibronic motion, has been considered since the earliest days of the optical laser. There are numerous proposals for these devices, and also evidence of stimulated phonon emission in paramagnetic systems at cryogenic temperatures (see [1,2] and references therein). However, phonon laser action in which stimulated emission of phonons overcomes mechanical loss, leading to coherent mechanical motion has only recently been demonstrated [1,2]. One of these systems operates in analogy to a two-level photon laser. Electronic transitions are replaced by phonon-mediated transitions between optical modes of a compound, whispering-gallery resonator (see figure 1). By optical pumping of the upper level, phonon gain is produced leading to coherent mechanical oscillation above a clear threshold (see figure 2). The interaction is provided by radiation pressure within the microcavity system, which provides a “dipole-like” coupling of the optical supermodes to the mechanical breathing modes of either resonator. The physics of this device is reviewed along with data showing threshold and line narrowing. Finally, we illustrate how these devices can be fully integrated as planar sources of vibrational coherence using the new technology of “optomechanical crystals” [3]. The possibility of a phonon laser, the analog of an optical laser except for vibronic motion, has been considered since the earliest days of the optical laser. There are numerous proposals for these devices, and also evidence of stimulated phonon emission in paramagnetic systems at cryogenic temperatures (see [1,2] and references therein). However, phonon laser action in which stimulated emission of phonons overcomes mechanical loss, leading to coherent mechanical motion has only recently been demonstrated [1,2]. One of these systems operates in analogy to a two-level photon laser. Electronic transitions are replaced by phonon-mediated transitions between optical modes of a compound, whispering-gallery resonator (see figure 1). By optical pumping of the upper level, phonon gain is produced leading to coherent mechanical oscillation above a clear threshold (see figure 2). The interaction is provided by radiation pressure within the microcavity system, which provides a “dipole-like” coupling of the optical supermodes to the mechanical breathing modes of either resonator. The physics of this device is reviewed along with data showing threshold and line narrowing. Finally, we illustrate how these devices can be fully integrated as planar sources of vibrational coherence using the new technology of “optomechanical crystals” [3].