Changing the spontaneous light emission rate and spectra of atoms or excitons through the modification of the photon density of states (DOS) has been the subject of significant efforts following Purcell’s proposal and demonstration in the microwave domain. This has been accomplished either by changing the dielectric boundaries close to the emitting species, or more fundamentally by embedding the emitter in an optical microcavity. Depending on the tuning of the emitter spectra with maxima or minima of the modified photonic DOS, the emission can be either enhanced or inhibited.
In the field of phononics, and specifically in the search of phonon ``lasing’’, for efficient monochromatic THz acoustic sources, and for the control of heat at the nanoscale, these ideas have been very little applied so far. We will describe recent pump-probe time resolved reflectivity experiments along these lines performed in a hybrid air-Ni metal-BaTiO3/SrTiO3 oxide mirror phonon cavity. These experiments demonstrate that the generated coherent acoustic phonon spectra of the impulsively excited metallic film can be inhibited or enhanced in the phonon cavity with respect to a Ni film directly grown on a SrTiO3 substrate. The experiments are compared with simulations that highlight the role of the phonon density of states in the coherent acoustic generation, extending concepts at the base of cavity-quantum-electrodynamics to the field of phononics. The prospects of observing the Purcell-effect in phonon systems based on the presented results will be addressed. On-going research on an alternative scheme based on cavities that confine simultaneously light and GHz-hypersound and that are interesting candidates for the observation of stimulated phonon emission will be also commented.