Tokiko Ohata

Edge Emission of Excitons in KI and RbI

The intensity of the edge emission in KI and RbI has been measured from 5 K to 100 K. The temperature dependence is interpreted in terms of a simple exciton decay model. Thermal quenching of the emission can be ascribed to the increase in the self-trapping probability of excitons. The edge emission is assumed to originate from a metastable free exciton state.

Indirect exciton luminescense and Raman scattering in CdI2

Abstract Intrinsic luminescence and Raman scattering in 4HCdI 2 have been investigated at 2 K. Weak emission bands observed near the absorption edge are attributed to the phonon-assistes indirect exciton luminescence. Several new Raman lines are observed under resonant excitation in addition to known lines. The symmetry of the phonon modes associated with the indirect transitions as well as with Raman scattering is discussed.

Luminescence of (Tl + ) 2 Centers in KI

New emission bands have been observed at 450 nm and 341 nm when a crystal of KI doped heavily with Tl + was excited with ultraviolet light near 285 nm at 5 K. An absorption band has been found at 284.5 nm from the excitation spectrum. Dependence of the emission intensity on the concentration of Tl + shows that this luminescence is due to (Tl + ) 2 centers.

Edge Luminescence of KI and RbI under UV Excitation

Emission bands of the edge luminescence of KI and RbI measured under excitation with ultraviolet light are more sharp and closer to the absorption edge than those induced by X-ray excitation. It is suggested from the temperature dependence of the emission intensity that the origin of the uv-stimulated emission is same as that of X-ray induced one.

Luminescence of Lead Ion Centers in Cadmium Iodide Single Crystals

Emission and excitation spectra of the luminescence associated with Pb 2+ centers in CdI 2 layered crystals have been investigated. Excitation in the absorption band of the center induces an emission band with fine vibrational structures at the energy near the absorption band, and a broad emission band with large Stokes shift. Measurements of the decay time and time-resolved emission spectra reveal that the former band consists of two parts originating from ( 3 E u , 3 A 2u ) and 3 A 1u states of Pb 2+ with D 3d symmetry. The low energy broad band is ascribed to the emission from a relaxed state of excitons of CdI 2 perturbed by Pb 2+ . Thermally activated transfer of energy is found to take place from the excited states of Pb 2+ into the perturbed exciton state.

Luminescence of KI:Br and RbI:Br

Emission spectra and decay times of luminescence in KI and RbI containing small amounts of Br - ions are investigated. The Br - ions induce a reduction of Ex emissionin KI, but an enhancement of it in RbI. The Ex emission in KI seems to be of extrinsic origin. The luminescence center associated with the Ex emission in RbI:Br is deduced to be an STE of RbI perturbed by nearby Br - ions. The states responsible for the Ex and π emission bands in RbI are discussed in relation to the model of on- and off-center STEs.

Quenching of luminescence by coloration in KI and RbI

Abstract Quenching of exciton luminescence caused by coloration with u.v. irradiation in KI and RbI has been studied. Luminescence intensity measured after coloration is found to show a remarkable dependence on temperature. A possible explanation is given for the mechanism of luminescence quenching on the assumption that the luminescence intensity is proportional to the probability of exciton self-trapping.

Optical Absorption and Luminescence Spectra of Pb 2+-Aggregated Centers in CdI 2

Change in absorption and luminescence spectra with the concentration of Pb ions doped in a CdI 2 crystal is investigated up to a few mol%. Absorption bands due to dimmer, trimmer and further aggregated centers are found to appear in the low energy side of Pb 2+ monomer absorption band. Excitation in these absorption bands induces the luminescence from relaxed excited states in the respective centers whose peak energy decreases with increase in the size of aggregated centers. Further increase in the concentration beyond 5 mol%, gives rise to an edge luminescence arising from an excitonic state formed in PbI 2 clusters.

Systematic studies on optical gain spectra in GaInN/GaN-MQWs

Abstract The origin of optical gain in Ga 1− x In x N/GaN multiple quantum wells with x =0.05, 0.10 and 0.16 was investigated systematically by using a variable excitation-stripe method under various excitation conditions, such as power density and temperature. It was found that the alloy inhomogeneity of In content increases the width of the gain spectrum and decreases its value. It is supposed that the stimulated emission observed at the high-energy end of spontaneous emission spectrum originates from the optical gain due to electron–hole plasma around the mobility edge for all the samples.

Thallous Dimer Centers in RbI

A new absorption band has been found in RbI containing thallium of high concentration. The dependence of the luminescence intensity on the thallium concentration reveals that the new band is due to absorption by dimer (Tl + ) 2 centers. The analysis of the polarization of luminescence leads to the conclusion that the Tl + –Tl + axis of a dimer center orients along a axis of the crystal.