S. Saikan

Phase-modulation technique for accumulated photon echo

A novel photon-echo technique based on the phase-modulation method has been developed that is applicable to the accumulated photon echo in scattering materials.

Optical memory based on heterodyne-detected accumulated photon echoes

Optical memory based on heterodyne-detected accumulated photon echoes has been demonstrated in dye-doped polymers. Compared with previously developed time-domain optical memory, writing and reading of higher bit data is possible owing to the superior time resolution of this method.

Hole-burning spectroscopy in dye-doped polymers by spatial modulation technique

Abstract The spatial modulation method also called the beam-overlap modulation method is shown to be suitable for highly sensitive persistent hole-burning spectroscopy. From the sample dependence of electron-phonon coupling strength studied by means of this technique in dye-doped polymers, it is suggested that the electron-phonon interaction is relatively strong in the dyes in which the intramolecular deformation occurs rather easily, and also in the host polymers which have large sidegroups on the polymer chain. Based on these results and on the data of an accumulated photon echo experiment, the ultrafast phase relaxation previously observed in nonlinear optical spectroscopy is attributed to the presence of strong phonon sidebands in the absorption spectrum.

Time dependence of hole spectrum due to dispersive burning kinetics in dye-doped polymers

Time evolution of a persistent hole spectrum has been investigated for dyes doped in polyvinyl alcohol films at 2 K. The zero‐phonon hole profile measured for resorufin using a single‐mode dye laser has been found to be nearly Lorentzian even in the case of relatively deep holes. The observed fluence‐dependent depth and width of the zero‐phonon hole are reproduced well by the numerical simulation based on a dispersive burning‐kinetics model. It is pointed out that a large distribution of the burning rate is responsible for the fluence‐dependent hole broadening. Time evolution of the phonon‐sideband hole simulated using the same model is also compared with the experimental data. It is shown that the low‐energy sideband hole is enhanced when the burning kinetics is dispersive.

Linear electron-phonon interaction in dye-doped polymers

Abstract The temperature-dependent variation of phonon sideband spectrum in dye-doped polymers has been investigated with Fourier transform femtosecond photon-echo spectroscopy. From the analysis taking into account the linear electron-phonon interaction, it is found that the ultrafast decay of the echo signals observed at higher temperatures is due to excitation from the thermally occupied phonon state in the ground state.

Observation of vibronic quantum beat in dye‐doped polymers using femtosecond accumulated photon echo

Fourier‐transform photon echo spectroscopy has been applied to the vibronic absorption band in dye molecule. The quantum beat between the vibronic lines has been observed in cresyl‐violet and tetraphenyl‐porphine doped in polymer films. The vibronic lines associated with the quantum beat have been identified with comparing the Fourier‐transformed spectrum of the echo signal and the persistent hole‐burning spectrum. The observed steep temperature dependence of the vibronic coherence has been discussed in comparison with the mild temperature dependence of vibrational coherence (sublevel coherence between vibronic states).

Linear electron-phonon coupling in dye-polymer systems: in search of weakly coupled systems

Abstract Recent experimental results on femtosecond accumulated photon echo in dye-polymer systems are reviewed. The aim of this research is to clarify the parameter that is most important in significantly affecting the linear electron-phonon coupling in these materials, and to find the materials which have extremely weak linear electron-phonon coupling. It has been found that the frequency of the phonon sideband spectrum nearly coincides with the frequency of the boson peak that is observed in the Raman scattering spectrum of polymers, and that the Debye-Waller factor is larger for the polymers with higher boson peak frequency, such as hydrogen-bonded polymers. However, there seems to exist a maximum of the boson peak frequency at around 30 cm−1 in organic polymers. As far as the dye-polymer system is concerned, the porphyrin substituted hemeproteins are, to the best of our knowledge, samples with very weak linear electron-phonon coupling.

Effects of dispersive burning kinetics on hole-burning spectrum in dye-doped polymers

Abstract The profile of the burnt zero-phonon hole, as well as the time dependence of the hole depth, has been measured at 2 K for rhodamine 640 in a polyvinyl alcohol film by means of a highly sensitive spatial-modulation technique. The result is well explained by a dispersive first-order reaction model in which the homogeneous width of the zero-phonon line and the finite spectral width of the laser light are taken into account. It is shown that the dispersive burning kinetics causes not only the non-exponential hole growth but also on the time-dependent spectral broadening of the burnt hole.

Improved time resolution in an accumulated photon echo by using a repetitively frequency-scanned dye laser

The time resolution of a heterodyne-detected accumulated photon echo has been improved significantly by using a dye laser whose lasing frequency is repetitively scanned. A time resolution of less than 30 fsec was obtained, even with a non-mode-locked cw dye laser. This technique is applicable provided that the absorption recovery time of the sample is longer than the repetition period of the frequency-scanning dye laser. The absorption recovery times of several dyes that were doped with polyvinyl alcohol have also been measured.

Peculiarities of non-linear spectroscopic ellipsometry of initially isotropic substances

Abstract We present novel non-linear spectroscopic techniques for the advantageous determination of the third-order complex susceptibility as well as spectroscopic parameters of isotropic media which exhibit light-induced anisotropy. These methods are quite simple and general and can be applied to a large variety of substances. Optically heterodyned polarization spectra have been obtained for dye solutions for the first time. They exhibit light-induced dichroism and birefringence within the absorption band.