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Physical review. B, Condensed matter

Temperature dependence of Debye-Waller factor in dye-doped polymers

S. Saikan, A. Imaoka, Yasuo Kanematsu, Kazuaki Sakoda, Kazuhiko Kominami, M. Iwamoto

Published Feb 15, 1990• 64 citations
Publisher

Abstract

The temperature dependence of the Debye-Waller factor in dye-doped polymers has been investigated with femtosecond accumulated photon echo. The superiority of polyvinyl alcohol as the host polymer for the optical-memory material is ascribed to the higher phonon sideband frequency in this sample. It is pointed out that the temperature dependence of the Debye-Waller factor, in particular the phonon sideband frequency, is of fundamental importance to finding out the best polymer for high-temperature optical memory based on photochemical hole burning.

DOI
10.1103/physrevb.41.3185

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This talk presents a review of our recent experimental results on femtosecond accumulated photon echo in dye-doped polymers. The aim of this research is to clarify the parameter that is most important in affecting seriously the linear electron-phonon coupling in these samples, and to find the materials which have extremely weak linear electron-phonon coupling. The latter research is needed for the improvement of the PHB memory materials. We have so far demonstrated experimentally that Fourier-transform spectroscopy [1] based on the femtosecond accumulated photon echo is most useful for these studies. Furthermore, the detection sensitivity for the accumulated photon echoes has been improved significantly with the use of the phase modulation technique [2].

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Physical Review A•Nov 1, 1988•20 citations
The temperature dependence of the Debye-Waller factor in dye-doped polymers has been investigated with femtosecond accumulated photon echo. The superiority of polyvinyl alcohol as the host polymer for the optical-memory material is ascribed to the higher phonon sideband frequency in this sample. It is pointed out that the temperature dependence of the Debye-Waller factor, in particular the phonon sideband frequency, is of fundamental importance to finding out the best polymer for high-temperature optical memory based on photochemical hole burning.
Optical dephasing of the zero-phonon line (ZPL) of organic dye molecules doped in polyvinyl alcohol (PVA) and several cross-linked PVA derivatives was studied by using the incoherent photon echoes and the photophysical persistent hole-burning. It was found in the incoherent photon echo measurements that optical dephasing time of the ZPL increases with increasing the length of a cross-linker introduced to the PVA backbone. The difference in the temperature dependence of the dephasing time was also observed between a dye doped in PVA with and without the cross-link. When a longer cross-linker was introduced, the phonon sideband component in the incoherent photon echo signal became dominant in contrast to that in PVA with a relatively short cross-linker, while the dephasing time of the ZPL was still longer than that in PVA without the cross-link. Hole-burning studies showed that the phonon sideband spectrum of a dye doped in the cross-linked PVAs is essentially similar to that in PVA. However, the depth of the zero-phonon hole burnt with the same burning power was found to depend on the PVA hosts with the cross-linkers. These our findings suggest that the introduced cross-link does not alter the electron–phonon coupling modes or pseudo-local mode, but it drastically reduces the effect of the two-level tunneling systems in PVA on the optical dephasing of a doped dye. As a result, the dephasing time of the dye in the cross-linked PVAs becomes longer than that in PVA without the cross-link. The decrease of the optical dephasing by introducing the cross-link can be interpreted by assuming a void space in PVA, which is created near to the cross-linker. In addition, absorption and fluorescence spectra in these systems were measured, which imply the occurrence of weak dye–dye interaction when the dye is doped in PVA with a much longer cross-linker.
Fourier-transform spectroscopy using the technique of femtosecond accumulated photon echo with incoherent light has been performed in dye-doped polymers. Several types of interference effect or quantum beat between the zero-phonon line and other spectral components including the phonon sideband, the vibronic lines, and the photoproduct state have been observed. Based on the density matrix incorporating the transition-frequency modulation, it is pointed out that the factorization approximation for the relaxation function holds rigorously in the accumulated photon echo. We also clarified that the Fourier cosine transform of the heterodyne-detected echo signal provides the hole-burning spectrum.
The temperature-dependent femtosecond dephasing of vibronic lines of a dye, Nile blue, doped in a polymer (polyvinyl alcohol) was studied by using an incoherent-photon-echo technique. It is, to our knowledge, the first detailed study on temperature-dependent dephasing for vibronic lines in a dye-doped polymer system. Photon echoes for the vibronic lines displayed an exponential decay with the dephasing times less than 100 fs, and the temperature dependence of the dephasing was found to follow a power law of T1.5{\mathit{T}}^{1.5}T1.5 in the temperature range from 9.5 to 130 K. By comparing the results with the behavior of the zero-phonon line, we discuss the effect of the interactions with so-called two-level tunneling system and phonons in the polymer host on the femtosecond dephasing of the vibronic lines.
This talk presents a review of our recent experimental results on femtosecond accumulated photon echo in dye-doped polymers. The aim of this research is to clarify the parameter that is most important in affecting seriously the linear electron-phonon coupling in these samples, and to find the materials which have extremely weak linear electron-phonon coupling. The latter research is needed for the improvement of the PHB memory materials. We have so far demonstrated experimentally that Fourier-transform spectroscopy [1] based on the femtosecond accumulated photon echo is most useful for these studies. Furthermore, the detection sensitivity for the accumulated photon echoes has been improved significantly with the use of the phase modulation technique [2].
The temperature dependence of vibronic quantum beats in dye-doped polymer films has been investigated using femtosecond accumulated photon echoes. It has been found that the vibronic quantum beat is not discernible above liquid-nitrogen temperature. It makes a striking contrast with the previous observation of the quantum beat in transient pump-probe experiments performed at room temperature. Based on the density matrix incorporating the fluctuation of the transition frequency, it is pointed out that there is no contribution of sublevel coherence to the accumulated photon echo, and the line broadening of the vibronic state or phonon sideband obscures the quantum beat at higher temperature. On the other hand, in the transient pump-probe experiment, the quantum beat is caused by the sublevel coherence, as was previously pointed out by Mitsunaga and Tang [Phys. Rev. A 35, 1720 (1987)]. Hence it is concluded that the influence of the frequency fluctuation on the sublevel coherence is relatively small despite the serious line broadening of the vibronic state.