Thermal-phonon resonance

Abstract

We have observed the resonance of thermal phonons confined in a small cavity by Brillouin scattering detected by an optical beating technique. Longitudinal resonance modes of sub-GHz phonons were found at every 760 kHz in the resonance cavity composed of two flat walls set face to face with 880 \ensuremath{\mu}m spacing. The optical beating technique offered a frequency resolution as high as \ensuremath{\sim}kHz to the Brillouin scattering measurement, which was thus able to detect the spectrum of phonon resonance. The spectrum observed is well described by a theory which takes the effect of finite correlation time of the phonon phase into consideration.