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A femtosecond laser pulse of a fiber laser is amplified in dye gain media. To evaluate the conditions for the amplification of a femtosecond laser pulse, the time-resolved measurements of gains in OXAZINE 750, DOTC, and LDS 821 dye solutions are carried out by using a TEA N2 laser (λ= 337.1 nm, FWHM= 0.6 ns), a TE N2 laser (λ= 337.1 nm, FWHM= 6 ns) and the second harmonic of a Nd:YAG laser (λ= 532 nm, FWHM= 5 ns), as the pump sources. The amplified output energy of the fs laser pulse is 1 μJ, and the pulse width is 180 fs (FWHM). These output energy and pulse width are comparable to those of a typical CPA laser system, and the cost performance is very high.

We have developed a pulsed laser with a pulsewidth of 70 ps in the full-width at half-maximum as a light source for calibrating photon detectors. It has a simple electric circuit that generates short electric pulses that are applied to a laser diode. By utilizing the properties of the diode, a laser beam with a pulsewidth that is one-tenth of the applied electric pulse can be generated. Since the electric circuit is a simple shaping circuit, the pulsed laser can be generated at low cost, and the device can be made compact, occupying only a few cubic centimeters. Such short-pulsed-light sources are useful in high-energy experiments and astronomical observation as the time resolution and read-out electronics of the photon detectors are being continually advanced. The performance of this pulsed laser and its application for the calibration of photon detectors are reported in this paper.

We have measured the absorption and Raman spectral change induced by the irradiation of e-beam. By the irradiation of e-beam on SWNTs thin films, the intensity of defect related Raman band increase, and the peak energy of IR absorption bands shift to the higher energy side. These results indicate that the origin of infrared band is due to the plasmon resonance of finite-length SWNT. We have estimated the effective tube length and defect density from IR absorption peak energy.

High-speed-photon detectors are some of the most important tools for observations of high energy cosmic rays. As technologies of photon detectors and their read-out electronics improved rapidly, the time resolution of some cosmic ray detectors became better than one nanosecond. To utilize such devices effectively, calibrations using a short-pulse light source are necessary. We have developed a pulsed laser of 80 picosecond width and adjustable peak intensity up to 100 mW. This pulsed laser is composed of a simple electric circuit and a laser diode. Details of this pulsed laser and its application for quality controls of photon detectors are reported in this contribution.

Optical and magnetic properties of graphene oxide (GO) have been intensively investigated because of the promising applications of GO-related materials in various technical fields. So far, the optical and magnetic properties of GO have been discussed independently. However, localized electronic states in reduced GO may simultaneously add optical transitions and spin moments in sp2 nanodomains in GO nanosheets. In the present study, the structural, optical, and magnetic properties of graphene oxide (GO) photoreduced in an aqueous solution are correlated on the basis of experimental and theoretical investigations. Experimental observations show that photoreduction leads to enhancement of visible absorption, quenching of photoluminescence, and emergence of magnetism. Detailed spectroscopic and microscopic characterizations indicate the presence of photoreduction-produced basal plane C—H bonding and carbon vacancies. Ab initio calculations suggest that the presence of these defects in sp2 nanodomains results ...

The temperature dependence of the optical response in the far-infrared (FIR) region of metallic and semiconducting rich single-walled carbon nanotubes (SWCNTs) was investigated by micro-Fourier transform infrared spectrometry with a focused beam of synchrotron radiation. The temperature dependence of the FIR spectra of both types of SWCNT showed negligibly small variations within a wide temperature range from 4 to 300 K. Upon comparison with a theoretical model for the diffusive region, it is speculated that these results might have been caused by a strong suppression of phonon scattering in relatively short CNTs with lengths of less than 1 \ensuremath{\mu}m.

An Er-doped ZnO nanoparticle/organic hybrid was synthesized in situ from zinc acrylate (ZA) and erbium acetate (EA) using methylhydrazine. Nano-sized Er-doped ZnO particles were formed in an organic matrix by hydrolysis and polymerization of ZA–EA at 80 °C. The crystallinity of the Er-doped ZnO particles in the hybrid was dependent upon the hydrolysis temperature and water amount. Analysis by transmission electron microscopy and energy dispersive X-ray analyzer revealed that crystalline ZnO nanoparticles doped with Er were dispersed in the organic matrix. The hybrid film sandwiched between fused silica plates was highly transparent. The Er-doped ZnO particle/organic hybrid showed a photoluminescence peak at 0.81 eV (1.54 μm) attributed to the transition of Er3+ ions.

Absorption spectra of high purity metallic and semiconducting single-walled carbon nanotubes separated by the density-gradient ultracentrifugation method have been measured in the wide energy region from 1 meV to 5 eV. In the high purity metallic nanotube sample, a strong and broad absorption band has been observed at 0.06 eV. This observation suggests that the optical properties of even high purity metallic nanotube bundles cannot be explained by the simple Drude conduction model. We discuss the origin of these absorption bands for metallic and semiconducting nanotube samples by considering the existence of a small energy gap in metallic nanotube bundles and plasmon resonance.

Electron spin-spin interaction in asymmetric coupled GaAs/AlGaAs quantum well (QW) is investigated through electron spin precession measurements. In the experiment, the precession (Larmor) frequency of electrons localized in thin and thick QWs are measured by means of polarization- and time-resolved photoluminescence measurements under magnetic field. The Larmor frequency observed in thin well was transformed to that of thick well with increasing excitation power density. This dependence can be reproduced by theoretical calculations, which take into account electron spin-spin interactions in GaAs wells.

A persistent hole-burning is observed in β-perylene microcrystallites, which were embedded in poly-vinyl alcohol. By laser light excitation at 22,535 cm−1 and at 10 K, the hole is found at the excitation photon energy. The mechanism of the persistent hole-burning is interpreted in terms of the resolution of microcrystallites into smaller microcrystallites. This is a novel observation of the persistent hole-burning in aromatic microcrystallites. When the specimen, which includes a hole, is annealed at high temperatures, the resolved microcrystallites restore back to the old position as had been. The β-perylene microcrystallite specimen that we have grown was as small as 1.5 nm in average diameter. They are one order smaller in number of molecules included, compared to those that have been reported on aromatic microcrystallites, anthracene for example. Due to this, we were able to observe the 0–0 transition energy, which varied according as the number of molecules involved in the microcrystallites. We also observed the 0–0 absorption (excitation) spectrum, which depends on the molecular arrays in the microcrystallites. The 0–0 transition of a single molecule in poly-vinyl alcohol matrix is anticipated to be located at 22,885 cm−1.

Eu-doped ZnO particle/organic hybrid was synthesized in situ from zinc acrylate (ZA) and europium acetate (EA) using methylhydrazine. Nanocrystalline Eu-doped ZnO particles were formed in the organic matrix by hydrolysis and polymerization of ZA–EA at 65°C. The crystallinity of Eu-doped ZnO particles in the hybrid was dependent upon the synthesis conditions. TEM and EDX analysis revealed that crystalline ZnO nanoparticles doped with Eu were dispersed in the organic matrix. The hybrid film sandwiched between fused silica plates was highly transparent. Eu-doped ZnO particle/organic hybrid revealed photoluminescence peaks attributed to the transition of Eu3+ ions.

In fluorene(80%)–anthracene(20%) mixed crystals, mainly observed luminescence is broad D2 bands, which originates from an excitation state localized on two anthracene molecules and is accompanied by lattice relaxation. Under high intensity excitation, several sharp luminescence lines emerged in the high energy side of the D2 0–1 bands and their intensity increased superlinearly with excitation intensity. The decay time of the luminescence lines is 30 ps, which is two orders of magnitude smaller than decay time under low intensity excitation. These results suggest that sharp luminescence lines are stimulated luminescence from D2 centers. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have investigated third-order nonlinear optical properties of bundled and isolated semiconducting single-walled carbon nanotubes (SWNTs) by means of Z-scan method, pump–probe method, and two-beam time-resolved degenerate four-wave mixing (DFWM) method. The figures of merit Im χ(3)/α in both bundled and isolated SWNTs samples were found to be enhanced with increasing tube diameter. The measured Im χ(3)/α value in the bundled SWNTs was an order of magnitude smaller than that in the isolated SWNTs. Both population relaxation time T1 and phase-relaxation time T2 for bundled samples were smaller than those in the isolated samples. These experimental results can be explained by an increase in nonradiative recombination rate and phase-relaxation rate in the bundled sample. The phase-relaxation time T2 is considered to have a significant role in the enhancement of Im χ(3)/α.

We report on spin precession observed in CdTe and GaAs bulks at room temperature by means of time-resolved Kerr rotation measurements under a high magnetic field. Electron g-factor in CdTe obtained from the spin precession measurements was significantly different from that in GaAs having similar band gap because of stronger spin-orbital interaction in CdTe. Theoretical analysis on temperature dependence of the g-factor was also conducted, based on 3-band k·p perturbation theory, to estimate the effects of remote bands other than the 3 bands. The dependence of the electron g-factor on temperature was numerically reproduced in low temperature region by considering thermal distribution of electrons. The effects of the remote bands are discussed in relation to the discrepancy between experiments and theoretical analysis in higher temperature region. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have investigated the phase-relaxation process of excitons in semiconducting single-walled carbon nanotubes (SWNTs) by a time-resolved degenerate four-wave mixing (DFWM) method. The phase-relaxation time (T2) estimated from the time evolution of the diffracted DFWM signals strongly depends on the temperature and excitation intensity, shorter T2 with higher temperature and excitation intensity. The experimental trend can be explained by the enhancement of phase-relaxation rate due to frequent exciton–phonon scattering at high temperature and exciton–exciton scattering under high-density excitation conditions. The T2 value at the weak excitation limit measured for semiconducting SWNT with a tube diameter of 1.1 nm at 2 K is estimated to be 350 fs. This T2 value is in good agreement with the homogeneous width estimated from microluminescence measurements. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have studied 1D exciton relaxation dynamics in semiconducting single-walled carbon nanotubes (SWNTs) by femtosecond pump–probe experiments. The time evolution of change in transmittance DT/T induced by photo-excitation varies depending on the tube diameter. The decay time decreases with a decrease in the tube diameter. Pressure measurements have been conducted to explore the relaxation mechanism. The deformation potential estimated from the pressure dependence of photoluminescence spectra increases with decreasing tube diameter. This means that the exciton–phonon interaction becomes stronger in the smaller diameter tubes. The diameter dependences of decay time and deformation potential suggest that the exciton–phonon interaction plays an important role in exciton nonradiative relaxation process in semiconducting SWNTs. r 2007 Published by Elsevier B.V.

The origin of near-infrared (NIR) luminescence from graphene oxide (GO) is investigated by photoluminescence (PL) excitation spectroscopy, time-resolved PL spectroscopy, and density functional theory based many body perturbation theories. The energy of experimentally observed NIR PL peak depends on the excitation energy, and the peak broadens with increasing excitation energy. It is found that the PL decay curves in time-resolved spectroscopy show build-up behavior at lower emission energies due to energy transfer between smaller to larger graphene nanodisc (GND) states embedded in GO. We demonstrate that the NIR PL originates from ensemble emission of GND states with a few nanometers in size. The theoretical calculations reveal the electronic and excitonic properties of individual GND states with various sizes, which accounts for the inhomogeneously broadened NIR PL. We further demonstrate that the electronic properties are highly sensitive to the protonation and deprotonation processes of GND states using both the experimental and theoretical approaches. ASTAR (Agency for Sci., Tech. and Research, S’pore)

Lande g-factor which characterizes the response of electrons to external magnetic field is one of the most important factors in semiconductor opto-magnetic effects. In this report effects of quantum confinements on the electron g-factor in semiconductor quantum wells are discussed. First we introduce Ivchenko's method of calculating g-factor based on k・p high-order perturbation method. Then we examine a simplified k・p method to obtain g-factor in quantum wells. It is found that the simplified method exhibits a good agreement with the experimental trends not only in the unstrained QWs but also in strained QWs.

The effects of quantum confinement on electron spin g-factor have been investigated in lattice-matched GaAs/AlGaAs quantum wells (QWs) and strained InGaAs/GaAs QWs. We demonstrate how the spin precession frequency changes depending on the GaAs well width and the indium fraction in InxGa1–xAs well by time-resolved photoluminescence measurements under a high magnetic field. The g-factor values obtained by the spin precession measurements are compared with precise calculation method proposed by Ivchenko. The calculated results agree with our experimental results in lattice matched QWs. In addition to this calculation, we have examined a simplified k·p perturbation calculation. It is found that the k·p calculation exhibits good agreement with the experimental trends both in the unstrained and strained QW systems. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on photoluminescence (PL) properties in the new type of Eu doped ZnO nanocrystals, nanocrystals/polymer hybrid structure, in which ZnO nanocrystal is covered and chemically bonded by organic polymer. The hybrid samples exhibit strong exciton PL band of ZnO, and weak defect PL. These results suggest that the polymer efficiently suppresses carrier recombination at the nanocrystal surface, preventing formation of defect centers. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Perylene microcrystallites with a wide range of the size were fabricated by a precipitation method. The microcrystallite size distribution was well expressed by a Gaussian function. The luminescence spectra and the luminescence decay times were measured at 7∼10 K. The luminescence decay depended strongly on the power of a laser used for photo-excitation. At weak excitation, the decay exhibited a few nano-second delay in the rise time but at the intense excitation, a sharp rise and a sharp decay (spike) were observed. Those results suggested the presence of strong interaction among microcrystallites. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical properties of single fluorene/phenanthrene crystals doped with various mole fraction of anthracene, 10–5 to a few %, were investigated at 10 K. In low concentration samples, sharp structures attributed to isolated anthracene molecules were observed. Inhomogeneous width in their 0-0 energy were found to be ∼20 cm–1. When concentration is higher, spectral species, which are distinctly different from the isolated molecule spectrum, were observed and their origins are discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have investigated exciton luminescence spectra, decay behaviors, and their temperature dependence in single-walled carbon nanotubes in micelles. The temperature dependence of luminescence spectra can be explained by the one-phonon process associated with the radial breathing mode in the single-walled carbon nanotube. The luminescence decay behavior suggests that the signal is composed of various exponential decays with different decay times. These experimental results are explained by the existence of trapping centers on the nanotube.

Ba2NaNb5O15 (BNN) thin films with a tungsten bronze structure were fabricated using a precursor solution that was synthesized from barium, sodium, and niobium alkoxides. Highly (002)-oriented BNN films were prepared successfully on Pt(100)/MgO(100) substrates at 700°C, using a BNN underlayer. X-ray pole-figure measurement showed that the BNN films that crystallized on Pt(100)/MgO(100) substrates had two in-plane orientations. The remanent polarization and coercive field of the BNN film (thickness of 1.0 µm) that was crystallized at 700°C were 12.3 µC/cm2 and 101 kV/cm, respectively, at –150°C (123 K). BNN films on fused silica substrates exhibited second harmonic generation upon irradiation with 1064 nm light.

We have investigated carrier relaxation dynamics in both metallic and semiconducting single-walled carbon nanotubes by means of femtosecond pump-probe experiments. The femtosecond excitation pulse bleaches the absorption bands of both metallic and semiconducting tubes. In the metallic tubes, the change in the absorption spectra vary from bleaching to broadening. The relaxation aspects of the spectral features change from internal thermalization to electron temperature cooling. In the semiconducting tubes, the nonradiative channel governes the recovery behavior via trapping at defects in the SWNTs.

Highly [110]∕[101]-oriented semiconducting iron disilicide β-FeSi2 continuous films were grown on Si(111) by molecular-beam epitaxy (MBE) using a β-FeSi2 epitaxial template formed by reactive deposition epitaxy. The optimum MBE growth temperature was determined to be about 750°C. At this temperature, the full width at half maximum β-FeSi2(220)∕(202) x-ray diffraction peak was at a minimum. Subsequent MBE overgrowth of an undoped Si layer was performed on the β-FeSi2 at 500°C, resulting in the Si∕β-FeSi2∕Si double heterostructure. After annealing the wafers at 800°C in Ar for 14h, 1.55μm photoluminescence (PL) was obtained at low temperatures. Time-resolved PL measurements elucidated that the luminescence originated from two sources, one with a short decay time (τ∼10ns) and the other with a long decay time (τ∼100ns). The short decay time was thought to be due to carrier recombination in β-FeSi2, whereas the long decay time was due probably to a dislocation-related D1 line in Si.

We have prepared aligned single-walled carbon nanotubes in polymer by mechanical stretching. The polarized absorption and Raman spectra of the aligned samples have been investigated. The optical transitions in nanotubes strongly depend on the angle θ between the polarization of the incident light and the nanotube axis: the absorbance measured at the peak energy shows a maximum at θ=0°, and they are suppressed at θ=90°. Raman spectra also show strong polarization dependence, which is mainly determined by the dependence of the oscillator strength of optical transitions in nanotubes.

This study investigates the possibility that a crack pair that is naturally formed in the course of sample preparation functions as a laser cavity. Lasing with longitudinal mode structures has been reported in mixed-crystal thin films composed of fluorene and anthracene without artificial cavity structure. In the present study, it was shown that lasing is observed only when an excitation spot covers parallel pairs of cracks. The longitudinal mode intervals of the stimulated emission spectra for several crack pairs were found to be proportional to the inverse of the crack spacing. Furthermore, the interval for the artificially fabricated metal cavity followed the same linear relation. These results indicate that the crack pairs can function as laser cavities.

We have investigated the photopolymerization and thermal decomposition of photochemical products with high density excitation (11–480 mW/mm2) by means of time-gated Raman scattering experiments in C60 single crystal. The temperature of laser-illuminated area was measured by Stokes and anti-Stokes Raman scattering of C60 Hg(1) mode, which shows the local temperature rise with increase of laser power density. The Raman intensity of Ag(2) mode rapidly decreases with irradiation time for I

Photopolymerization kinetics of C 60 crystals has been investigated by means of Raman scattering measurements changing sample temperature, laser irradiation intensity and time. It is found that the observed Raman spectrum is composed of monomer, dimer and higher photoproduct components. The dimer is formed as an intermediate phase during the course of the photopolymerization process. The variation of Raman intensity with laser irradiation time has been well explained by a kinetic model taking into account photocreation and thermal decomposition of monomers, dimers and trimers. Photochemical reaction coefficients of dimers and trimers have been obtained from the kinetic model analysis.

We have investigated luminescence spectra, decay behaviors and their temperature dependence in C70 crystals prepared by different annealing procedures. The spectra have been measured with one-photon excitation, two-photon excitations and excitation below the HOMO‐LUMO band gap. We have found two types of defect-related luminescence band with different activation energies: the radiative recombination of Frenkel-like excitons weakly localized at monomolecular-type defects existing near the sample surface and the radiative recombination of excitons trapped at bimolecular-type defects. q 2000 Elsevier Science Ltd. All rights reserved.

We have investigated temporal evolution from the phototransferred metastable state to the ground state, i.e., from the single-valence (SV) to the mixed-valence (MV) states, in the perovskite-type g...

We have grown self-assembled Er-doped InAs quantum dots (QDs) on GaAs (0 0 1) by organometallic vapor-phase epitaxy (OMVPE). InAs QDs are on average 7 nm in height and 20 nm in diameter, and the areal density is 6]1010 cm~2. They are independent of Er doping. Characteristic photoluminescence (PL) due to intra 4f-shell transitions of Er3‘ has been successfully observed for Er-doped InAs QDs. The luminescence appears only under the excitation with photon energy higher than the GaAs bandgap. It is discussed in relation to an energy-transfer model. ( 2000 Elsevier Science B.V. All rights reserved.

We have found a photoinduced phase transition (PIPT) in the mixed-valence (MV) gold complex ${\mathrm{Cs}}_{2}{\mathrm{Au}}_{2}{\mathrm{Br}}_{6}$ from the MV state (ambient pressure phase) to the single-valence (SV) phase (high-pressure phase) in the pressure region from 6.4 to 6.8 GPa. We have observed an accumulation of the photoirradiation effect of each pulse shot, and have ascribed it to stabilization of the photogenerated SV clusters due to the cooperative Jahn-Teller instability of ${\mathrm{Au}}^{2+}$ ions. The PIPT shows a distinct threshold behavior against the photon density as well as the shots number, which has been interpreted in terms of the critical nucleation of the photoinjected SV clusters. We further found that the phototransformed SV phase is not stable below 5.5 GPa, and changes into the former MV phase after a long dead time of $\ensuremath{\sim}{10}^{4}$ s.

We have investigated absorption spectra of charge-transfer (CT) complexes of C60 with various aromatic amines with different ionization potentials. An absorption band due to CT transition is observed in the visible region; with increasing ionization potential of amines, the peak energy shifts to the higher energy side and the oscillator strength of CT transition increases. We extract degrees of charge transfer by analyzing the transition energy of the CT band as a function of ionization potential and Raman spectra. The experimental results show that the CT complexes studied here have a neutral character in their ground state.

The dependence of electron spin g -factor on magnetic field has been investigated in GaAs/AlGaAs quantum wells. We have estimated the electron g -factor from spin precession frequency in time-resolved photoluminescence measurements under a magnetic field in different configurations; the magnetic field perpendicular ( g ⊥ ) and parallel ( g ∥ ) to the quantum confinement direction. When the angle between the magnetic field and the confinement direction is 45°, we have found that g -factor varies depending on the direction of magnetic field and the circular polarization type of excitation light ( σ + or σ − ). These dependences of g -factor exhibit main features of Overhauser effect that nuclear spins react back on electron spin precession. The value of g ⊥ and g ∥ corrected for the nuclear effects agree well with the results of four-band k·p perturbation calculations.

We have performed femtosecond reflectivity ΔR/R and transmissivity ΔT/T measurements in Nd2CuO4 thin film at 80 and 300 K. We have derived time variations of Δϵ′ and Δϵ′′ from transient data of ΔR/R and ΔT/T. Assuming a Lorentz oscillator model consisting of three oscillators, we obtain spectra of the real part ϵ′ and imaginary part ϵ′′ of dielectric function from the measured reflectivity and absorption spectra in the visible-uv region. The absorption bands in the visible region are assigned to in-plane charge-transfer (CT) excitations, and relaxation times of CT excitations are found to be 0.6 and 1.0 ps.

Atomic-force microscopy (AFM) is used to study InAs quantum-dot structures grown by molecular-beam epitaxy (MBE) on vicinal GaAs(001) surfaces misoriented in the [010] direction by 1, 2, 4, and 6°. It is shown for a chosen misorientation direction that a vicinal GaAs(010) surface is covered with a network of stepped terraces. The thickening of the network of terraces with increasing misorientation angle leads to the suppression of adatom surface diffusion and makes it possible to achieve higher densities and more uniform ensembles of quantum dots, while simultaneously decreasing the probability of their coalescence.

We have studied formation and energy structure of triplet states of charge-transfer excitons in C 60 thin film by means of pump and probe spectroscopy in a vast time domain from 100 fs to 100 ms. In solid C 60 , absorption bleaching which lasts up to several hundred milliseconds is observed slightly below the absorption band due to charge-transfer excitons (CT excitons). This provides a spectroscopic evidence for existence of an inter-molecular triplet state due to CT excitons in solid C 60 . The energy splitting between singlet and triplet CT exciton states is 0.09 eV, and the lifetime of triplet CT excitons is ∼50 ms.

We report on the first observation of the evap- oration of InAs quantum dots (QDs) on the GaAs surface under moderate heating in UHV. InAs layers 2.3 ML thick were MBE-grown on GaAs (001) surfaces exactly oriented and misoriented in the (010) direction and annealed at 350C in UHV without an As flux. Ambient AFM studies have re- vealed the formation of craters in the center of InAs QDs grown on the GaAs surface. The inner walls of the craters show clear faceting along the main crystallographic planes. Evaporation craters develop through the center of the QDs down to the GaAs substrate. Complete evaporation of the QDs leaves the GaAs surface patterned with tiny wells at the former QD positions. A tentative explanation of the observed effects is given, taking into account the strain distribution in the strained system of InAs QDs on the GaAs surface.

The temperature dependence of absorption spectra for thin films of double-exchange ferromagnet ${R}_{0.6}$Sr${}_{0.4}$MnO${}_{3}$ has been investigated with variation of the one-electron bandwidth $W$. With decreasing temperature, which corresponds to the increase of the magnetization $M$, a large amount of spectral weight transfers from the high- $(\ensuremath{\sim}$3 eV) to low- $(l~$1 eV) energy region. We ascribe these transitions to the interband transition between the bands split by the on-site exchange interaction $J$ and the intraband transition within the lower band, respectively. This suggests that optical excitations in the visible region strongly couple with local spin alignments in these doped manganites.

We have investigated absorption and luminescence spectra and temporal behaviors of luminescence in solutions and solids of C 70 . The luminescence bands observed for C 70 in solution originate from the S 1 –S 0 transition. The luminescence decay kinetics of C 70 single crystal exhibits two-component decay with decay times of ∼400 ps and ∼1.4 ns. The fast component is assigned to the intra-molecular S 1 –S 0 transition, and the slow one is attributable to the self-trapped state of charge-transfer exciton. It is found that there are two relaxation channels of photoexcitations in C 70 crystals: intra-molecular and inter-molecular relaxations.

β-BaB2O4 (β-BBO) thin films were successfully synthesized by the sol-gel method using metallo-organic compounds. A stable BBO precursor solution was prepared from barium metal and boron triethoxide or 2,4,6-triethoxycyclotriboroxane in a mixture solvent of ethanol and 2-ethoxyethanol. As-precipitated powder formed by hydrolysis of the precursor solutions crystallized to ψ phase, which was transformed to β phase at higher temperatures. The transformation temperatures of powders from ψ to β phase of the ethoxide system and the boroxane system were 600 and 680°C, respectively. The calcination of precursor films in a mixture gas of water and oxygen was found to decrease the crystallization temperature of β-BBO films on Pt sheet substrates. The precursor films prepared from the ethoxide system and the boroxane system crystallized to β-BBO on Pt (111)/glass substrates at 500 and 550°C, respectively. The BBO films on Pt(111)/glass substrates showed the strong (006) preferred orientation. The β-BBO films on Pt(111)/glass substrates showed the second harmonic generation (SHG) of the 532 nm light on irradiation with 1064 nm light. The SH power from the BBO films was correlated with the fundamental power through the square-law proportionality based on the theory. The SHG efficiency of the BBO films was dependent upon the film thickness.

We have investigated photodoping effects in insulating YBa 2 Cu 3 O 6.2 and Nd 2 CuO 4 , which are parent materials of high T C superconductors, by means of femtosecond pump-probe spectroscopy using white continuum light. The absorption changes are observed in the wide energy region after the irradiation of the femtosecond laser pulse. The spectral features of the absorption change are in good agreement with the change by the doping with changing the chemical composition, and this indicates that the electronic structures in the CuO 2 planes are drastically changed by femtosecond laser pulses. The photo-induced change in the electronic structure suggests the formation of the optically induced in-gap state with the decay time of ∼ 1 ps. Furthermore, we have investigated the photodoping effect by changing the photon energies of the pumping pulse.

β-BaB 2 O 4 (β-BBO) thin films have been successfully synthesized by the sol–gel method from a BBO precursor solution. A stable BBO precursor solution was prepared from barium metal and boron triethoxide in a mixed solvent of ethanol and 2-ethoxyethanol. The precursor films crystallized into β-BBO on glass (Corning 7059) and fused silica substrates at 550 °C. BBO films on glass (Corning 7059) and fused silica substrates showed a highly (006) preferred orientation. The absorption edge of β-BBO films on fused silica substrates was at

We have measured the exciton and carrier dynamics in the high purity semiconducting (S-) and metallic (M-) single-walled carbon nanotubes (SWNTs) in the isolated and aggregated (bundled) forms. The exciton relaxation decay times are measured by using the pump-probe spectroscopy. For bundled samples, the relaxation time becomes shorter than that for isolated SWNTs sample, because of the existence of inter-tube relaxation. We estimate the relaxation rates from S-SWNT to S-SWNT and S-SWNT to M-SWNT using the decay times for isolated SWNTs, high purity S-SWNTs bundle, and doped S-SWNTs in high purity M-SWNTs bundle. For S-SWNTs, inter-tube relaxation plays an important role in the relaxation dynamics. However, for M-SWNTs, the inter-tube relaxation is not so important, and the transition energy and intensity of exciton in M-SWNTs is strongly affected by the photoexcited carriers which plays like as photo doping.

We report huminescence spectra and their temporal behaviors for C 60 single crystals. Two types of luminescence bands are observed. The origin of weak luminescence band can be assigned to the intramolecular HOMO—LUMO transition. We discuss the origin of the main luminescence band connecting with a self-trapped state of a charge-transfer exciton.

We have studied relaxation dynamics of photoexcitations of C 60 and C 70 molecules in solution and in solid by means of a femtosecond pump and probe spectroscopy. Comparing the results for both C 60 and C 70 , we discuss especially relaxation processes in C 70 . With the excitation of the dipole allowed transition at 3 eV, the photoexcitations of C 70 molecules in solution are relaxed into the triplet state in ∼ 1 ns. For C 70 solid, however, the fast relaxation occurs in 1.2 ps, which is interpreted in terms of the self-trapping of charge-transfer (CT) excitons.

We have studied relaxation dynamics of photoexcitation in C 60 and C 70 molecules and solids by means of a femtosecond pump and probe spectroscopy. With the excitation of the dipole allowed transition at 3 eV, the photoexcitations in C 60 solution relax into the triplet state in 900 ps. For solid C 60 , however, the fast relaxation occurs in 1 ps, which is interpreted in terms of the self-trapping of charge-transfer (CT) excitons in the simple cubic lattice. Such a self-trapped CT exciton seems to be a precursor to the photopolymerization. A similar dynamical behavior has been observed for solid C 70 .