Yasuo Kanematsu

Sensitivity enhancement of fiber-laser-based stimulated Raman scattering microscopy by collinear balanced detection technique

We propose the collinear balanced detection (CBD) technique for noise suppression in fiber laser (FL)-based stimulated Raman scattering (SRS) microscopy. This technique reduces the effect of laser intensity noise at a specific frequency by means of pulse splitting and recombination with a time delay difference. We experimentally confirm that CBD can suppress the intensity noise of second harmonic (SH) of Er-FL pulses by 13 dB.The measured noise level including the thermal noise is higher by only ~1.4 dB than the shot noise limit. To demonstrate SRS imaging, we use 4-ps SH pulses and 3-ps Yb-FL pulses, which are synchronized subharmonically with a jitter of 227 fs. The effectiveness of the CBD technique is confirmed through SRS imaging of a cultured HeLa cell.

Efficient optical Kerr shutter for femtosecond time-resolved luminescence spectroscopy

We have measured optical Kerr effect responses, transmission spectra, and emission properties due to two photon absorption for 17 kinds of optical glass materials and have evaluated their capabilities as an optical Kerr shutter (OKS) for time-resolved luminescence spectroscopy. There is a tendency that the larger the refractive index and dispersion are, the higher the electronic response of optical Kerr effect becomes. Transmission efficiency as high as several percents with the time resolution of 350 fs is obtained using a highly Bi2O3-doped glass plate under 200 kHz repetitive pulse excitation. The time-resolved luminescence spectrum of Zn-tetraphenylporphyrin dye solution is demonstrated using the combination of the OKS and a monochromator equipped with a charge coupled device detector.

Polymer dependence of boson peak frequency studied by hole burning and Raman spectroscopies

Abstract Raman scattering spectra in polymers and phonon sideband spectra observed in hole burning experiments show that the peak frequency of the phonon sideband spectrum corresponds to the low-frequency fracton edge, the so-called boson peak in amorphous materials. The polymer dependence of the phonon sideband spectra indicates that the polymer which has hydrogen bonding has a higher boson peak frequency. The absence of a high-frequency fracton edge in the phonon sideband spectra, which has been observed in the Raman spectra, is attributed to the different frequency dependence of localized-electron-phonon coupling versus dielectric modulation in Raman scattering.

Femtosecond spectral snapshots based on electronic optical Kerr effect

On the basis of an optical Kerr-gate method, we have constructed a high performance system for femtosecond spectral snapshots to measure ultrafast behavior of time-dependent spectrum of weak luminescence. Optical components and their configurations have been investigated to optimize the system. We have measured nonlinear refractive indices and two-photon absorption coefficients of cubic crystals and glasses by the z-scan technique in order to evaluate their performances for an optical Kerr gate. SrTiO3 has been found to be highly efficient and suitable as a Kerr-gate material. Simulations based on the Gaussian-beam propagation were performed to find appropriate configurations to obtain high time resolution. Observation for time-resolved luminescence spectra of β-carotene has revealed that time resolution of 200 fs is achieved in a wide range of wavelengths, which is limited by the pulse width of the gating light.

Intramolecular energy transfer in covalently linked polypyridine rhenium(I)/ruthenium(II) complexes

Abstract A novel series of covalently-linked polypyridine Re(I)/Ru(II) binuclear complexes [LRe(I)(CO) 3 MebpyCH 2 CH(OH)CH 2 MebpyRu(II)(bpy) 2 ] (L = Cl, 4-methylpyridine, N-methylimidazole, or acetonitrile) 1 , has been synthesized. The emission spectra of 1 (excitation wavelength: 360 nm) showed a nearly complete quenching of Re I →π * (bpy) MLCT emission and the enhancement of Ru II →π * (bpy) MLCT emission. The rate constants (k en ) of the intramolecular energy transfer ranged from 1.7 × 10 8 to 1.2 × 10 9 s −1 . The relation between k en and the energy gap was discussed in terms of the energy gap law.

Determination of weighted density of states of vibrational modes in Zn-substituted myoglobin

Abstract Site-selective fluorescence spectroscopy has been performed for Zn-substituted myoglobin in the lowest optical absorption band at 4 K. The single-site fluorescence spectrum as well as the distribution of the zero-phonon transition energy have been obtained. Furthermore, the density of states of vibrational modes of myoglobin weighted by the coupling strength between the chromophore and the polypeptide chain has been determined from the analysis of the obtained spectrum. This is the first case in which the weighted density of states of low-frequency vibrational modes has been determined in biological materials without employing any model shape functions. The results are compared with those for dye-doped polymers.

Optical properties of Sm-doped ZnS nanocrystals

In order to understand the spectral characteristics of ZnS: Sm nanocrystals, in which a novel optical memory effect has been observed, optical properties have been studied between 2 and 300 K for the samples prepared by a solution method. The broad fluorescence and excitation bands observed in the visible region are tentatively assigned to the transitions between the 4f 6 and 4f 5 5d states of Sm 2 + and also to the transitions of excitons bound to Sm 2+ . Temperature dependence of the intensity of the lowest-energy emission band and the dependence of its peak position on the excitation wavelength have been explained well by these assignments.

Femtosecond optical Kerr-gate measurement of fluorescence spectra of dye solutions

Abstract Utilizing the ultrafast electronic response of Kerr effect of glass material as an optical gate, we have constructed a system for the time-resolved fluorescence measurement in the 100-fs time region. We have demonstrated that the ultrafast Kerr-gate technique combined with multi-channel spectral detection enables us to the direct measurement of the spectral shape. Because of highly efficient gating and simultaneous acquisition of all the spectral components, the time-resolved fluorescence spectra of high S/N ratio have been obtained for several dye solutions. Although wavelength-dependent time delay of the system is found to remain, direct recognition of the dynamic Stokes shift in the S 2 →S 0 fluorescence of malachite green in ethanol has been made by comparing the obtained time-resolved spectra with those of the ultrafast fluorescence of β-carotene in hexane.

Coherent oscillations in ultrafast fluorescence of photoactive yellow protein.

The ultrafast photoinduced dynamics of photoactive yellow protein in aqueous solution were studied at room temperature by femtosecond fluorescence spectroscopy using an optical Kerr-gate technique. Coherent oscillations of the wave packet were directly observed in the two-dimensional time-energy map of ultrafast fluorescence with 180 fs time resolution and 5 nm spectral resolution. The two-dimensional map revealed that four or more oscillatory components exist within the broad bandwidth of the fluorescence spectrum, each of which is restricted in the respective narrow spectral region. Typical frequencies of the oscillatory modes are 50 and 120 cm(-1). In the landscape on the map, the oscillatory components were recognized as the ridges which were winding and descending with time. The amplitude of the oscillatory and winding behaviors is a few hundred cm(-1), which is the same order as the frequencies of the oscillations. The mean spectral positions of the oscillatory components in the two-dimensional map are well explained by considering the vibrational energies of intramolecular modes in the electronic ground state of the chromophore. The entire view of the wave packet oscillations and broadening in the electronic excited state, accompanied by fluorescence transitions to the vibrational sublevels belonging to the electronic ground state, was obtained.

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.