Jun′ichiro Nakahara

Optical Spectra of Hexagonal Ice

Reflectivity spectrum of single crystal of hexagonal ice is measured at 80 K in the energy range between 6 and 28 eV. Spectra of absorption coefficient, dielectric constants and effective number of optical electrons are determined. Reflectivity spectrum of amorphous ice is measured for a comparison. Electronic states responsible for the structures found in the spectrum of hexagonal ice are discussed.

Edge Emissions and Broad Band Emissions in Thallous Halides

The emission spectra are measured in pure TlBr and TlCl crystals as well as in the mixed crystals of TlBr(Cl), TlBr(I), TlCl(Br) and TlCl(I). Sharp emission lines near the absorption edge are observed in TlBr and TlCl. High energy lines among them are examined with the aid of the exciton state calculation and are interpreted as the radiative decay of the indirect forbidden X 6 + × R 6 - excitons. In the halogen-doped crystals, the emissions due to the zero phonon decay of the X 6 + × R 6 - excitons are observed. The broad band emissions observed in the long wavelength region in the halogen-doped crystals are assigned to the decays of the relaxed excitons localized at the added halogen ions.

Epitaxial growth of zinc-blende ZnSe/MgS superlattices on (001) GaAs

We report the growth of zinc‐blende ZnSe/MgS superlattices (SLs) on GaAs (001) substrates. The SLs were grown with metalorganic vapor phase epitaxy by selecting appropriate precursors for Mg and S. MgS naturally forms rocksalt structures, but zinc‐blende MgS layers were grown. The lattice constant of MgS was estimated to be 5.59 A. X‐ray diffraction measurements show that the ZnSe/MgS SLs are grown coherently to the GaAs substrates up to the total thicknesses of ∼3000 A.

Inhomogeneous Broadening of Mn2+ Photoluminescence in CdMnTe.

Photoluminescence spectra of Mn 2+ in CdMnTe are investigated using tunable laser in a temperature range from 15 K to 80 K. The shift of the photoluminescence peak energy with the excitation energy is observed. From analysis using the shift and the photoluminescence width, we obtain the temperature dependencies of homogeneous and inhomogeneous broadening widths of the d-d transition. In addition, the effects of high hydrostatic pressure on the electron-phonon coupling strength and the homogeneous and inhomogeneous widths are investigated.

Excitonic luminescence up to room temperature in a ZnSe/MgS superlattice

Optical properties of a ZnSe/MgS superlattice (SL) were studied by reflection and photoluminescence (PL) spectroscopies. Excitonic structures were clearly observed in the reflection spectra up to room temperature, and the exciton absorption peak energy and linewidth were well coincident with the PL peak energy and linewidth. The origin of the lowest luminescence peak was indicated to be C1-HH1 exciton up to room temperature. The lineshape broadening due to longitudinal-optical-phonon scattering of excitons is much decreased in this SL compared to bulk ZnSe.

Emission from the Excitonic Molecules in Thallous Halides

Two emission lines which show nonlinear growth to the excitation intensity are found near the absorption edges in TlCl and in TlBr. They are investigated in a wide range of the excitation intensity at 4.2 K. From their intensity dependence on the excitation intensity, their line shapes and their energies, it is proposed that the higher energy emission line among is due to the radiative decay of the molecule of two X 6 + × R 6 - excitons by leaving one X 6 + × R 6 - exciton and one M-point LA-phonon through the forbidden indirect process. The lower energy emission is found to be due to the radiative decay of the excitonic molecule but its origin is not cleared. Theoretical calculation on the line shape of the emission from the molecule of forbidden indirect excitons is made.

Effect of Charge Density Waves on Reflectance Spectra of TaS3 and NbSe3

We report new reflectance results in the plasmon region of orthorhombicTaS 3 (single crystal) through the Peierls transition temperature and near the plasmon region of monoclinic NbSe 3 as a function of temperature. The optical spectra yield the dielectric constant due to core electrons, mass density and electron relaxation time. Furthermore, we discuss the modification of one electron excitations and plasmon collective excitations through charge density wave (CDW) transitions and assign these anomalies in TaS 3 to its normal-incommensurate (215±10 K) and incommensurate-commensurate (170±20 K) transitions in TaS 3 . In NbSe 3 , no modifications are observed through the normal-incommensurate CDW transition temperature (140 K).

Raman Spectra in AlxGa1-xAs under High Pressure

Raman spectra have been measured in Al x Ga 1- x As as a function of alloy composition and pressure. The Raman spectra are interpreted as a 2-mode behavior (persistence type) for LO and TO phonons and a 1-mode behavior (amalgamation type) for LA and TA phonons as being theoretically predicted. With increasing x, the AIAs-like LO and TO modes shift to higher frequencies and GaAs-like modes shift to lower frequencies. Under high pressure these modes shift to the higher frequencies. We analyze the Raman spectra of TO modes as the disorder activated lines from the whole Brillouin zone. The low energy structures are assigned to the disorder activated TA and LA modes by the pressure dependences of their energies and by their line shape. The spectral assignments of these modes in Al x Ga 1- x As are consistent with their composition dependence.

Pressure dependence of local structure in liquid carbon disulfide

High pressure x-ray diffraction measurements on liquid carbon disulfide up to 1.2 GPa are performed by using an energy dispersion method. The results are compared with a molecular dynamics calculation with usual Lennard-Jones potential. They give very good agreement for all pressures measured. It becomes clear that the liquid structure changes like hard core liquid up to the pressure just below crystallizing point. The relation between structural change and optical response at high pressure is discussed.

Optical Dephasing of β-Carotene Molecule in Liquid and Solid Phases of Ethanol

Fluorescence spectra of β-carotene molecules in ethanol have been measured in a temperature range from 20 K to 240 K. Assuming that dephasing of the system is characterized by slow modulation, we obtained temperature dependence of homogeneous and inhomogeneous broadening widths analyzing an almost linear dependence of the fluorescence peak energy on the excitation energy. Theoretical calculations of fluorescence to Raman scattering intensity ratio using the homogeneous and inhomogeneous widths reproduce well experimental results. It supports that, within a wide temperature range from liquid to solid phases of solvent, the essential feature of the optical dephasing process of the system is characterized by the slow modulation.