Light Beating Spectroscopy of Brillouin Scattering in Solid Polymer

Hyper-resolution of the optical beating technique revealed a very accurate shape of the Brillouin spectrum, which had been deformed by the insufficient resolution of Fabry-Perot technique. The curve observed in liquid CS2 near room temperature at the scattering angle 10.7° showed a Rayleigh peak whose height is about 100 times higher than the two Brillouin components. This spectrum shape, which is far from the triplet structure, is theoretically understood in terms of the Landau-Placzek ratio which predicts the ratio of the scattered power of Rayleigh to Brillouin components. This optical beating technique was, for the first time, used for a solid specimen; phonon peaks were observed in acrylic resin over a range from 90 to 460 MHz. The phonon velocities thus determined showed a strong dispersion of Δν=40 m/s above the ultrasonic velocities obtained in the MHz range by the pulse-echo method. The phonon decay time was also determined from the width of the Brillouin peaks and correlated with the ultrasonic absorption. The results of velocity and absorption consistently suggested a broad relaxation effect expanding over 4 decades of frequency centered around 1 MHz.