Forced Brillouin Spectroscopy Using Frequency-Tunable Continuous Wave Lasers

Abstract

We have developed a new method of phonon spectroscopy using forced Brillouin scattering: A scanning interference pattern produced by intersecting two cw lasers generates the density variation (acoustic phonons) through the thermal expansion for a light-absorbing liquid. When the dispersion relation of phonons is satisfied, phonons are generated resonantly in the liquid. Continuous tuning of the frequency difference between the two lasers enables us to measure a resonance spectrum of light-excited phonons using light scattering phenomena. We demonstrate that this resonance spectrum is equivalent to the Brillouin spectrum of thermal phonons both experimentally and theoretically.