Hideyuki Kunugita

We observed intersubband transition in GaN/AlN multiple quantum disk (MQD) in nanocolumns by absorption spectra. We also investigated ultrafast relaxation dynamics at 1.55 mu m by time-resolved saturable absorption. We have found that the temporal response of nanocolumns is almost the same as the multiple-quantum-well (MQW) sample. This indicates that the delay caused by multiple scatterings in MQD nanocolumns is negligible. The saturation intensity for the Brewster configuration in MQD was also estimated. (C) 2007 Elsevier B.V. All rights reserved.

The dynamics of photoexcited carriers in standard TiO2 photocatalytic powders has been studied by investigating the time decay and temperature dependence of the photoluminescence. A biexponential decay curve in a time-resolved signal suggests that there are different formation processes of the self-trapped exciton states. The peak position of the luminescence spectrum shifted to a higher energy at room temperature. On the basis of these experimental results, we propose a relaxation model of photoexcited carriers.

We observed coherent phonon oscillations in zinc blende and wurtzite wide-gap semiconductors (ZnTe, ZnS, ZnO). We clarified the different relaxation mechanism of the coherent oscillations between LO phonon in zinc blendes and non polar mode in ZnO. Resonant effect of coherent phonon generation is also studied.

We have investigated the 2s-2p splitting of excitons in an organic-inorganic hybrid quantum well material, (C4H9NH3)PbBr4, by measuring one-photon and two-photon excitation spectra. Generally, energy level of 2s is lower than that of 2p for usual Wannier excitons because of the polaron effect. For the hybrid system, however, we have found that the energy level of 2p is lower by 20 meV than that of 2s. This large and opposite splitting of 2s-2p excitons can be explained by considering image charge effect.

We have investigated the detailed ultrafast electronic relaxation dynamics and coherent nonlinearity at optical communication wavelength in GaN/AlN multiple quantum wells (MQWs), using spectral-integrated and resolved two-color pump-probe techniques in a wide energy range. First, we found that the ISBT relaxation dynamics consists of the ultrafast (similar to 140 fs) intersubband relaxation and slow (similar to 1.3 ps) carrier cooling process. Second, we have observed coherent spectrum oscillations, which show that the ISBT dephasing time is more than 100 fs at a room temperature. Finally, we have observed the polarization-dependent absorption in waveguide structures.

We have investigated the relaxation process of photo-excited carriers in anatase TiO2 films, in which the concentration of the oxygen vacancy is controlled by annealing in ultra high vacuum. A broad luminescence band peaked at 2.32 eV is ascribed to a recombination of self-trapped excitons (STEs). The luminescence decay curve was composed of several time constants corresponding to the direct and indirect formation of STEs. We have confirmed that the longer annealing, i.e. higher concentration of the oxygen vacancy, leads to an increase in the indirect STEs.

Photoluminescence and spin-flip Raman scattering of excitons in an organic-inorganic quantum well material (C4H9NH3)(2)PbBr4 have been investigated at low temperature. The estimated exchange energy of the exciton is greatly enhanced by strong confinement and by image charge effects, and ranges from 28 to 32 meV. These values are one or two orders of magnitude larger than those for quantum wells made from the more usual inorganic materials. The ratio of the oscillator strength of the dipole-allowed exciton to that of the dark (dipole-forbidden) exciton is estimated to be of order 800.

We have investigated the ultrafast coherent dynamics of the intersubband transition in GaN/AlN multiple quantum wells, using a spectral resolved two-color pump-probe technique. We have found a significant spectrum change as a function of the delaytime tau. For negative delaytimes corresponding to tau < 0, coherent spectrum oscillations have been observed. At tau similar to 0, asymmetric dispersion has been observed. For positive delaytimes, tau > 0, no measurable spectrum change has been observed. From the analysis of these results, we have estimated that intersubband transitions dephasing time is more than 100 fs measured at room temperature.

In polyacetylene and polydiacetylene oligomers, photoluminescence (PL) is induced from the dipole-forbidden state by vibronic coupling. The assumption that the PL in polythiophene (PT) and poly(p-phenylene vinylene) (PPV) also includes this induced component, consistently explains many experimental results. In this work, we focus on the temperature dependence of the time-integrated and time-resolved PL spectra of a PT derivative and unsubstituted PPV, and demonstrate the validity of the above model for their temperature dependence. Applying this model to the hitherto unresolved drastic PL spectral change of unsubstituted PPV, we propose that the spectral change and the spectral shape itself can be understood as resulting from a contribution to the PL from the 2A(g) state coupled with a change in the ordering of the 2A(g) and the 1B(u) states at low temperatures.

We have investigated intersubband relaxation in GaN/AlN multiple quantum wells using a two-color pump-probe technique in a wide energy range around 800 meV (1.55 mu m). We have observed not only absorption bleaching signals but also, for the first time, induced absorption signals with an ultrafast and a slow component. Absorption bleaching signals are attributed to a phase space filling of the upper subband and a carrier cooling process in the lower subband, with relaxation times of 140 fs and 300-400 fs, respectively. The ultrafast induced absorption corresponds to the absorption of electrons during intra-subband relaxation and the slow component of 1.3 ps corresponds to the carrier cooling process. We have also found that the intrinsic absorption width of highly doped materials is as large as the inhomogeneous width, because the dispersion difference of subbands is enhanced by exchange interaction.

Photoinduced effects in a single crystal of bilayered manganites, La2-2xSr1+2xMn2O7 (x = 0.38), were investigated in a wide range of temperatures by pump-probe measurement at a photon energy of 1.6 eV. In a ferromagnetic metallic state, significant enhancement of positive rise in differential reflectivity with a slow relaxing time of 100 ps was observed just below T-C = 127 K, indicating that the reflectivity change with the slow relaxation time constant is induced by laser heating. We have also observed an unconventional fast relaxing component that has a time constant of the order of 10 ps. This fast relaxing component, whose absolute value has an asymmetric peak at T-C, is presumably due to short-range correlation of Jahn-Teller distortion. (C) 2004 Elsevier Ltd. All rights reserved.

Organic-inorganic quantum well materials, (CnH 2n+1NH3)2PbX4 (X=I, Br), form self-organized two-dimensional (2D) systems, where inorganic semiconductor well layers are sandwiched by organic barrier layers. Owing to strong quantum confinement and dielectric enhancement effect, ideal 2D-Wannier exciton exists in these materials [1]. It has been founded that the binding energies of the exciton (Eex) and the biexciton (Ebi) are extremely large (Eex ∼360meV and Ebi ∼40meV for (C 6H13NH3)2PbI4 [1], E ex ∼480meV and Ebi ∼60meV for (C4H 9NH3)2PbBr4 [2]), which are the largest values ever reported. Thanks to the large binding energies, these materials are quite suitable to investigate various properties of 2D-exciton system. In this study, we have investigated the fine structure of the excitons in (C6H13NH3)2PbI4and (C4H9NH3)2PbBr4 using photo-luminescence (PL) measurement under magnetic field and spin-flip Raman scattering (SFRS) measurement.

We observed coherent phonon oscillations in ZnTe by means of a transmission electro-optic sampling using noncollinear optical parametric amplifier. We investigated the excitation energy dependence of the coherent phonon amplitude.

We have investigated intersubband relaxation dynamics in GaN/AlN multiple quantum wells by the two-color pump-probe technique. We have clarified the ultrafast relaxation scenario which includes thermalization and cooling processes, and found that relaxation is influenced by a hot-phonon effect.

We have investigated the ultrafast relaxation dynamics of intersubband transition (ISBT) in GaN/AIN, using a two-color pump-probe technique, in a wide energy range around the optical communication wavelength. We suggest that the origin of the signal depends on the relation between the pump and probe pulse energies. We have observed an ultrafast induced absorption signal and a slow negative component which are due to the absorption of electrons during intra-subband scattering and a carrier cooling process with a hot-phonon effect, respectively. Moreover, we clarify the origin of the inhomogeneous broadening width of the ISBT and of the intrinsic absorption width of ISBT from the detailed analyses of the result. We have reproduced the relaxation dynamics by a numerical calculation to confirm this interpretation of ISBT relaxation dynamics.

Steady-state and time-resolved photoluminescence (PL) measurements on vapor deposited films of a non-ionic bisazomethine dye have been performed. In the films, it is possible to control the ratio between J-aggregate and crystalline phases of the dye by means of exposure to chloroform vapour, and thus the origin of PL can be determined from comparison between several films. In the films, PL was emitted exclusively from the crystalline phase. Although the origin of PL was not from the J-aggregate phase, some features of the observed PL were very similar to those reported for J-aggregates of ionic dyes. We also found that these features in the vapor deposited films were caused by a competition between free excitons and excitons trapped at defect sites.

Using a degenerate pump-probe technique on the Brewster configuration at room temperature, we investigate the dynamics of the intersubband transition in GaN/AlN multiple-quantum wells. The relaxation dynamics is found to consist of ultrafast (similar to140 fs) and slower (similar to1.3 ps) components. We estimate the third-order susceptibility for the Brewster configuration to be similar to5.5x10(-18) m(2)/V-2, which indicates that its value becomes similar to2.2x10(-16) m(2)/V-2 when a light beam is parallel to the well. (C) 2004 American Institute of Physics.

We have observed intersubband transition dynamics in GaN/AlN. The relaxation dynamics consists of ultrafast and slower components. The origin of these dynamics is discussed. © 2003 Optical Society of America.

We performed pump-probe transient reflectivity measurements at d-d transition of bilayered perovskite manganite La2-2xSr 1+2xMn2O7 (x=0.38). Signals that suggest coexistence of spin-glass state and ferromagnetic metallic state were observed. © 2003 Optical Society of America.

We have confirmed biexciton formation in an organic-inorganic hybrid quantum-well material (C4H9NH3)(2)PbBr4 by photoluminescence and two-photon absorption measurements. The biexciton has extremely large binding energy, 60 meV, which to our knowledge is the largest value ever reported for a semiconductor. By analyzing the spectrum of biexciton luminescence, the biexciton gas temperature was found to be much higher than the bath temperature due to a higher local temperature arising from the large biexciton binding energy. (C) 2003 Elsevier Ltd. All rights reserved.

無機有機複合型量子井戸物質の励起子非線形性におけるスピン緩和の寄与を議論した。

Time-resolved photoluminescence (PL) measurements on highly ordered and disordered films of a polythiophene (PT) derivative were compared. Although delocalization lengths of π electrons and the strengths of interchain interaction are quite different between the two films, a biexponential decay and a change in spectral shape, from one with a large 0-0 transition to one with a large 0-1 transition, were commonly observed. Therefore, we conclude that PL spectra of PT consist of two components having different spectral shapes and different lifetimes, and that these components originate from intrachain odd- and even-parity states that are coupled by electron-phonon interaction. This model explains the greatly reduced fluorescence yield for the ordered film as well as the time-resolved PL measurements. © 2003 The American Physical Society.

Time-resolved photoluminescence (PL) measurements on highly ordered and disordered films of a polythiophene (PT) derivative were compared. Although delocalization lengths of pi electrons and the strengths of interchain interaction are quite different between the two films, a biexponential decay and a change in spectral shape, from one with a large 0-0 transition to one with a large 0-1 transition, were commonly observed. Therefore, we conclude that PL spectra of PT consist of two components having different spectral shapes and different lifetimes, and that these components originate from intrachain odd- and even-parity states that are coupled by electron-phonon interaction. This model explains the greatly reduced fluorescence yield for the ordered film as well as the time-resolved PL measurements.

We studied biexciton dynamics of an layered perovskite-type quantum-well material (C6H13NH3)2PbI4. We estimated biexciton binding energy to be 60 meV. The equilibrium temperature of biexciton is 160K which is much higher than the lattice temperature.

We observed polariton beats and optical precursors in propagation experiments at exciton resonance in CuCl, and obtained good agreement between experimental results and numerical calculations using a two-oscillator model. The observation of more than thirty periods of beatings enables us to determine the polariton parameters and the dephasing constants. At higher intensities we observed a suppression of the polariton beat and found that the suppression is well reproduced by excitation induced dephasing.

Pulse propagation experiments were performed just at the Z3-exciton resonance of CuCl. Polariton beats and an optical precursor were observed clearly. The exciton in CuCl has very large oscillator strength and long decay time. With increasing input-pulse intensity, polariton beats are strongly suppressed. These experimental results were confirmed with Lorentzian oscillator model.

An overview is given on the biexcitons with extremely large binding energy (=66 meV) in an organic-inorganic hybrid quantum-well material, (C4H9NH3)2PbBr4. This material belongs to the layered perovskite-type quantum-well materials (CnH2n+1NH3)2- PbX41-5 which have excitons with large binding energy. The dynamics of such a stable exciton-biexciton system are outlined.

The time-resolved photoluminescence spectrum of a polythiophene derivative has been analyzed from the viewpoint of the vibronic structure and its dynamics. We found that the photoluminescence (PL) originates from two vibronic coupled states which consist of the odd-parity 1B(u) component and the even-parity 2A(g) component. Since the fluorescences from the two states have different time constants and different vibronic structures, the PL shows a biexponential decay and a spectral change.

無機有機複合型量子井戸物質を用いた光信号の超高速シリアル－パラレル変換について発表した。

We report a simple method of a serial-to-parallel conversion that does not use a Fourier-transform system. The method is based on directly picking up a narrow temporal range of the skewed input signal by using a nonlinear wave mixing with the reference pulse. We show theoretically that the pick-up method is less influenced by the relaxation times of the nonlinear material. We demonstrate our method experimentally using a self-organized quantum-well material, and confirm that T-bits/s pulses are clearly converted into spatial patterns with conversion rates of 140 GHz.

Transient nondegenerate four-wave mixing was performed in the femtosecond domain on a stable exciton-biexciton system. For excitation with two spectrally narrow pulses of frequencies omega (1) and omega (2) that have no spectral overlap with each other, only a frequency-mixing signal at 2 omega (1)-omega (2) was observed. The polarization dependence of the frequency-mixing signal intensity changed dramatically with increasing frequency difference between the incident pulses. Our results demonstrate that the frequency-mixing signal is strongly correlated with the exciton dynamics and the dramatic change of its polarization dependence is caused by the exciton-exciton interactions.

Transient nondegenerate four-wave mixing was performed in the femtosecond domain on a stable exciton-biexciton system. For excitation with two spectrally narrow pulses of frequencies ω1 and ω2 that have no spectral overlap with each other, only a frequency-mixing signal at 2ω1−ω2 was observed. The polarization dependence of the frequency-mixing signal intensity changed dramatically with increasing frequency difference between the incident pulses. Our results demonstrate that the frequency-mixing signal is strongly correlated with the exciton dynamics and the dramatic change of its polarization dependence is caused by the exciton-exciton interactions. © 2001 The American Physical Society.

Summary form only given. Time-resolved photoluminescence and pump-probe spectroscopies have been employed to study the spin relaxation processes of excitons in bulk semiconductors and semiconductor quantum wells (QWs). Extensive studies have shown that the spin dynamics in QWs differs from that in the bulk. In spite of many studies on excitons with large Bohr-radius such as in GaAs-based QWs, studies on excitons with small Bohr-radius are lacking. Since spin relaxation greatly depends on the exchange interaction between electron and hole in the exciton, the spin dynamics is expected to change with the coupling strength between the electron and hole. Therefore, experiments for excitons with small Bohr-radius are desired for full understanding of spin phenomena in QWs.

We report a time-to-space conversion technique using a material which has a large chi ((3))(similar or equal to 10(-6) esu) and a fast response time (<7 ps) at room temperature. The material is a self-organized quantum-well system consisting of inorganic well layers and organic barrier layers. We achieve a high conversion sensitivity even for nJ-order optical pulses. We demonstrate serial-to-parallel conversion of nJ-order Tbits/s signals at room temperature with conversion rates of 140 GHz. (C) 2000 American Institute of Physics. [S0003-6951(00)00448-4].

We measured the time-resolved photoluminescence of highly ordered and disordered films of a polythiophene derivative, poly(3-[2-((S)-2-methylbutoxy)ethyl]thiophene). The formation of self-trapped excitons (STE's) within 10 ps after photoexcitation and the subsequent migration of the STE's are common behavior in both films. The spectral narrowing of the phonon sidebands within 10 ps after photoexcitation is associated with the formation process of STE's and the spectral changes on a few hundred ps time scale are explained by the STE migration and the dependence of the electron-phonon coupling on the effective conjugation length.

We measured the time-resolved photoluminescence of highly ordered and disordered films of a polythiophene derivative, poly(3-[2-((S)-2-methylbutoxy)ethyl]thiophene). The formation of self-trapped excitons (STE's) within 10 ps after photoexcitation and the subsequent migration of the STE's are common behavior in both films. The spectral narrowing of the phonon sidebands within 10 ps after photoexcitation is associated with the formation process of STE's and the spectral changes on a few hundred ps time scale are explained by the STE migration and the dependence of the electron-phonon coupling on the effective conjugation length.

非縮退型四光波混合信号の偏光依存性を調べることにより、非線形性への励起子分子の寄与を明らかにした。

非縮退型四光波混合法を用いて、無機有機複合型量子井戸物質における励起子間相互作用を明らかにした。

四光波混合過程を用いた光信号の時空間変換のデモンストレーションを行なった。

We measured the absorption and \chi((3))\ spectra around the excitonic resonance for Langmuir-Blodgett (LB) film of polythiophene derivative, poly(3-[2((S)-2-methylbutoxy)ethyl]thiophene) at 10 K and room temperature. We derived a theoretical distribution function for the effective conjugation length taking the molecular weight distribution into consideration, and fitted the calculated absorption and \chi((3))\ spectra to the experimental ones. From the fits, we estimated the average effective conjugation length to be 21 thiophene rings at 10 K, and 15 thiophene rings at room temperature.