Hyper-resolution Brillouin-rayleigh Spectroscopy by Optical Beating Technique

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The light scattering system with a scanning Fabry-Perot interferometer and a high-sensitivity photo-multiplier has long been the general standard of all the Brillouin scattering experiments ever since Fleury and Chiao first succeeded in the observation of a Brillouin triplet in the middle of the 1960's. Every endeavor has been made to improve the resolution of the experiment, which has not generally been considered satisfactory. A very sophisticated system in which a computer controls multipass Fabry-Perot etalon or etalons in tandem, for example, has been developed. To reduce the additional line broadening introduced by the spectrum width of the light source, a single longitudinal mode laser with a frequency-stabilizing device has been utilized, the resultant lack of output power being compensated by the increased detection sensitivity of a cooled photo-multiplier tube followed by a photon counter. The Brillouin scattering system of today is thus becoming more and more complicated tho … More ugh its essential part still stays within the experiment of Fleury and Chiao. Nevertheless, all these efforts to achieve higher resolution seems to have been rewarded with rather limited success.On the other hand, the idea of a Brillouin scattering experiment base on a completely different principle has been proposed, i.e., the technique of light beating spectroscopy in which the Fabry-Perot etalon is replaced with a spectrum analyzer. Various experiments have been tried along this line, but most of them were unsuccessful with only one exception. Recently we succeeded in the development of a new system of hyper-resolution Brillouin scattering in which the frequency resolution was increased by 10^3-10^4 times. In this study, we further improved the light scattering system with which we conducted some sophisticated experiments, including (1) observation of phonon relaxation phenomenon in polymer solids, (2) discovery of thermal phonon resonance and (3) investigation of asymmetry in Brillouin triplet observed under non-equilibrium state. Less