Aurelia Lupei

The nature of nonequivalent Nd3+ centers in CNGG and CLNGG

Abstract Results on the spectroscopic investigations and modeling of Nd3+ nonequivalent center structure in calcium niobium gallium garnet (CNGG) and calcium lithium niobium gallium garnet (CLNGG) are presented. The low temperature absorption, emission under selective excitation and lifetimes on a large variety of samples with Nd3+ varying between 0.1 and 5 at.% are analyzed. At least five different types of centers have been separated. Based on spectral data along with previous reports on the composition of the crystals a model of Nd3+ five-center structure in CNGG and CLNGG disordered crystals is proposed. The relative intensities for all the centers can be explained consistently by the structural possibilities of occupation of the first cationic octahedral sphere around the Nd3+ site. Starting from an unique average composition of this sphere our modeling suggests a correlated placement of the cations in the first octahedral sphere around Nd3+: the larger charge of Nd3+ as compared to the substituted Ca2+ leads to a local depletion of Nb5+ ions, accompanied by a corresponding enhancement of the lower-charge cations ( Ga 3+ , Li + ) or vacancies as compared with the undoped crystals.

Mechanisms of energy transfer between Nd3+ ions in YAG

Abstract A systematic presence of non-equivalent crystal field Nd3+ sites as well as a non-exponential decay of the 4F3/2 level luminescence is observed for melt-grown YAG crystals containing up to 1.5 at% Nd3+. The non-exponential part of the decays is analyzed and assigned to direct donor-acceptor energy transfer processes between Nd3+ ions in various sites; no migration among donors occurs in this concentration range. Energy transfer mechanisms are proposed and a connection between these processes and the concentration-dependent decrease of luminescence efficiency is discussed.

Local perturbations due to rare-earth (R3+) doping

The origin of some complex structure in the absorption and emission spectra of rare-earth ions in different host lattices were analyzed in terms of local perturbations caused by doping. The modifications in the strength of the crystal field effect resulting from the doping were investigated in the rare-earth oxide, oxyfluoride, oxychloride and ethyl sulfate hosts. The size effect on the dopant ion in the cubic C-type rare-earth oxide hosts was discussed and a parallel was made to the pressure effect on the emission spectra of the Eu 3+ ions. The complexity of the emission spectra of the Eu 3+ -doped rare-earth calcium oxoborates EuCa 4 O(BO 3 ) 3 and GdCa 4 O(BO 3 ) 3 :Eu 3+ was used to illustrate the inhomogeneous distribution of the dopant. Another source of complex spectra, the pair formation, was discussed for the R 3+ -doped rare-earth oxide and garnet hosts.

Efficient sensitization of Yb3+ emission by Nd3+ in Y2O3 transparent ceramics and the prospect for high-energy Yb lasers.

Very efficient energy transfer from Nd3+ to Yb3+ in transparent Y2O3 ceramics in the temperature range 10-300 K is demonstrated. It is inferred that this shows potential for the construction of high-energy Yb3+ lasers under diode or flash-lamp excitation of Nd3+.

Comparison of Nd:YAG single crystals and transparent ceramics as laser materials

Spectroscopic and emission decay investigation on single crystal Nd:YAG (with up to 3at.%Nd) and transparent ceramic (up to 9at.%Nd) samples indicate that the state of Nd ions (the sites occupied by the Nd ions, the crystal field and electron-phonon interactions, energy transfer between the Nd ions) are identical. Coupled with the similarity of the thermo-mechanical properties and with specific features of ceramics, such as a high production yield, possibility to produce very large components with uniform Nd doping up to very high concentrations, reduced production cost, these studies show that the transparent ceramic Nd:YAG can be used for construction of lasers. These conclusions are supported by efficient laser emission data.

Thermal shifts of Sm 3+ lines in YAG and cubic sesquioxide ceramics

A comparative analysis of the thermal shift of Sm3+ ion zero-phonon lines in YAG and sesquioxide (Y2O3 and Sc2O3) ceramics is presented. The Sm3+ lines in YAG show small red shifts, whereas in sesquioxides large blue shifts in absorption (up to ~9 cm−1 Y2O3 or ~6 cm−1 Sc2O3), and blue or red shifts in emission are observed for the C2 centers, while the C3i magnetic-dipole allowed lines exhibit small red shift. The data are analyzed in terms of competition between the dynamic (due to electron-phonon interaction) and the static shifts produced especially by the thermal local structural changes.

Spectroscopic and de-excitation properties of (Cr,Nd):YAG transparent ceramics

High resolution spectroscopy brings new data on the spectroscopic and energy transfer properties of the transparent (Cr,Nd):YAG ceramics. The role of joint superexchange and electric dipole interactions in the Cr-to-Nd energy transfer is evidenced and the calculated emission quantum efficiency is essentially determined by the Nd concentration, 52% for 1 at.%Nd and 77% for 2 at.%Nd. The implication of Cr and Nd concentrations on the laser emission and heat generation under broad-band pumping is discussed

Inhomogeneous broadening by point defects of RE3+ optical lines in YAG

Most of the RE3+ optical lines in YAG, at low temperature and concentrations, present two interesting features: a multisite structure and Lorenz lineshape of the inhomogeneous broadened lines. The satellite structure of RE3+ ions optical spectra in high temperature YAG present a group of P satellite lines of almost equal intensity that was assigned to nonstoichiometric occupation of Al3+ octahedral sites (a) by Y3+. Up to now, no reliable explanation of the anisotropic effects of Y3+(a) on RE3+ spectra was given. In this paper an attempt is made to explain the multisite structure and lineshapes in terms of the strain field (induced by point defects) theory. The model is illustrated for the absorption line 4I15/2 implies 4S3/2 of Er3+ (1.at.%) doped in YAG.

Effects of up-conversion energy transfer on the decay shapes of the4 F3/2 level of Nd3+ in YAG at high-pump intensities

Experimental data presents a pump intensity dependence of the 4F3/2Nd3+ level emission in various hosts; these effects were assigned to the up-conversion processes. A new formalism that takes into account both up- and down-conversion processes and the statistical distribution of acceptors around donor in the case of direct processes is presented. This involves a time dependent of transfer function and two interaction types: dipolar and superexchange. The profile of excitation beam is considered in order to compare theoretical calculations with experimental data. Calculations concerning quantum efficiency as a function on the ratio between up- and down- conversion rates are presented.

Energy transfer effects in the VUV-to-VIS quantum cutting in praseodymium-activated phosphors

The effect of energy transfer on the VUV-to-VIS quantum cutting in Praseodymium-activated phosphors for Xe dimer luminescence lamps is discussed. From the modeling of emission process in presence of energy transfer it is inferred that the failure to observe efficient energy transfer-assisted quantum cutting in these phosphors is caused by the low efficiency of transfer with respect to the radiative de-excitation of the metastable VUV state 1So of Pr at the low Pr concentrations specific to these systems.