Lifetime of THz Acoustic Nanocavity Modes

We present an ultrahigh resolution Raman study of the lifetime of 1 THz acoustic phonons confined in nanocavities. We demonstrate that the cavity Q factor can be controlled by design. Anharmonicity contributes only marginally to limit the cavity phonon lifetime, even at room temperature, while thickness fluctuations in the scale of 1/10 of a unit cell are the main limitation for the performance of THz phonon cavities.