The spectroscopic investigation of the highly transparent rare earth (Pr3+, Nd3+, Ho3+, Er3+, Tm3+, Yb3+) doped Sc2O3
ceramics produced by the solid-state synthesis technique indicate that these materials could substitute the single crystals
in construction of solid-state lasers. These studies indicate also that the rare earth doped transparent ceramics could
extend considerably the variety and performances (new active systems, wavelength ranges or emission schemes) of these
lasers.
The information on the variety, nature and structure of the centers formed by the rare earths ions doped in the transparent laser ceramics of garnets and cubic sesquioxides, acquired from high-resolution spectroscopy and emission decay is analyzed. The quantum states (energy levels, transition probabilities) of several doping rare earth ions, their distribution at the available lattice sites, the interactions between ions, and energy transfer processes are also presented. It is inferred that from spectroscopic point of view these materials could substitute the melt-grown single crystals in construction of solid-state lasers and extend considerably their capabilities.
Highly transparent Nd- or (Nd,Yb)-doped Sc203 ceramics are produced by the method of solid-state mixing of oxides. High-resolution spectroscopy indicates the spectroscopic and structural properties of the doping ions in these ceramics are similar with those of single crystals, although the maximum doping could be much higher. Very efficient Nd-to-Yb energy transfer is observed, which can be used for sensitisation of Yb laser emission.
The high resolution and polarized spectroscopic investigation of Nd3+ in Mg-compensated strontium lanthanum aluminate Srl-xLax-yNdyMgxAl12-x019 (Nd: ASL) function on composition makes possible the elucidation of the nature of non- equivalent centers and enables the selection of composition and pumping conditions that grant the efficient quasi-three-level laser emission around 900 nm. Based on a proper selection of these conditions, 900 nm laser emission with slope efficiencies of 0.74 and 0.84 at 792 nm 4F5/2 and respectively 865 nm 4F3/2 pumping of Nd3+ is demonstrated.
The interpretation of complex Yb3+ optical spectra in non-linear gadolinium calcium oxoborate GdCa4O(BO3)3-GdCOB crystals is still contradictory. The electronic structure of two Yb3+ centers in GdCOB is analyzed, including a crystal field parametric calculation for the main center. The possible models for vibronics intensities are discussed. The presence of a minor center whose relative intensity is of ~10%, indepenent on Yb3+ content, and Gaussian line-shapes are connected to the RCOB crystal disordered structure proposed recently from X-ray data.
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.
KEYWORDS: Quantum efficiency, Ions, Neodymium, Nd:YAG lasers, Energy transfer, Crystals, Data modeling, Luminescence, Temperature metrology, Energy efficiency
The paper discusses the problem of Nd concentration dependence of the emission quantum efficiency in Nd:YAG at low pump intensities, based on a modeling which uses the emission decay data. The measurements on crystalline or ceramic Nd:YAG samples with Nd concentrations up to 9at.%Nd show a marked dependence on concentration, induced by the energy transfer self-quenching. The parameters of ion-ion interactions that determine the transfer are used for an analytical modeling of the concentration dependence of the emission quantum efficiency, which gives a very good description of the experimental data.
New aspects of RCOB structure were obtained from Eu3+ absorption/emission spectra in GdCOB and EuCOB host lattices. The spectra of Eu3+ in powders and single crystals are comparatively analyzed. The Eu3+ spectra show at least three non-equivalent centers whose static spectral characteristics were used to assign structural modes. An explanation of the difference between GdCOB: Eu3+ and EuCOB spectra in terms of a disordered occupancy of rare earth and calcium sites is given.
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.
Strontium lanthanum aluminates crystals (ASL) doped with Nd3+ with formula Sr1-xNdyLax-yMgxAl12-xO19 are interesting crystals for lasers operating in i.r. (~900 nm range) or blue spectral ranges. This paper uses high-resolution spectroscopy, at low temperatures, for investigation of the influence of composition and Nd3+ doping of ASL crystals on emission properties. Preliminary spectral characteristics of two non-equivalent centers were determined. From these data and their dependence on composition, structural models for the two centers are proposed.
The structural data obtained from of high resolution optical spectroscopic investigations on Nd3+, Eu3+ and Yb3+ ions in rare earth calcium oxoborate crystals RCOB, mainly GdCOB and YCOB, but also EuCOB, are compared with those obtained from other studies such as X-ray diffraction or differential thermal analysis. The dependence of non-equivalent center intensities and on R3+ (dopants) ionic radii is in qualitative accord with X-ray diffraction data. The glassy like behavior of 4F3/2 Nd3+ emission under selective excitation, as well as the shift of one line of 5Do → 7F1 Eu3+ emission are explained in terms of cationic structural disorder of RCOB crystals.
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.
The garnets with disordered structure such as calcium niobium gallium garnet (CNGG) doped with Nd3+ are interesting materials for diode pumping. Spectroscopic investigations on Nd3+:CNGG have revealed a multisite structure and this is reflected in laser emission characteristics. The purpose of this paper is to present several new spectral aspects, a modeling for the main nonequivalent centers, energy transfer characteristics and an attempt to connect the laser characteristics to centers structure. Samples were grown by Czochralski method with Nd3+ from 0.1 to 5 at.% and the high resolution spectral and decay measurements were performed at 10 K. Similar considerations were made in the case of Nd3+ doped in calcium lithium niobium gallium garnet (CLNGG).
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.
An analysis of low temperature lineshapes for various absorption lines of several RE3+ ions from the beginning Nd3+ and end of lanthanide series Er3+, Tm3+ embedded in the same crystal - YAG known as important laser systems for 1-3 micrometers emission is presented. New aspects of the dependence of electron-phonon coupling on RE3+ ions and matrix are observed from the analysis of many transitions of these ions at low temperature. These data show that eh electron-phonon coupling presents a symmetric behavior in the lanthanide series, larger toward beginning and end. With only very few exceptions, less than 2 percent, the broadenings or splittings of zero-phonon liens could be connected to optical Raman phonons of undoped lattice. That means that the parity of 4fn states could still be considered a good parent quantum number, even if the local symmetry has no inversion, and the near resonant coupling involves the even part of the electron-phonon coupling operator. Besides, the one phono relaxation involve the undoped lattice phonons. For ions with small spin-orbit coupling, the vibronic contribution to the line intensity given by ir phonons through a Van Vleck mechanism could be important for spin forbidden transitions.
Laser crystals with disordered structure present a series of advantages for diode pumping or short pulse generation as compared with ordered crystal. Such a system proved to be calcium lithium niobium gallium garnet (CLNGG) doped with Nd3+. The papers present the growth and x-ray analysis of specially doped CLNGG. Very few spectroscopic data on this system have been previously published. The paper present low temperature absorption spectra, site selective excitation and lifetime measurements of Nd3+ in CLNGG. At least four distinct nonequivalent centers are observed, the spectral lines for every center presenting a disordered Gaussian shape. based on structural, spectral static and emission dynamics data an attempt of modeling the structure of these centers is proposed.
This paper presents a theoretical treatment of electron-phonon interaction problem in the case of quasi resonant interaction for rare earths ions in laser crystals. Expressions for the absorption or emission lineshapes (asymmetries or splittings of zero-phonon lines) are obtained. The theory is applied to explain some spectral features of Yb3+ transitions in YAG; it allows also the estimation of the vibronic coupling strengths, phonon energies and the `unsplit' electronic levels. A new electronic energy level scheme of Yb3+ in YAG is proposed.
The effects of co-doping with Cr3+ on the absorption and emission characteristics of double doped Cr3+, Nd3+:YAG (yttrium aluminum garnet) are analyzed. The multisite structure due to co-doping (samples with a large range of Cr3+ content were used) is separated from that in single doped crystals. At least seven new lines have been separated in high resolution spectra and a tentative assignment in terms of structural data is proposed. These data suggest that the energy transfer data should be analyzed in terms of inhomogeneous system. However, the measurements indicate strong migration effects. The global donor and acceptor data at 300 K show a linear dependence on donor concentration, but the decays cannot be described in terms of published theoretical models.
The sensitization with Yb3+ of other rare earth ions emission is intensively used in the last years for increasing excitation efficiency of infrared (diode laser) pumped solid state lasers. It proved as a good sensitizer for Pr3+ emission in various matrices. The spectral investigations for the elucidation of the nature of multisite structure of Pr3+ in YAG and of the Yb3+ co-doping on Pr3+:YAG are presented in this paper. In Pr3+:YAG two types of multisites have been observed: perturbed Pr3+ sites by nonstoichiometric defects and strongly coupled Pr3+ - Pr3+ near neighbor pairs. Pr3+ spectra in Yb3+, Pr3+:YAG show new lines assigned to near neighbor Pr3+ - Yb3+ pairs and Pr3+ - Yb3+ - Yb3+ triads.
The mutual static crystal field perturbations produced by near sensitizer and activator ions shift the energy levels and modify the radiative and energy transfer characteristics. The selective and discrete nature of these effects transforms the ensembles of sensitizer and activator ions into inhomogeneous systems and modify the spectral and temporal characteristics of emission from activator ions in sensitized laser crystals. These effects are exemplified for Cr3+ or Fe3+ sensitized emission of Tm3+:YAG.
The various processes leading to loss of efficiency and to heating effects in YAG:Nd laser crystals are discussed. It is thus shown that there are several unavoidable intrinsic sources of loss such as the spectral inhomogeneities in the melt grown crystals and the concentration quenching of emission and these impose upper limit quantum efficiency function on the activator concentration. The estimations made within these models are in a good agreement with the data on quantum efficiency of emission or on fractional thermal loading.
Spectral data on Pr3+ ion in Czochralski grown GGG Gd3Ga5O12) crystals are analyzed. Pr3+ spectra, investigated by high resolution absorption, site selective excitation and emission contain besides the main lines assigned to Pr3+ in dodecahedral c-sites, at least four other types of non-equivalent centers. Three satellites have been assigned to Pr3+ in dodecahedral c-sites perturbed by a non-stoichiometric defect, Gd3+ in octahedral sites, whose concentration was estimated from absorption. A reduction of local symmetry is observed from the spectra. It is connected with X-ray measurements that show forbidden {222} reflections in cubic ideal garnets, but with similar small intensities. Other lines are assigned to Pr3+(c)- Pr3+(c) near pairs, one of them showing a very strong quenching of 3P0 emission. Some of the mechanisms of 3P0 emission quenching and their influence on quantum efficiency are discussed.
The spectroscopic properties of F3+ ions in tetrahedral sites in garnets, together with the good packing of these sites around the dodecahedral position occupied by rare-earth ions, favor a very efficient energy transfer from Fe3+ to TM3+. Since Fe3+ does not introduce additional losses, this ion is a potential sensitizer for Tm3+ laser emission.
The possibility of sensitization of rare-earth laser active ions with infrared (especially two- micron) emission by Fe3+ is discussed. The spectroscopic and luminescence decay measurements on Fe, Tm or Fe:Tm doped GGG or YAG show that a very efficiency energy transfer from Fe3+ in tetrahedral sites to Tm3+ takes place showing prospects for new sensitization schemes.
The characteristics of energy processes in concentrated Er3+ system and their effects on the stationary three micron laser emission are discussed. It is shown that in certain conditions, valid for a broad range of concentrations, the laser equations could be solved analytically, the solution allowing to study the influence of various parameters on the laser emission. This revealed that a crucial role in determining a positive pump effect is played by a figure of merit that depends on the ratios of up-conversion rates from 4I11/2 and 4I13/2 levels and of the fractional thermal populations for the crystal field components involved in emission. In given pump conditions, the restrictions imposed on the figure of merit limit the emission wavelength range and the efficiency. The emission in Er3+ systems co-doped with Cr3+ is also discussed. Cr3+ co- doping not only improves the pump efficiency, but modifies the whole chain of energy transfer processes, leading to the extension of the laser wavelength range.
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