Ikuko Akimoto

High carrier mobility in ultrapure diamond measured by time-resolved cyclotron resonance

We have performed time-resolved cyclotron resonance measurements in ultrapure diamond crystals for the temperature range of T = 7.3 – 40 K and obtained the temperature-dependent momentum relaxation times based on the cyclotron resonance widths for optically generated electrons and holes. The relaxation time follows a T − 3 / 2 law down to 12 K, which is expected for acoustic-phonon scattering without impurity effect because of the high purity of our samples. The deviation from the law at lower temperatures is explained by the impurity scattering and the breakdown of the high-temperature approximation for the phonon scattering. We extract the carrier drift mobility by using the directly measured effective masses and the relaxation times. The mobility at 10 K for 600 ns delay time after optical injection is found to be μ e = 1.5 × 10 6 cm 2 / V s for the electrons, and μ l h = 2.3 × 10 6 cm 2 / V s and μ h h = 2.4 × 10 5 cm 2 / V s for the light and heavy holes, respectively. These high values are achieved by our high-sensitivity detection for low-density carriers (at <1011 cm−3) free from the carrier-carrier scattering as well as by the suppression of the impurity scattering in the high-purity samples.

Femtosecond time-resolved spectroscopy of photoinduced ionic-to-neutral phase transition in tetrathiafulvalen-p-chloranil crystals

Recent experimental results are reviewed of the photoinduced phase transformation between ionic and neutral phases in tetrathiafulvalen-p-chloranil crystals. Spectroscopic studies have revealed different features of Frenkel-type and charge transfer (CT)-type excited states in inducing photo-induced structural phase transition; the transition can be induced only above threshold-excitation intensities in the case of CT excitation. The thresholds imply that non-linear processes of CT excited states are crucial in the transition. On the other hand, time-resolved studies have demonstrated a dynamical aspect of the transition; optical reflectance signals that probe the evolution of the transition show dumped-oscillatory changes before establishing N-phase domains at around 100 ps after excitation. Mechanism of the transition has been discussed based on these results.

X-ray-Triggered Thermoluminescence and Density Functional Theory Characterization of a gem-Diphenyltrimethylenemethane Biradical

Thermoluminescence (TL) from the excited triplet state of a gem-diphenyltrimethylenemethane biradical (34••*) is triggered by X-irradiation at 77 K followed by annealing to ~140 K. The new X-irradiation method reported here is simpler and more convenient than the previously employed γ-irradiation method. The TL spectrum of 34••* is similar to the photoluminescence spectrum of the 1,1-diphenylethyl radical (5•). The results of density functional theory (DFT) and time-dependent-DFT calculations of the ground state biradical 34•• suggest that no significant electronic interaction takes place between the diphenylmethyl and allyl radical moieties owing to its twisted geometry. Accordingly, the results also suggest that the excited state biradical 34••* has a similar molecular geometry and electronic structure as the triplet ground state. Both the experimental and computational results obtained for 34•• and 5• confirm that the main fluorophore of 34••* is the diphenylmethyl radical moiety.

Fluorescent silica nanoparticles modified chemically with terbium complexes as potential bioimaging probes: their fluorescence and colloidal properties in water

It generally requires a complicated reaction protocol for the synthesis of silylated fluorescent lanthanide complexes. In this study, a silylated terbium complex was prepared by a very simple procedure through formation of a Schiff base between a terbium complex bearing a formyl group and 3-aminopropyltriethoxysilane. Using the silylated terbium complex, highly fluorescent silica nanoparticles modified chemically with terbium complexes (Tb-SNPs) were efficiently synthesized by a reverse micelle method with Triton X-100 as a surfactant in cyclohexane. The fluorescence properties of Tb-SNPs were remarkably improved by photostability, pH dependence and fluorescence lifetime, compared to free terbium complexes. Also, Tb-SNPs hardly aggregated under aqueous conditions with different salt concentrations and pHs. From these results, it was found that Tb-SNPs are applicable under physiological aqueous conditions. Furthermore, as an application model, Tb-SNPs were used as the fluorescent label for the imaging of African green monkey kidney cells.

Initial-Condition Sensitivity and Nonlinear Relaxation in Photo-Induced Ionic-to-Neutral Phase Transition in Tetrathiafulvalen- p -Chloranil Crystals

Recent experimental results of the photoinduced ionic-to-neutral (NI) phase transition in tetrathiafulvalen- p -chloranil (TTF-CA) crystals are reviewed with emphasis on the state-sensitive features and nonlinear properties. Frenkel-type and charge transfer (CT)-type excited states can induce the NI transition but with different characteristics; the transition can be induced only above threshold-excitation intensities in the case of CT excitation, whereas it is induced without any threshold for Frenkel-type excitation. The threshold that implies nonlinear processes of CT excited states is strongly state-dependent and temperature-dependent. Femtosecond time-resolved studies have resolved three distinctive sequential steps in the dynamics; formation of the precursor of N-phase domains, the local proliferation of photoinduced changes, and the process of forming N-phase orders. Origin of the nonlinear processes and the mechanism of the NI transition have been discussed based on these results.

Alcohol additive effect in hydrogen generation from water with carbon by photochemical reaction

We have found a novel photochemical activity of carbon powder to generate hydrogen-included gas by irradiating nanosecond laser pulse on carbon aqueous suspension at room temperature. In this study, we investigate alcohol additive effect to improve the gas generation efficiency in Bincho-tan charcoal aqueous suspension. We find that the generated gas volume with 50 wt % ethanol addition increases twice as much as that without ethanol. Components of the generated gas are analyzed to be 50% of hydrogen, 20% of carbon monoxide, and others, being similar to that generated with pure water suspension. Possible roles of ethanol are discussed as an agent in photochemical reactions and/or as a hole scavenger reagent in a photocatalytic reaction.

Two optical routes of cold carrier injection in silicon revealed by time-resolved excitation spectroscopy

We reveal two routes of optical carrier injection in pure silicon by means of high-resolution excitation spectroscopy on nanosecond cyclotron resonances. Free carriers are generated either by the band-to-band transition assisted by phonon emission, or via two-body collisions of excitons. The first route was previously masked by a strong excitonic response in steady-state optical spectra at low temperatures. Furthermore, valley polarization is achieved for the cold carriers created by the second route with optimized excess energy. These optical carrier injection routes are crucial to initialize the momentum and valley degrees of freedom of carriers in order to enable versatile applications of indirect-bandgap semiconductors.

Application of Liquid Laser Ablation: Organic Nanoparticle Formation and Hydrogen Gas Generation

Laser ablation is induced by a heating process of materials through the absorption of laser light and results in an explosive expansion of materials. For materials located in liquid, in contrast to those in vacuum, laser ablation proceeds under rather mild conditions via a cycle of heating and cooling by mediated solvent; therefore, it is applicable for organic solids to fragment into nanoparticles. Alternatively, for effective light absorbers, the irradiated site becomes the reaction centre of a photochemical reaction even in liquids, resulting in hydrogen gas generation. In this chapter, two topics of laser ablation in the liquid phase are presented: nanoparticle formation of organic materials and hydrogen gas generation from solid carbon in water. Thereby, the extended abilities of liquid laser ablation to transform ordinary materials into functional ones are introduced.

White light emission from solid carbon in water during hydrogen generation by VIS nanosecond laser pulse irradiation

W found a novel method of hydrogen generation from a mixture of water and solid carbon by visible nanosecond laser pulse irradiation at room temperature and under ambient atmosphere. The generated gas contains roughly 50% of hydrogen and 20% of carbon monoxide, meaning that the carbon was oxidized by the laser irradiation. Speed of the gas generation was improved by ethanol addition. We tentatively attributed the gas generation mechanism to photochemical reactions of the solid carbon with water, in analogy to the classical coal gasification which usually progress under high pressure and high temperature (HPHT). It has been unclear, however, whether the HPHT condition was achieved at the site and the moment of laser pulse irradiation or not. According to the nonlinear laser power dependence of generated gas volume and pulse width dependence, we have supposed that the photochemical reactions occur by multi-photon absorption and sequential excitation in carbon material in nanosecond duration. In this presentation, we focus on the spectroscopic phenomenon during the hydrogen generation. Optical emission of continuous spectrum was observed over visible range in both sides of the excitation wavelength for excitation power over a threshold. We discuss a possibility of non-equilibrium temperature elevation.T talk reviews high resolution phase shifting methods which have the capability of accommodating the requirements inherent in multiple beam holographic interferometry. These methods facilitate the use of multiple PZTs in an optical setup, offer the flexibility of using arbitrary phase steps and spherical beams, and provide the ability of making simultaneous multidimensional deformation measurements. The main sources of errors in implementing phase stepping are caused by the miscalibration of the piezo electric device (PZT) and by the presence of non-sinusoidal waveforms (consequence of CCD nonlinearity or multiple reflections inside the laser cavity). The proposed phase stepping methods render the holography related interferometry techniques insensitive to the error sources mentioned above. Experimental results show the feasibility of the proposed methods.

Dynamics of the Photoinduced Ionic-to-Neutral Phase Transition in Tetrathiafulvalene- p -chloranil Studied by Femtosecond Time-resolved Reflection Spectroscopy

Changes in reflection spectra were probed to study the dynamics of photoinduced transformation from ionic to neutral phases in tetrathiafulvalene- p -chloranil crystals. The results have revealed the formation of the precursor of metastable N -phase domains, dynamical lattice relaxation of and proliferation during phase transformation.