Nobuko Naka

Photo-induced Insulator–Metal Transition in an Organic Conductor α-(BEDT-TTF)2I3

Photo-switching between a charge-ordered insulating (CO) state and a metallic (M) state has been successfully realized in an organic conductor α-(BEDT-TTF) 2 I 3 at low temperatures. The large phot...

Condensation of excitons in Cu2O at ultracold temperatures: experiment and theory

We present experiments on the luminescence of excitons confined in a potential trap at milli-Kelvin bath temperatures under continuous-wave (cw) excitation. They reveal several distinct features like a kink in the dependence of the total integrated luminescence intensity on excitation laser power and a bimodal distribution of the spatially resolved luminescence. Furthermore, we discuss the present state of the theoretical description of Bose–Einstein condensation of excitons with respect to signatures of a condensate in the luminescence. The comparison of the experimental data with theoretical results with respect to the spatially resolved as well as the integrated luminescence intensity shows the necessity of taking into account a Bose–Einstein condensed excitonic phase in order to understand the behaviour of the trapped excitons.

Thin Films of Single-Crystal Cuprous Oxide Grown from the Melt

We have grown thin films of cuprous oxide (Cu2O) from the melt in a small gap between paired substrates of MgO or Al2O3 single crystals. The films are characterized by scanning electron microscopy, X-ray diffraction and optical spectroscopy. The thickness of the films is found to range from 16 nm to 20 µm. The domain structure with dimensions of 0.1–1 mm is found and attributed to single crystals oriented into different directions. The narrow peaks in rocking curves and in azimuthal angle rotations indicate the formation of highly oriented single-crystal films. Clear peaks due to yellow, green, blue, and violet excitons are observed in optical spectra. The positions of the peaks depend on the thickness of the film, implying strain-induced shifts of the exciton levels.

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.

Photo-induced current-modulation in zeolite (AFI) crystals containing single wall carbon nanotubes (SWCNs)

Photo-induced electric current modulation is studied in AFI-SWCNs systems by visible and near infra-red laser light at room temperature. Remarkable polarization anisotropy in the modulation is observed. λ-dependence of the polarization anisotropy is analyzed by a simple model based on the optical anisotropy observed in absorption spectra.

Resonant creation of indirect excitons in diamond at the phonon-assisted absorption edge

We have developed a new scheme for generating cold and high-density excitons in diamond by resonant excitation at the phonon-assisted absorption edge. Both the thermalization of excitons between fine-structure states and the effective temperature are examined with the tuning of the photon energy. The down-conversion rate for the excitons in the fine-structure states is found to be 3.5 ns independent of the photon energy, which means that the lowest-energy dark exciton can be created within a time scale comparable to its lifetime via the generation of a higher-energy bright exciton. With careful choice of excitation photon energy, we have achieved an electron-hole pair density of while maintaining the effective excitonic temperature below 15 K.

Dynamics of paraexcitons generated in a 3D confined potential well by two-photon resonance excitation in Cu2O

Abstract Time-resolved emission spectra of orthoexcitons in Cu2O are measured at 1.6 K under two-photon selective excitation in space and energy. SHG scattering and luminescence associated with orthoexcitons can be discriminated by their time domain and directionality. The spatial distribution of the excitons in a stress-induced trap is estimated from time-integrated spectra. Paraexcitons are found to reach the bottom of the potential well while orthoexcitons diminish on the way. These are consistent with the measured decay time of emission bands associated with the excitons.

Coherent optical processes of 1s ortho-excitons in Cu2O by two-photon excitations

Abstract Emission spectra of natural single crystals of Cu 2 O are examined under a two-photon excitation of 1s ortho-excitons of yellow series in degenerate regime at 2K. It is found that a second harmonic generation (SHG) via an electric quadrupole transition followed by a two-photon electric dipole transition and a hyper-Raman scattering process accompanied with 1LO ( Γ 12 − )-phonon are enhanced resonantly by the 1s ortho-exciton level. The integrated intensities of the SHG light is analyzed as a function of the excitation photon energy giving a phenomenological damping constant of the 1s ortho-exciton.

A new aspect of the Bose-Einstein condensation of 1s-exciton system in Cu2O

Abstract Bose-Einstein statistics of is ortho- and para-excitons in commercially available natural single crystals of Cu 2 O is examined by spectral shape analysis of their phonon assisted emissions under a band-to-band excitation. It is confirmed that the density of the ortho-excitons increases with the rise of the effective temperature along the phase transition boundary as the power density of the excitation light increases. It is newly found that the density of the para-excitons stays at the critical density of the BEC at 3K in the excitation power density between 10 2 W/cm 2 and 6 × 10 4 W/cm 2 .

Optical Nano-Tomography on Photosensitive Single-Wall Carbon Nanotube Arrays in Zeolite Crystals

Single-wall carbon nanotubes of the smallest diameter formed in microchannel arrays of Zeolite single crystals (IUPAC cord: AFI) have attractive potential for fundamental and device-oriented optical researches. Here we report on very photosensitive characteristics of the material at room temperature. We find that tightly focused irradiation of the 488 nm Ar+-ion laser light causes remarkable suppression of the visible emission from nanotubes. The photo-induced suppression occurs under low-level irradiation less than 1 µW. By the nano-tomography performed with Nanofinder, a three-dimensional spectro-tomographic system based on a confocal laser microscope, the size of the affected region is estimated to be 1 µm×1 µm×2 µm. This is comparable to the effective volume of the photo-irradiation spot. We discuss the origin of the photo-irradiation effects in view of the effects of the thermal conduction along the tube axis and of the local electronic effects within the photo-irradiated region.