T. Sonehara

Temperature dependence of the Brillouin frequency shift in crystals

In order to find a theoretical expression which most faithfully reproduces the temperature dependence of the Brillouin frequency shift in crystals, we have performed stimulated Brillouin spectroscopy in two crystals of TeO2 and PbMoO4 as test crystals by using a high precision Brillouin spectrometer with the frequency resolution of 20 kHz. In general, the temperature dependence of a Brillouin shift in crystals not undergoing phase transitions exhibits a linear negative slope at high temperatures and an asymptotic approach with zero slopes at low temperatures. We have experimentally confirmed that this temperature dependence stems from the temperature change of the elastic constant rather than the thermal changes of the refractive index and the volume expansion coefficient. It has been found that the Lakkad function based on Debye model and the Varshni function based on the Einstein model both reproduce quite well the temperature dependence of the Brillouin frequency shift in crystals.

Frequency-modulated stimulated Brillouin spectroscopy in crystals

We have developed a Brillouin spectrometer based on frequency modulation (FM) spectroscopy in order to enhance the detection sensitivity and to detect separately the real and imaginary parts of the Brillouin spectrum. With this spectrometer, we have measured Brillouin spectra at room temperature in a variety of single crystals including SiO2, CaF2, LiNbO3, deuterated L-arginine phosphate, PbMoO4, TeO2, langatate (La3Ta0.5Ga5.3Al0.2O14), and KRS-5. To determine the Brillouin linewidth and shift from the observed FM spectrum, we have derived a spectral formula for the FM-stimulated Brillouin spectrum by taking into account several contributions from both electrostrictive and absorptive Brillouin scattering and polarization ellipticity of pump or probe waves. This formula reproduces the observed FM Brillouin spectra quite well.

kHz stimulated Brillouin spectroscopy

We have developed a high precision stimulated Brillouin spectrometer. Since this spectrometer is based on the stimulated light scattering, Brillouin measurements are possible without any signal reduction even at extremely low temperatures. The use of stable lasers for the pump and probe waves and a real-time frequency-monitor gives a high frequency resolution reaching 20 kHz, which is, to the best of authors knowledge, the highest resolution so far reported for Brillouin spectrometers. The frequency-monitor has been achieved through the measurement of the beat frequency between the two lasers by using a microwave frequency counter. With this spectrometer, we have succeeded to measure Brillouin spectra in single crystals of TeO2 from room temperature to liquid helium temperature. Furthermore, in a sample with [110] surfaces, we have observed a spectral splitting due to the multireflection of phonons at crystal surfaces.

Frequency-modulated stimulated Brillouin spectroscopy in LiTaO3.

By using a high-precision FM Brillouin spectrometer with the spectral resolution of 20 kHz, we have succeeded in determining the Brillouin linewidth of single-crystal LiTaO(3). The large directional anisotropy for the attenuation rate of the longitudinal elastic wave and the extremely narrow Brillouin linewidth of 1.60 MHz have been observed at room temperature.

Separation of electrostrictive and absorptive stimulated Brillouin spectra

From the symmetry analysis of frequency modulated (FM) stimulated Brillouin spectrum in As40Se60 glass, we demonstrate a separate detection of electrostrictive and absorptive FM Brillouin spectra as well as the detection of real and imaginary parts of Brillouin spectrum due to FM spectroscopy.