Seishiro Saikan

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.

Two-photon excited fluorescence line narrowing spectroscopy in dyes without inversion symmetry

Two-photon excited fluorescence line narrowing (FLN) spectroscopy has been demonstrated in dyes without inversion symmetry in order to develop site-selective FLN spectroscopy which does not suffer from the contamination of excitation stray light. The broadening of zero-phonon line width, which is prominent at low temperature, has been ascribed to the transient heating of sample due to the infrared absorption.

Note: Higher resolution Brillouin spectroscopy by offset stabilization of a tandem Fabry-Pérot interferometer

A simple modification to a Sandercock-type tandem Fabry-Pérot interferometer is demonstrated. By adding an independent reference laser with temperature tunability, narrow Brillouin lines that are tens GHz shifted from the Rayleigh line can be recorded with much higher frequency resolution than in the original system.

Power-Law Quasielastic Light Scattering Observed in Relaxor Pb(Mg1/3Nb2/3)O3

The low-frequency light-scattering spectrum in Pb(Mg1/3Nb2/3)O3 single crystal is investigated at 297 K. A quasielastic scattering is found, and its line shape is identified as a “power-law” type, instead of a single Lorentzian or superposition of two Lorentzian curves. The power-law spectrum implies the existence of a wide distribution of relaxation times in relaxors.

Quasielastic Light Scattering and Second Sound in SrTiO3

We study the light-scattering spectrum in SrTiO 3 in the low frequency range (< 1000 GHz). Two components of quasielastic peaks were observed at relatively high temperatures. We found that the collective phonon-excitation in a crystal accounts for both quasielastic components. The anomalous broad Brillouin component in SrTiO 3 , which was reported previously, can also be accounted for by our model, and it has been strongly indicated that the origin of the broad doublet is “second sound”. The broader quasielastic component arises from the interactions between two second sounds, and it can be understood as the “Mountain mode” in crystals.

Heitler experiment in J-aggregate: Difference between FLN and hole-burning spectra

Abstract We have observed Heitler effect in the fluorescence line narrowing spectrum of J-aggregate for temperatures below 5K. The spectrum shows complete vanish of luminescence and predominant Rayleigh scattering. From the observed hole-burning spectrum and the Rayleigh spectrum, we discuss the difference between spontaneous and stimulated optical processes.

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.

Novel Method for the Derivation of 1-Phonon Function from Fluorescence Line Narrowing Spectrum

Theoretical formulation for fluorescence line narrowing (FLN) spectrum is reinvestigated by using the dynamical theory of electron-phonon interaction. We have found that the formulation becomes very simple in the case of sufficiently large inhomogeneous broadening. With the use of this formulation, 1-phonon function which is defined as a product of linear electron-phonon coupling constant and the density of states of phonon, can be obtained directly from Fourier transform of experimentally observed FLN spectrum. To demonstrate the feasibility of our method, we derive 1-phonon function from the observed FLN spectrum in a sample of tetracene doped in polyisobutylene. By using thus obtained 1-phonon function, the temperature dependence of FLN spectrum can be numerically reproduced without any adjustable parameters. We further consider the influence of inhomogeneous width on the present method for the derivation of 1-phonon function.