Takeshi Yogi

Thermal Phonon Resonance in Solid Glass

Thermal phonon resonance was observed in a stiff solid material of fused silica, from which a lower intensity of scattered light is expected than from liquids. The improved detection sensitivity of the optical beating Brillouin spectroscopy technique enabled us to observe the spontaneous elastic resonance of the solid sample attributable to thermal density fluctuation with a frequency resolution of 1 kHz. The accuracy in determining phonon velocity was then increased up to 10-5. The high frequency resolution also revealed the fine structure of the resonance peak train brought about by the lateral mode vibration, and the eigen frequencies of the compound mode are in good agreement with those theoretically estimated. Volume and shear elasticities were uniquely determined from the resonance spectra of longitudinal and shear phonons. Phonon absorption at about 100 MHz was also determined from the resonance peak width of the Brillouin component.

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-Perot etalon 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 ...

Millisecond Brillouin scattering spectroscopy

The millisecond Brillouin spectroscopy was developed and applied for the real-time observation of phonons in solid and gas materials. The Brillouin spectra of solid (polymethylmethacrylate) and gas (air) at 300 K are observed over the wave number range from k=8.1×104 m−1 to k=5.3×105 m−1. The measurement time is only 100 ms, which is by far shorter than that for the conventional method from several minutes to several hours. The technique would provide us with a useful tool for the dynamic observation of thermal phonons in solid and gas.

Simultaneous Observation of Longitudinal and Shear Phonons in Solid Glasses by Optical Beating Brillouin Spectroscopy

The propagation of shear acoustic phonons in the solid materials of fused silica and BK7 was observed by optical beating Brillouin spectroscopy in the wavenumber range of 7×104<k<9×105 m-1. Remarkably improved detection sensitivity of the optical beating Brillouin spectroscopy system enabled the simultaneous observation of longitudinal and shear phonons in solid samples. The accurate estimation of the volume and shear elasticities in the isotropic solid material is thus possible. The measured ratio between the intensities of longitudinal and shear peaks is well explained by the equipartition law of the thermal fluctuation energy to these independent phonon modes with different elastic susceptibility.

Development of submillisecond Brillouin spectroscopy with optical beating technique

We developed a rapid measurement system for Brillouin scattering spectroscopy, which requires the data acquisition time of only 100 μs. The optical beating detection system was improved to directly observe and accumulate the incoherent signal carrying the information of the power spectrum of the scattered light. The temporal evolution of the mechanical properties of material can be measured with high time resolution. It is also possible to apply it to the light absorbing sample, since the short time radiating of the laser beam does not cause serious temperature change in the sample. The performance was actually demonstrated for the light absorbing material.

Rotational relaxation in diatomic gas at high temperature observed with Brillouin scattering spectroscopy

It has recently become possible to observe Brillouin scattering spectra of gases at high temperatures as well as their rotational–translational (R–T) relaxation at room temperatures with optical beating Brillouin scattering spectroscopy. In this study, the sound velocity in nitrogen gas was measured at high temperature and a R–T relaxation in the frequency region of 100 MHz was observed. The obtained relaxation frequency value was found to be experimentally reasonable when compared to theoretical values. To our knowledge, optical beating Brillouin spectroscopy is the only method used to observe the total change in sound velocity in R–T relaxation at high temperature. Employing the spectroscopy allowed for a discussion of the temperature and frequency dependence on mechanical properties such as the specific ratio; that is, the internal energy degrees of freedom of the molecules.

Rotational relaxation in H2 gas observed with optical beating Brillouin spectroscopy

The optical beating Brillouin spectroscopy used in this study possesses quite high frequency resolution, up to 1 kHz, which enabled us to observe the Brillouin peak of hydrogen gas in the megahertz frequency region. In this study, we measured the sound velocity of hydrogen gas in the 10 MHz frequency region with optical beating Brillouin spectroscopy and observed rotation-translation relaxation. The obtained value of the relaxation frequency is reasonable. This method would enable us to observe rotation–translation relaxation of other gases in a nondestructive, noncontact manner.

Chirp Control of Free Carrier Injection in GaAs Using Femtosecond Optical Pulses

Control of photoinjection of free carriers in bulk GaAs at room temperature was achieved by changing the chirp of the excitation light pulses having a duration in the 10 fs regime. It was observed from pump-probe measurements that the transmittance increases for negatively chirped pump pulses, which is opposite to the trend observed with other materials. The result is explained by a combination of an intrapulse pump-dump process and band-gap renormalization, and shows the possibility of a new way to control the ultrafast dynamics of many-body systems in semiconductors.

Optical beating Brillouin scattering spectroscopic measurements of high-temperature gas

The sound velocity of air at high temperature was measured in the 10 MHz frequency region using optical beating Brillouin scattering spectroscopy. The sound velocity dependence on temperature was observed, and the molecular weight of the gas was obtained and is in agreement with the literature value. The present experiment will enable the observation of gas phase rotation-translation (R-T) relaxation, for example, the R-T relaxation of nitrogen at high temperature that occurs in the 100 MHz region. The high-temperature sound velocity could also be used to measure the molecular fraction of the gas in this experiment.

High resolution Brillouin scattering in solid and gases

The experimental system of our optical beating Brillouin spectroscopy was improved, which can observe the phonon propagation with the resolution of 1 kHz. The increased sensitivity and frequency resolution were demonstrated in the observation of phonon spectrum in solids of fused silica and also in gases, nitrogen, and freon CHClF2: for their weak light‐scattering ability, these substances are very tough specimens for Brillouin scattering studies, and the classical spectrometer of Fabry‐Perot etalon has been so far used for the frequency analysis. The phonon peaks were observed with the present light beating system in nitrogen and freon near 300 K over the wave‐number range from 2π=6.20×103 to 3.09×105 m−1, the pressure range from 14.8 to 177 kPa. The observed spectra were well fitted with the Lorentzian curve representing the phonon lifetime, and the phase velocity. The phase velocity of phonon in fused silica agrees well with that obtained by ultrasonic measurement, while the phonon dispersions obtained...