| Budget Amount *help |
¥3,200,000 (Direct Cost: ¥3,200,000)
Fiscal Year 1998: ¥700,000 (Direct Cost: ¥700,000)
Fiscal Year 1997: ¥2,500,000 (Direct Cost: ¥2,500,000)
|
|---|
| Research Abstract |
We have found that the modulated four-wave-mixing (FWM) signal observed in NdGaO_3 crystal at 585 nm results from the quantum beats between the several absorption lines based on the detailed spectroscopic study at low temperature, which, in the beginning of this project, we attributed to the result of phonon generation. The FWM signal consists of interest features that the absorption lines are predominantly homogeneously broadened even at low temperature. We have performed the detailed analysis of the signal and published its results in Phys. Rev. 13. however, these results are different subject from that of this project.
We have performed the study on phonon generation and phonon coherence in two dielectric, SrTiO_3, BaTiO_3 by mean of the phonon generation with Raman excitation. BaTiO_3 crystal had too worse quality to get the good signal to noise ratio.
The excitation source for phonon generation is the output from a Ti-sapphire regenerative amplifier with the pulse energy of 800 (muJ
… More
) and the pulse duration of about 130 (fs), corresponding to the peak power of 6.2x10^9 (W). The output from the regenerative amplifier is split into two strong excitation beams (wave vector k_1, k_2) and a weak probe beam (wave vector k_3). We excite the sample with the BOXCARS configuration. The signal in the direction of k_3 + k_2 - k_1 is measured as a function of the delay time tau between the excitation and probe beam.
(1) In order to examine the time resolution and sensitivity of our excitation and detection system, we measured the signal in CS_2. The signal exhibited the double-exponential decay curve with the time constants, 0.4 (ps) and 0.7 (ps). The signal merged from the K err effect of CS_2 and the measured time constants correspond to that of relaxation of orientational polarization in CS_2 molecule. Based on this examination, the time resolution of our system was concluded to be 0.2 (ps).
(2) When we measured the signal in SrTiO_3 crystal at room temperature, we obtained the very strong signal in the direction of k_3 + k_2 - k_1 at tau = 0, which is greater by about one order of magnitude than that in CS_2. This signal emerges mainly from the nonlinear electronic polarization in SrTiO_3 and indicates that the SrTiO_3 crystal can be one of the efficient nonlinear materials with a large susceptibility. However, we could not observe the signal and its modulation for tau > 0, which is expected to appear if the phonon generation takes place. We could not obtain the signal due to the phonon generation even when the polarization of the two excitation beams is made perpendicular to excite the E_2-phonon mode. However, we recently observed that the small signal at tau > 0 appears doe to the phonon generation at 10 K, with the polarization of the excitation beams perpendicular to each other. We are going to study the dependence of this signal on the polarization of the excitation beams and temperature. Less
|
