G.P. Morgan

Quantum size microcrystals grown using organometallic vapor phase epitaxy

Needle‐shaped quantum size microcrystals as thin as 10 nm have been selectively grown by employing reduced pressure organometallic vapor phase epitaxy using trimethylgallium and arsine as source materials. The microcrystals grown within a SiO2 window area have their growth axes along the [111] direction. Transmission electron diffraction analysis shows that the crystal structure of microcrystals is consistent with the zinc‐blende structure of GaAs. The mechanism for growing the needle‐shaped crystals is similar to a vapor‐liquid‐solid (VLS) equilibrium phase growth model. From photoluminescence measurements at 4.2 K, it is found that the microcrystals show a very distinct spectra for free exciton and neutral acceptor‐bound exciton recombinations, meaning good crystal quality.

Spectroscopic effects of small 4T2-2E energy separations in 3d3-ion systems

Abstract The energy of the 4A2 → 4T2 transition of 3d3 ions in octahedral sites decreases with increasing temperature, with a consequent reduction in the energy separation, Δ, between 4T2 and 2E levels. This can significantly affect the spectroscopic properties of Cr3+-doped materials with small Δ values. These materials are of interest because of their possible use in tunable laser systems. Some experimentally measured values of this shift are presented, the mixing of 2E and 4T2 electronic-vibrational states when Δ is small is estimated, and the effects of this mixing on the spectroscopic properties are discussed.

On the quantum efficiency of chromium-doped glasses

Abstract It is argued that in chromium-doped glasses the environment about the chromium ion can undergo a much greater deformation, when the electronic state of the ion changes from 4 A 2 to 4 T 2 , than is the case for most crystalline materials. An increased deformation enhances the probability for multiphonon relaxation. The case of Cr 3+ ions in low field sites in oxide glasses is considered. The large deformation shows up as a large Stokes shift of the 4 T 2 ↔ 4 A 2 transitions, and it is argued that the enhanced multiphonon relaxation is responsible for the low quantum efficiency of broadband-emitting chromium-doped glasses.

Spectroscopic analysis of the sharp line luminescence of GSGG:Cr3+

Abstract Laser spectroscopic techniques have been employed to study the sharp line luminescence of a sample of GSGG: Cr3+ (Gd3Sc2Ga3O12) (dopant concentration 8.1×10+19 cm-3). We are able to identify four distinct Cr3+ centres, which we attribute to a small degree of disorder in the distribution of Sc3+ and Ga3+ ions among the sites in the garnet crystal. The existence of another possible Cr3+ centre, in a low field site, is considered.

Luminescence from β-Ga2O3: Cr3+

Abstract Cr3+ substitutes readily for Ga3 + in β-Ga2O3 and the luminescence at room temperature from this material is broadband and originates mainly on the 4T2 level. We have examined this luminescence over a wide range of temperature to determine the radiative efficiency of the material and the relative positions of the 2E and 4T2 emitting levels.

Temperature dependence of the luminescence of GSGG:Cr3+

Abstract The crystalline material GSGG:Cr3+ is currently of interest as a broad-band, room temperature laser material. The small separation between the 4T2 and 2E electronic levels of Cr3+ in this system can result in interesting spectroscopic behaviour. We have investigated the temperature dependence of the CW and transient luminescence and have found it to be consistent with a model for the dominant Cr3+ site in which the lowest energy 2E and 4T2 levels are approximately coincident in energy at low temperature.

Optical characterization of GaAs quantum wire microcrystals

Abstract Optical properties of quantum-size GaAs wire crystals grown by organometallic vapor-phase epitaxy (OMVPE) are measured. The typical size of the wire-shaped microcrystals is 1–5 μm long and 10–200 nm wide. Photoluminescence measurements at 4 K reveal spectral features dominated by free carrier to acceptor impurity recombination. A free exciton recombination line is also observed and is more intense relative to other features than that observed from a conventional OMVPE epitaxially grown layer. Small spectral shifts (0.5 meV) of the free exciton and the acceptor-bound exciton recombination lines are considered to be due to the effects of quantum confinement on the energy levels of the system.

Level-crossing in chromium-doped materials

Abstract A coincidence in energy of the 2E and 4T2 excited levels of Cr3+ appears to occur in the material GSGG:Cr3+, and as a result the material exhibits a number of interesting luminescence features. The mixing of the 2E and 4T2 eigenstates by the spin-orbit coupling operator is analyzed using a simple model which takes account of the vibrational nature of the eigenstates. It is shown that this model is capable of explaining many of the observed luminescence features of the material.

The nature of the 3P0 fluorescence quenching processes in LaF3: Pr3+

The temporal decay profiles of 3P0 fluorescence from five single crystals of LaF3 with Pr3+ dopant concentrations from 1% to 80% have been measured at 2 K. The quenching of fluorescence from this level is discussed in terms of both long-range multipolar and short-range superexchange cross-relaxation interactions. The superexchange interaction provides the most consistent fit to the observed data and is believed to be dominant, however it is concluded that no single interaction provides a complete description of the fluorescence decay.

Cascade stimulated emission in the sodium dimer

Abstract We have observed many stimulated emission lines near 910 nm, 1030 nm and 690 nm, when diatomic sodium molecules are pumped by a pulsed laser at 354.7 nm. A mechanism of cascade emission, which may be responsible for laser action in the 910 nm and 690 nm region, is proposed and discussed.