山田 泰裕

We studied the trion (charged exciton) formation and recombination dynamics in hole-doped (7,5) single-walled carbon nanotubes (SWCNTs) by performing femtosecond transient absorption spectroscopy. The doping of SWCNTs with holes leads to a fast decay component from an exciton to a trion, and the trion decays with a lifetime of a few picoseconds. The experimental results can be explained by a quantized model accounting for the dark exciton and trion states and the hole number distribution in a SWCNT. Our findings show that the optical responses of SWCNTs can be manipulated by doping of SWCNTs with a small number of holes. (C) 2013 AIP Publishing LLC.

We have studied the photocarrier localization and recombination dynamics in Cu2ZnSnS4 single crystals at room temperature. The band-gap energy and tail states below the band edge were evaluated by a combination of photoluminescence (PL), PL excitation, photocurrent, and femtosecond transient reflectivity spectroscopy. The photocarriers are rapidly localized to shallow tail states within a typical time constant of several picoseconds to a few tens of picoseconds. The sub-nanosecond PL decay dynamics indicate the importance of multiple carrier trapping processes in the shallow tail states. Therefore, it is concluded that the tail states dominate the optical responses of Cu2ZnSnS4 single crystals. (c) 2013 AIP Publishing LLC.

We report photoluminescence (PL) spectra of typical wide-gap perovskite semiconductors, SrTiO3, KTaO3, BaTiO3, LiNbO3, and LiTaO3. In SrTiO3, KTaO3, and BaTiO3, undoped crystals show no PL at room temperature, while broad PL bands appear by Ar+ irradiation. The room temperature PL properties of SrTiO3, KTaO3, and BaTiO3 are very similar to each other, but are different from those of LiNbO3 and LiTaO3. At low temperatures, only Ar+-irradiated SrTiO3 crystals show sharp band-edge PL, corresponding to the band-to-band radiative recombination of free electrons and holes. Unique PL behaviors in SrTiO3 are discussed, compared to other perovskite oxides. (c) 2011 Elsevier B.V. All rights reserved.

Transparent inorganic luminescent materials have attracted considerable scientific and industrial attention recently because of their high chemical durability and formability. However, photoluminescence dynamics of ns2-type ions in oxide glasses has not been well examined, even though they can exhibit high quantum efficiency. We report on the emission property of Sn2+-doped strontium borate glasses. Photoluminescence dynamics studies show that the peak energy of the emission spectrum changes with time because of site distribution of emission centre in glass. It is also found that the emission decay of the present glass consists of two processes: a faster S1-S0 transition and a slower T1-S0 relaxation, and also that the energy difference between T1 and S1 states was found to be much smaller than that of (Sn, Sr)B6O10 crystals. We emphasize that the narrow energy gap between the S1 and T1 states provides the glass phosphor a high quantum efficiency, comparable to commercial crystalline phosphors.

The exciton decay dynamics in hole-doped single-walled carbon nanotubes (SWCNTs) has been studied by femtosecond pump-probe transient absorption (TA) spectroscopy. By the doping of SWCNTs with holes, a fast decay component with the lifetime of a few picoseconds appears in TA signals, which corresponds to exciton decay through the Auger recombination between an exciton and a hole and the trion formation. We revealed that this exciton decay rate is quantized by the number of holes in a single SWCNT. The number of holes of a hole-doped SWCNT is successfully evaluated on the basis of TA decay dynamics.

We studied the band-edge photoluminescence (PL) of electron-doped SrTiO3 and strongly photoexcited SrTiO3 crystals. Two band-edge PL peaks are clearly observed at low temperatures. The PL peak energies coincide with the high- and low-temperature onsets of the optical absorption, corresponding to the phonon-assisted band-to-band optical transition. This result is the direct evidence that the free electrons and holes exist in the conduction and valence bands, and radiative recombination of free electrons and free holes causes the band-edge PL. (C) 2011 Elsevier B.V. All rights reserved.

The dynamical behavior of photoexcited states of TiO2 governs the activities of TiO2-based solar cells and photocatalysts. We determined the lifetimes of photoexcited electrons and holes in rutile and anatase TiO2 single crystals by combining advantages of time-resolved photoluminescence, photoconductance, and transient absorption spectroscopy. Electrons and holes in rutile show exponential decays with the lifetime of a few tens of nanoseconds, while non-exponential decays are observed in anatase, indicating the presence of multiple carrier trapping processes. We revealed the generic features of the carrier recombination processes in rutile and anatase TiO2. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754831]

We studied photoluminescence (PL) spectrum and dynamics of SrTiO3 nanoparticles. A broad blue PL band is observed at around 2.9 eV in nanoparticle thin films. The excitation-density dependence of the blue PL intensity indicates that nonradiative Auger recombination dominates the carrier recombination process under strong photoexcitation. The intrinsic Auger recombination rate in SrTiO3 nanoparticles is much larger than that of SrTiO3 bulk crystals. The origin of enhanced Auger recombination in SrTiO3 nanoparticles is discussed. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3626594]

We summarized our recent optical studies on perovskite semiconductor SrTiO3 crystals. Photocarrier recombination dynamics of undoped, electron-doped, and vacancy-doped SrTiO3 crystals were studied by photoluminescence (PL) and transient absorption (TA) spectroscopy. By high-density photoexcitation and electron doping, the blue PL is clearly observed in SrTiO3 crystals at room temperature. The PL spectrum and dynamics are sensitive to the photoexcited carrier density and the chemically doped electron density. The PL and TA decay dynamics under high-density excitation are determined by nonradiative Auger recombination. It is demonstrated that the PL lifetime can be used as a probe for determining spatial profiles of carriers and defects in SrTiO3. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We studied the temperature dependence of the photoluminescence (PL) spectrum and dynamics of rutile-type TiO2 single crystals. Rutile TiO2 shows sharp exciton-PL lines at low temperatures. A broad blue PL band appears at around 2.8 eV under high-density photoexcitation using a pulse laser at room temperature. We discussed the temperature-dependent photocarrier dynamics of rutile TiO2 in terms of Auger recombination and single-carrier trapping processes. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The photoinduced dynamics of the lattice deformation and the relaxed excited state in SrTiO3 is studied by stationary- and transient-birefringence and pump-probe measurements in comparison with the doping-induced ferroelectric lattice deformation in Ca-doped SrTiO3. Ca-doped SrTiO3 is found to exhibit first-order-like structural deformation with a small threshold UV intensity (similar to 10(-3) mW/mm(2)), and it is also found to exhibit a shift in the ferroelectric transition temperature under UV illumination. The photoinduced lattice deformation has the same order of magnitude as the ferroelectric lattice deformation, and it is not affected by the electric field. The occupation probability in the relaxed excited state is found to increase significantly at low temperatures. These experimental results suggest that the photoinduced giant dielectric response of SrTiO3 is consistent with the conductive-region model rather than the ferroelectric cluster model. SrTiO3 is a peculiar material in which dielectricity of the Maxwell-Wagner type appears in a stoichiometric material only under UV illumination.

We studied photoluminescence (PL) spectrum and dynamics of undoped and electron-doped SrTiO3. At low temperatures below 100 K, we observed two band-edge PL peaks and a broad PL band in strongly photoexcited SrTiO3 and electron-doped SrTiO3 samples. In electron-doped samples, the peak energy of the broad PL band depends on the excitation photon energy at low temperatures: it changes from 2.5 to 2.9 eV with the excitation photon energy. On the other hand, the PL dynamics is independent of the excitation photon energy and is explained by a simple model involving single-carrier trapping and Auger recombination. From the PL spectrum and dynamics under different excitation photon energies, the PL processes in SrTiO3 are discussed. (C) 2011 American Institute of Physics. [doi :10.1063/1.3577613]

We report the observation of blue photoluminescence (PL) from highly photoexcited rutile TiO2 single crystals at room temperature. Under extremely high-density excitation, we found an anomalous dependence of the PL dynamics and intensity on the excitation density. By considering the nearly degenerate conduction bands of rutile, we revealed that the PL behavior is determined by the electron density in the lower-energy conduction band. The PL decay dynamics is well explained by the simple model involving two-body radiative recombination and three-body Auger recombination processes. We discuss the origin of the blue PL band.

We report the observation of band-edge photoluminescence (PL) in highly photoexcited SrTiO3 and electron-doped SrTiO3 at low temperatures. Two band-edge PL peaks coincide with the low- and high-temperature onsets of optical absorption. This clearly shows that band-edge PL peaks correspond to indirect band-to-band radiative recombination involving phonon emission and absorption processes and allows a determination of the band gap. The PL peaks redshift with increasing carrier density, indicating band-gap shrinkage. The temperature dependence of the band-edge PL and optical absorption spectra are also discussed in conjunction with phonon-assisted optical transitions.

The doping-induced ferroelectric phase transition in Ca-doped SrTiO3 is investigated by observing the birefringence and coherent phonons. The structural phase-transition temperature is determined by the birefringence measurement. Coherent phonons of the soft modes are studied by using ultrafast polarization spectroscopy. Two phonon modes are observed to be softened toward the ferroelectric phase-transition point at 28 K and other two phonon modes are observed to be softened toward the structural phase-transition point at 180 K. Another structural deformation at 25 K is found below the ferroelectric phase-transition temperature. From the temperature dependence of phonon frequencies under an ultraviolet (UV) illumination, a shift of the ferroelectric phase-transition point toward the lower temperature side is found. A decrease in phonon frequencies after the UV illumination is also found.

We report unusual photoluminescence (PL) behaviors in highly photoexcited SrTiO3 crystals at low temperatures. The PL spectrum and dynamics show abrupt changes below 150 K in both nondoped and electron-doped SrTiO3 samples. We clarified that the PL dynamics in both nondoped and electron-doped SrTiO3 is well described by the same simple model involving single-carrier trapping, radiative bimolecular recombination, and nonradiative Auger recombination. The unusual temperature dependence of PL dynamics is caused by the crossover from Auger recombination at high temperatures to single-carrier trapping at low temperatures. We discuss the temperature-dependent PL dynamics in conjunction with the high carrier mobility of SrTiO3 at low temperatures.

The photo-induced effects in quantum paraelectric materials, strontium titanate SrTiO3 and Ca-doped SrTiO3, are investigated. The observed birefringence for Ca-doped SrTiO3 shows that an optical anisotropy along the [100] axis is generated by the ultraviolet (UV) illumination. The transient birefringence in a pulsed electric field is studied to probe the dynamical property of the polarization under dark and UV illumination. Under the UV illumination, the change of birefringence is reduced, and the dielectric response of relaxation type was observed both in SrTiO3 and in Ca-doped SrTiO3. It was found that the ferroelectric phase transition temperature is shifted toward the lower temperature side under the UV illumination. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We studied photocarrier recombination processes in highly excited SrTiO3 crystals using pump-probe transient absorption (TA) and photoluminescence (PL) spectroscopy at room temperature. TA signals of nondoped SrTiO3 crystals clearly appear in the visible and infrared spectral region under intense interband photoexcitation, and TA spectra show Drude-like photon-energy dependence. Both TA and PL decay curves are well explained by the same simple rate equation including three-body Auger recombination and single-carrier trapping. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3238269]

The dielectric response of ferroelectric Sr1-xCaxTiO3 (SCT) under photoexcitation has been investigated. We observed photoinduced Debye dispersion, which is similar to that observed in nominally pure SrTiO3. The temperature dependence of photoinduced Debye dispersion in SCT indicates that the origin of photoinduced Debye dispersion is the interfacial polarization between conductive and insulative regions. We assumed that conductive regions with mobile photocarriers are partly generated by photoexcitation and showed that the experimental results are well reproduced by the conductive-region model. We also observed a photoinduced reduction in ferroelectric phase transition temperature. This can be quantitatively explained by the reduction of Coulomb interaction between Ca-induced-dipoles due to the screening effect by mobile photocarriers.

We report on the optical determination of the carrier-density profiles near surfaces in SrTiO3 crystals by means of photoluminescence (PL) dynamics measurements. The PL dynamics under band-to-band excitation depends strongly on the excitation photon energy for different optical penetration depths. In nondoped and Ar+-irradiated SrTiO3 crystals, we evaluate the depth profile of carriers near the surface based on the lifetime of the Auger recombination of electrons originating from oxygen vacancies with photocarriers. Our PL spectroscopy clarifies that in nondoped SrTiO3, the near-surface oxygen-deficient region is a few tens of nanometers in depth.

We have investigated photo-induced effects in SrTi((O1-xOx)-O-16-O-18)(3), which undergoes a ferroelectric transition at low temperatures. The ferroelectric transition temperature is lowered by inter-band excitation with ultraviolet light due to the screening of the Coulomb interaction by photo-induced carriers. Considering a microscopic mechanism, we concluded that the critical temperature shift is explained by the hardening of the TO mode with the screening effect. It is suggested that some dipoles with a nanometric size exist in O-18-isotope-excbanged SrTiO3(STO18) and the cooperative interactions among them bring about ferroelectric ordering. (c) 2004 Elsevier B.V. All rights reserved.

Dielectric properties are studied in quantum paraelectric SrTiO3 under UV-light illumination from 100 Hz to 1 MHz at low temperatures. We demonstrate that the complex dielectric constant is strongly enhanced by interband photo-excitation. Dispersion measurements confirm that the photo-induced Debye-type relaxator enhances the dielectric constant. The anomalous temperature and excitation-density dependence of the relaxation time suggests that photo-induced polar domains with finite lifetime lead to the novel dielectric properties.