Light beating spectroscopy of Brillouin scattering in gases and solids

The experimental system of optical beating spectroscopy was improved, and the increased sensitivity and frequency resolution were demonstrated in the Brillouin scattering experiments in solids, crown glass (BK7), and polymethylmetacrylate (PMMA), and also in gases, air, and nitrogen. For their weak light scattering ability, these substances are very tough specimens for Brillouin scattering studies, and the classical spectrometer of Fabry-Pérot étalon has been so far used for the frequency analysis. The phonon peaks were observed with the present light beating system in BK7 at room temperature over the wave number range from k∕2π=1.03×104to8.84×104m−1 (corresponding frequency from 61.6to548MHz), and the spectra were fitted with Gaussian curves since a condenser lens in the incident light path caused a large instrumental width that overwhelmed the intrinsic phonon width. The spectra in PMMA were analyzed with Voigt functions. The dispersion relations obtained in these solids were in good agreement with the literature values of ultrasonic velocity. The spectra in air were well fitted with Lorentzian curve representing the phonon lifetime, and the absorption as well as the phase velocity was obtained.