Hideki Ichida

Bound-biexciton photoluminescence in CuCl thin films grown by vacuum deposition

We report on the photoluminescence properties of CuCl thin films with the thicknesses of 20 and 100 nm under intense-excitation conditions produced with a pulsed nitrogen laser. We have clearly observed two photoluminescence bands with a superlinear (almost quadratic) excitation-power dependence. The high-energy band is attributed to the well-known free-biexciton photoluminescence. The excitation-power dependence of the low-energy band exhibits a saturation behaviour when the free-biexciton band grows remarkably. In addition, the energy and shape of the low-energy band do not change with the excitation power, which is contrary to the characteristics of the free-biexciton photoluminescence reflecting the thermal distribution in the energy dispersion. From these facts, we conclude that the origin of the low-energy band is a bound biexciton. Furthermore, we find from the temperature dependence of the PL properties that the thermal dissociation of the bound exciton induces the instability of the bound biexciton.

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.

Selective Photoinduced Energy Transfer from a Thiophene Rotaxane to Acceptor

An energy transfer process was investigated using cyclodextrin-oligothiophene rotaxanes (2T-[2]rotaxane). The excited energy of 2T-[2]rotaxane is transferred to the sexithiophene derivative which is included in the cavity of β-CD stoppers of 2T-[2]rotaxane.

Transient vibronic structure in ultrafast fluorescence spectra of photoactive yellow protein.

The ultrafast photo‐induced dynamics of wild‐type photoactive yellow protein and its site‐directed mutant of E46Q in aqueous solution was studied at room temperature by femtosecond fluorescence spectroscopy using the optical Kerr‐gate method. The vibronic structure appears, depending on the excitation photon energy, in the time‐resolved fluorescence spectra just after photoexcitation, which winds with time and disappears on a time scale of sub‐picoseconds. This result indicates that the wavepacket is localized in the electronic excited state followed by dumped oscillations and broadening, and also that the initial condition of the wavepacket prepared depending on the excitation photon energy affects much the following ultrafast dynamics in the electronic excited state.

Ultrafast Dynamics of Photoactive Yellow Protein via the Photoexcitation and Emission Processes

Pump‐dump fluorescence spectroscopy was performed for photoactive yellow protein (PYP) at room temperature. The effect of the dump pulse on the population of the potential energy surface of the electronic excited state was examined as depletion in the stationary fluorescence intensity. The dynamic behavior of the population in the electronic excited state was successfully probed in the various combinations of the pump‐dump delay, the dump‐pulse wavelength, the dump‐pulse energy and the observation wavelength. The experimental results were compared with the results obtained by the femtosecond time‐resolved fluorescence spectroscopy.

Theory of the lifetime of an exciton incoherently created below its resonance frequency by inelastic scattering

When an exciton in semiconductor is scattered and its energy is decreased far below the resonance energy of the bare exciton state, it has been considered that an exciton-polariton is created immediately by the scattering process because there is no exciton level at that energy. However, according to the recent time-resolved measurements of P emission originating from inelastic exciton-exciton scattering, it looks rather natural to consider that the exciton-polariton is created in a finite time scale which is restricted by a coherence volume of the exciton after the scattering. In this interpretation, the exciton remains in this time scale far below its resonance energy as a transient state in a series of processes. We propose an expression of the P-emission lifetime depending on the coherence volume of the scattered excitons through the conversion process from them to the polaritons. The coherence volume of the scattered excitons appears in the calculation of the inelastic scattering process on the assumption of a finite coherence volume of the bottleneck excitons. Time-resolved optical-gain measurements could be a way for investigating the validity of our interpretation.

Entire view of coherent oscillations in ultrafast fluorescence for photoactive yellow protein

Remarkable oscillatory components are observed in the 2-dimentional time-wavelength map of ultrafast fluorescence for photoactive yellow protein, by using the optical Kerr gating system with 180-fs time resolution and 5-nm spectral resolution.

Hierarchical construction of SHG-active polar crystals by using multi-component crystals

Organic salts composed of chiral amines and sulfonic acid with high hyperpolarizability allowed the construction of polar crystals with incorporated guest molecules. The polarity of the crystals was precisely regulated by employing suitable guest molecules. As a result, the crystals generated a strong second harmonic generation property.

Single-shot time-frequency imaging of phonon-polariton dispersion in ferroelectric LiNbO 3

Single-shot observation of phonon-polariton dispersion in LiNbO3 is demonstrated using optical Kerr-gate method with an echelon mirror. Forward- and backward-propagating phonon-polaritons are observed in the time-frequency two-dimensional image.

Ultrafast Dynamics of Electron-Hole Plasma Coupled to Optical Phonons in a ZnO Thin Film

We have investigated ultrafast photoluminescence dynamics of electron-hole plasma coupled to longitudinal optical phonons in a ZnO thin film. The dynamical change of the electron-hole-pair density is characterized by time-resolved photoluminescence spectra measured with an optical-Kerr-gating method.