Mitsuru Ishikawa

Femtosecond-pulse laser chirp compensated by cavity-mirror dispersion

The duration of pulses generated from a simple colliding-pulse mode-locked cw dye laser is measured as a function of cavity-mirror dispersion. The optimum amount of mirror dispersion of ø (omega) congruent with + 1.8 x 10(-28) sec(2) and a suitable mirror coating for upchirp compensation are identified. The adjustment of mirror dispersion only, without additional dispersive elements, generates continuous trains of pulses as short as 50 fsec.

Femtosecond spectral hole-burning of crystal violet in methanol. New evidence for ground state conformers

Abstract The absorption spectrum of crystal violet in methanol solution showed two spectral holes around 550 and 600 nm when using tunable femtosecond-pulse excitation. The formation of the 550 nm hole was delayed by 500 fs after the 600 nm hole was formed when the pumping wavelength was 598 nm. These findings support the presence of ground state conformational isomers, which have been the subject of controversy since 1942. A possible mechanism for the hole formation is proposed using a model having two ground states and one excited state.

Picosecond fluorescence spectroscopy of purple membrane in Halobacterium halobium with a photon-counting streak camera

Abstract Fluorescence lifetimes and spectra of native and deionized purple membranes of Halobacterium halobium at 22°C were measured to be blue-shifted transient previously found by absorption spectroscopy is attributed to bacteriorhodopsin in the lowest excited-singlet state. Ultraweak fluorescence of the light-adapted purple membrane with 2.5 × 10 −4 quantum yield could be detected even though the excitation pulse energy at 570 nm was reduced to 0.88 pJ (72 μW average power).

Picosecond-millisecond dual-time-base spectroscopy of fluorescent photointermediates formed in the purple membrane of Halobacterium halobium

Abstract Dual-time-base fluorometry to measure both fluorescence lifetimes and the reaction kinetics of their origins was applied to the identification of two fluorescence (>660 nm) species T 9ps and T 62ps ( τ F =9±2 and 62±2 ps, respectively) in the purple membrane of Halobacterium halobium in the photostationary state. Photocycles were driven with a 532-nm nanosecond laser and the τ F s were measured at delay times ( t d =0.05–90.5 ms) by a gated streak camera synchronized to a 626-nm picosecond laser. The t d dependencies of the fluorescence of T 9ps and T 62ps were identical with the behaviors of the photointermediates O and Q, respectively.

Solvation effect on the picosecond fluorescence lifetimes and anisotropy decays of a hydrogen-bonded biologically active furocoumarin derivative: 4,5′,8-trimethylpsoralen

Abstract The origin of non-single exponential fluorescence decays of hydrogen-bonded 4,5′,8-trimethylpsoralen (TMP) with trichloroacetic acid (TCA) in cyclohexane was elucidated from picosecond time-resolved fluorescence lifetime and anisotropy decay measurements. The lifetime of the hydrogen-bond-free TMP ( T 1 ) was 14.0±1.5 ps, and thereby the hydrogen-bonded TMP was identified at TCA concentrations lower than 10 −3 M, having a still longer lifetime: T 2 =360±15 ps. A further increase in T 2 with increasing concentration of TCA coincided with a swelling of molecular volume of the hydrogen-bonded TMP. Moreover, time-dependent red-shifts of fluorescence maxima were observed at TCA concentrations higher than 10 −2 M, which suggests dipolar relaxation between the hydrogen-bonded TMP and excess TCA in the swelling solvation sphere.

Femtosecond pump-probe studies of CdSe microcrystals embedded in a germanium dioxide glass matrix

We report here results of experiments on the transient absorption spectra of CdSe microcrystals embedded in a germanium dioxide glass matrix. The transient absorption spectra were measured by means of femtosecond pump-probe method. In the sample with the smaller microcrystal size within the category of the strong confinement regime (the individual particles confinement regime), the absorption bleachings due to the carrier dynamics in the quasi-zero- dimensional quantum confined states were observed. In the sample with the larger microcrystal size within the category of the weak confinement regime (the exciton confinement regime), the absorption bleachings due to the hot carrier in the 3D electronic states were studied at various excitation intensities.