G.F. Imbusch

The optical spectroscopy of LiGa5O8:Ni2+

Abstract The optical spectroscopy of nickel-doped LiGa 5 O 8 is presented. Absorption and luminescence measurements taken over a wide temperature range indicate that the dopant nickel ion is doubly charged and occupies a site of near-octahedral symmetry. We observe a luminescence transition in the near-infrared which is assigned to the 3 T 2 ( 3 F)→ 3 A 2 ( 3 F) transition of the Ni 2+ ions. No visible luminescence is observed. Luminescence decay times as a function of temperature have been measured to determine the presence and extent of nonradiative transitions in this system. The near-infrared luminescence appears to have a high quantum efficiency up to temperatures of around 350 K above which nonradiative processes occur. The high quantum efficiency and broad smooth shape of the luminescence makes this an interesting transition from the viewpoint of tunable laser action.

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.

The optical spectroscopy of chromium in ED-2 silicate glass

The technique of fluorescence line narrowing is used to probe the inhomogeneously broadened 2E-4A2 transition of Cr3+ in a silicate glass. The resultant sharp zero-phonon line is accompanied by a vibrational sideband which carries information about the range of vibrational modes in the glass and about the strength of the orbit-lattice interaction. The homogeneous broadening of the zero-phonon line is measured as a function of temperature and compared with the analogous broadening in Cr-doped crystals. The broadening of the zero-phonon line in the glass at high temperatures is attributed to a Raman relaxation mechanism such as is observed in crystals. At low temperatures, however, there is an additional broadening mechanism which has an approximately linear dependence on temperature and which has no analogue in crystalline materials.

The analysis of thermoluminescence glow curves

Information about the behavior of traps in a luminescent material is usually derived by fitting the glow curves in the thermoluminescence spectrum of the material to a general formula. From the fit one seeks to obtain values for the depth of the traps, the frequency factors governing the release of electrons from the traps, and some indication of the rates of trapping and retrapping. This study investigates how successful this fitting process is in providing reliable values for the trap parameters. The relevant rate equations are used to numerically generate simulated glow curves for specific values of trap parameters. These glow curves are then analyzed by the usual fitting process, and values for the trap parameters are derived from the fitting process. We comment on the comparison between the derived parameter values and the correct values. From these comparisons we attempt to obtain some useful insights to assist in the interpretation of experimentally observed thermoluminescence spectra.

Luminescence and fluorescence line narrowing studies of Y3Al5O12:Mn4+

Abstract A luminescence study of the Y 3 Al 5 O 12 :Mn 4+ system is presented. Luminescence is due to 2 E → 4 A 2 transitions on Mn 4+ ions which substitute for Al 3+ ions in distorted octahedral sites. The technique of fluorescence line narrowing (FLN) is used to interrogate some of the details of this transition. The zero-phonon R lines are found to be a composite of transitions from Mn 4+ ions in different sites, the strongest line coming from Mn 4+ ions in regular Al 3+ sites. The ratio of R line to sideband in the emission from Mn 4+ ions in these regular sites is found to be unusually low, and this is taken as an indication of an ion-lattice coupling for the Mn 4+ ion which is significantly stronger than for the case of the Cr 3+ ion in Y 3 Al 5 O 12 . Measurements of the luminescence decay rates as a function of temperature are given and we attempt to relate these data to models for nonradiative processes.

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.

Luminescence from LiGa5O8:Co

Abstract The absorption and luminescence of LiGa 5 O 8 :Co are studied at various temperatures between liquid helium temperature and 500K. The spectra are assigned to transitions on Co 2+ ions in tetrahedral sites. The luminescence transition is spin-allowed and is characterized by a very fast radiative decay rate.

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 study of LiGa5−xFexO8

Abstract Absorption, excitation, and luminescence measurements are presented for LiGa5−xFexO8 with concentrations x from 0.015 to 0.26. The emission and excitation data indicate that the red luminescence observed is due to the 6A1(6S)←4T1(4G) transition in the d5 configuration for Fe3+ in tetrahedral sites. Single-crystal samples allow direct absorption measurements for the first time and these indicate that most of the Fe3+ substitutes for Ga3+ in octahedral sites. Fine structure in the zero-phonon line and in the sideband is reported.

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.