The optical spectra of Yb3+ in YAG and Y2O3 and Ce3+: YAG show complex structure with contributions of vibronic satellites that extend beyond the spectral range delineated by the pure electronic transitions. The rather strong vibronic satellites are connected to the enhanced electron-phonon interaction of the RE3+ ions at the ends of 4f(n) series. Thus, tentatively we suggest that some of reported satellites in infrared absorption of Ce3+ in YAG crystals could be vibronics. The optical spectra of our Yb-doped YAG and Y2O3 ceramics are similar to those reported for single crystals. The vibrotlic lines intensities comparable with other lines of Yb makes separation of the zero-phonon lines from vibronics very difficult, particularly for Y2O3 which has two cationic sites. Measurements of optical spectra of Yb- and (Nd,Yb)doped ceramics under direct excitation in Yb3+ or via energy transfer from Nd3+ and comparison with the phonon spectra enables such separation and identification of Stark level structures. The particularly large intensities of the vibronic lines in vicinity of the zero-phonon lines can be connected to near resonant effects between the phonon energies and the Stark splitting of manifolds, that are also responsible for the broadening of electronic lines corresponding to transitions to high energy levels of F-2(5/2,7/2) manifolds (even at 10 K by phonon emission). This analysis indicates that the observed one micron spectra of Yb-doped YAG and Y2O3 ceramics belong exclusively to Yb3+. (C) 2016 Elsevier B.V. All rights reserved.

Comparative high resolution spectroscopic investigation between 10 and 300 K of the less investigated Pr3+ and Sm3+ ions in the YAG crystals and transparent ceramics evidenced that the satellite structures around the main optical lines corresponding to the ions in the dodecahedral c-sites belong either to ions whose crystal field is perturbed by adjacent lattice irregularities or to vibronics. In case of Pr: YAG such irregularities are mainly caused by presence of doping ions in near sites (n.n. and n.n.n. pairs) in both types of materials, while in the melt-grown crystal an additional irregularity is the presence of excess Y3+ ions that occupy part of the octahedral Al3+ sites. The new data on perturbed Pr3+ pair centers remove the ambiguity in identification of several lines that were previously attributed to transitions of the isolated Pr3+ ions forbidden in the D-2 symmetry of YAG ceramics and rule out the forced assumption of lowering of symmetry. In case of Sm3+: YAG ceramic the main perturbed center corresponds to n.n. pairs and plays major role in utilization of this material as ASE suppressor for the Nd: YAG laser. The presence of electron-phonon interaction was evidenced by shift or broadening of lines or by resolved weak vibronic structures. For selected optical transitions, such as the 10 K Sm3+ (4)G(5/2) -> H-6(5/2,7/2,9/2) emission in YAG the presence of enhanced electron-phonon effects was evidenced by strong alteration of the shape of electronic lines, such as apparition of asymmetries or even doubling of electronic lines whose Stark gap to other transitions is in quasi-resonance with Raman phonons of YAG. zero phonon lines at energies close to Raman phonons of YAG of T-2g symmetry. The shape of the lines is analyzed in terms of the theory of the quasi-resonant electron-phonon coupling between a pure electronic state and vibronics. (C) 2017 Elsevier B.V. All rights reserved.

Disordered Sm3+ doped Y3ScxAl5-xO12 (Sm:YS(x)AG) polycrystalline ceramics with compositional parameter x = (0.05, 0.5, 1, 2) and different Sm3+ ions concentrations were obtained by solid state reaction method. XRD investigations show the linear increase of the lattice constant with compositional parameters x. New spectroscopic data obtained from high resolution optical absorption and emission spectra of Sm3+ doped YS(x)AG ceramics reveal composition effects on Sm3+ optical spectra: modification of the shapes and shifts of the lines, presence of the additional satellites and unresolved multicenter structure, etc. The analysis of these data allowed us to establish a partial structure of Sm3+ energy levels in Sm:YS(x)AG (x = 2) ceramics. The emission kinetics of the (4)G(5/2) level for different Sm3+ concentrations and Sc content in Sm:YS(x)AG were measured and analyzed. (C) 2016 Elsevier B.V. All rights reserved.

The paper presents a critical review of the actual knowledge on Nd:YAG and brings new data on the spectroscopic properties and energy transfer self-quenching processes and on their implication in laser emission.

The paper investigates the characteristics of sensitization processes of Nd3+ emission in YAG ceramics under broad band pumping by co-doping with Cr3+ and the prospect of using Nd3+ and Cr3+ for sensitization of emission of Yb3+. It is evidenced that the energy transfer from Cr3+ to Nd3+ involves both direct and weak migration-assisted processes and is thus dependent on the concentrations of both species. It is also found that the ion–ion interaction responsible for the direct transfer contains besides the dipole–dipole coupling strong superexchange contribution that dominates the transfer to the Nd3+ ions up to the third coordination sphere and has major implication in sensitization. Investigation of (Cr, Nd, Yb)-doped YAG ceramics shows that Cr3+ can sensitize the emission of Yb3+ both via the chain Cr–Nd–Yb or by direct Cr–Yb energy transfer. The prospect of utilization of these processes in the solar-pumped laser is discussed.

New spectroscopic data obtained from high resolution low temperature absorption and emission spectra of Nd3+ in mixed scandium aluminum garnets Y3ScxAl5-xO12 - (x = 0-2) translucent ceramics revealed transition dependent composition effects: modification of the shapes (Lorentz at x = 0 and 2, quasi-Gauss at x = 1, x-dependent asymmetric for other x values, with obvious multicenter structure for low x), widths and shifts of the lines. Nd3+ electronic structure dependence on structural changes with composition is analyzed in terms of nephelauxetic effect and maximum splitting of manifolds: Sc3+ co-doping reduces the nephelauxetic effect, and the increase of F-4(3/2) splitting from 85 cm(-1) (x = 0) to 98 cm-1 (x = 2) denotes the lowering of local symmetry. The multicenter structure and inhomogeneous broadening of Nd3+ lines is attributed to crystal field distributions determined by the random occupancy of the octahedral sites by Sc3+ and Al3+. For low x (0.2) the resolved two satellites S-1, S-2 that accompany Nd:YAG lines are correlated to anisotropic crystal field perturbations produced by the n.n. Sc3+ by analogy to those determined by Y3+-antisites (excess of Y3+ ions that enter in octahedral sites of the melt-grown YAG crystals). The temperature evolution of the Nd3+ spectral characteristics (line intensity, shift, broadening) in the 10-300 K range is analyzed in terms of thermal population of the Stark levels, of the effect on electron-phonon interaction and on lattice expansion. The relevance of the spectroscopic properties on the laser emission characteristics in these systems is discussed. (C) 2015 Elsevier B.V. All rights reserved.

New aspects on the Ce3+ in YAG transparent ceramics, obtained from absorption and emission spectra in the 10-300 K range, under different visible excitation wavelengths, along with emission decays are presented. The spectra and lifetime of regular Ce-Y(3+) center are similar to those reported for single crystals. In the attempt to reveal perturbed Ce3+ centers by defects, the 5d --> 4f emission spectra under pulsed 532 nm (5 ns) excitation (below the Ce3+ absorption band) were recorded. The spectra and decays (at different wavelengths) are function on temperature, revealing at least three types of Ce3+ new emitting centers. The low temperature spectra, different from those of Ce-Y(3+) center, are dominated by a set of strongly perturbed centers with very short lifetimes. At least two other types of perturbed centers were separated by using wavelength and temperature dependences of decays. The spectral characteristics of these perturbed Ce3+ centers by lattice defects are analyzed in terms of the effects of the structural changes induced by Ce3+ doping on the interaction with defects, by using recent reported results. (C) 2014 Elsevier B.V. All rights reserved.

High resolution spectroscopic measurements from low to room temperatures of Sm3+:Lu2O3 microcrystalline translucent ceramics, prepared by solid-state reaction method, were performed. The presence of Sm3+ in both crystallographic sites of C-2 and C-3i symmetry in Lu2O3 was demonstrated. Though the spectra are dominated by electric-dipole transitions of C-2 centers, Sm3+ in centrosymmetric sites of C-3i symmetry was detected in magnetic-dipole allowed transitions (Delta J = 0, +/- 1) with significant oscillator strengths, (4)G(5/2) -> H-6(5/2), H-6(7/2) in emission and H-6(5/2) -> F-4(5/2) in excitation spectra by a quasi-selective excitation time-resolved method at 300 K. Spectral parameters such energy level schemes, concentration dependence of emission kinetics and emission quantum efficiency specific to each center are provided. (C) 2013 Elsevier B. V. All rights reserved.

Spectroscopic investigation of the radiation-resistant Nd:GSGG crystals and ceramics reported in this paper evidences that the absorption line at 883 nm corresponding to the unusual situation of quasi-degenerate absorption transitions 4I9/2(2) →4F3/2(1) and 4I9/2(3) →4F3/2(2) can be used for efficient direct diode laser pumping of this material, with stable absorption over an extended temperature range. It is inferred that the reduction of the quantum defect at this wavelength of pump compared to the traditional 807 nm pumping could improve the laser parameters and reduce drastically the heat generation, leading to a considerable extension of the power scalability. This possibility is demonstrated for the first time in the case of the 1061 nm laser emission in a continuous-wave and in the repetitive active acousto-optic and passive Q-switched laser emission. In all regimes this manifests in the reduction of the laser threshold, increase of slope efficiency and extension of the average power range. Additionally, in the active Q-switching this enables increased pulse energy and reduced pulse duration. The direct pumping could revitalize the utilization of Nd:GSGG for construction of lasers with storage of population inversion or working in an ionizing radiation environment.

A comparative analysis of the low temperature energy level structure of Sm3+ ion in YAG and sesquioxide (Y2O3, Lu2O3, Sc2O3) microcrystalline ceramics, based on new and previously reported data is presented. The Stark level structure of Sm3+ ion is discussed in terms of nephelauxetic and crystal field effects, determined by structural characteristics of the centers (of D-2 symmetry for YAG and C-2 or C-3i for sesquioxides). The Sm3+ electronic structure of C-2 centers in sesquioxides presents some peculiarities (barycenters positions, F-4(3/2) splitting along the series, etc.), that are correlated to J mixing effects, important for low symmetry systems and increasing from Y2O3 to Sc2O3. The energy levels of Sm3+ in centrosymmetric C-31 centers in Sc2O3, obtained by a time-resolved technique, along with the previous reported data on sesquioxides are also analyzed. (C) 2013 Elsevier B.V. All rights reserved.

The spectroscopic bases of suppression of amplified spontaneous emission (ASE) of Nd3+ in YAG or Y2O3 ceramic lasers by Sm3+, from cryogenic to room temperature, are presented and the mechanisms of thermal shifts are analyzed.

The optical spectroscopic properties of RE3+ (Nd, 1 at. % or Yb, 1 to 10 at. %)-doped calcium-lithium-niobium-gallium garnet (CLNGG) single crystals and ceramics in the 10 K–300 K range are analyzed. In these compositionally disordered materials, RE3+ substitute Ca2+ in dodecahedral sites and the charge compensation is accomplished by adjusting the proportion of Li+, Nb5+, and Ga3+ to the doping concentration. The crystals and ceramics show similar optical spectra, with broad and structured (especially at low temperatures) bands whose shape depends on temperature and doping concentration. At 10 K, the Nd3+ 4I9/2 → 4F3/2, 5/2 and Yb3+ 2F7/2 → 2F5/2 absorption bands, which show prospect for diode laser pumping, can be decomposed in several lines that can be attributed to centers with large differences in the crystal field. The positions of these components are the same, but the relative intensity depends on the doping concentration and two main centers dominate the spectra. Non-selective excitation evidences broad emission bands, of prospect for short-pulse laser emission, whereas the selective excitation reveals the particular emission spectra of the various centers. The modeling reveals that the nonequivalent centers correspond to RE3+ ions with different cationic combinations in the nearest octahedral and tetrahedral coordination spheres, and the most abundant two centers have 4Nb and, respectively, 3Nb1Li in the nearest octahedral sphere. At 300 K, the spectral resolution is lost. It is then inferred that the observed optical bands are envelopes of the spectra of various structural centers, whose resolution is determined by the relative contribution of the temperature-dependent homogeneous broadening and the effects of crystal field disordering (multicenter structure, inhomogeneous broadening). The relevance of spectroscopic properties for selection of pumping conditions and of laser design that would enable utilization of the broad optical bands for efficient laser emission and reduced heat generation is discussed.

The Nd3+ and Sm3+ spectroscopic characteristics (energy levels and emission wavelengths) dependences on the host structure and composition are analyzed. These ions are interesting for visible emission: blue obtained by doubling the Nd3+ laser emission in 900 nm range and orange -red by direct pumping of Sm3+ ion. The hosts are different oxide materials: sesquioxides ceramics Y2O3, Sc2O3, garnet ceramics Y3Al5O12 - YAG and hexaaluminates Sr1-xLaxMgxAl12-xO19 - ASL single crystals. The data are discussed in terms of the nephelauxetic effect and the crystal field strengths. The role of both effects on the emission wavelengths is outlined.

The Sm3+ optical spectra in Y2O3 translucent ceramics are dominated by electric-dipole transitions connected to the centers of C2 symmetry, similar to those reported for single crystals. However, quasi-selective time-resolved excitation and emission spectroscopy investigation evidenced, for the first time, additional lines in the magnetic dipole allowed 6H5/2 → 4G5/2, 4F5/2 and 4G5/2 → 6H5/2, 7/2 transitions, associated with the centrosymmetric C3i centers. An energy level scheme for C3i and an improved one for C2 centers are proposed. The 4G5/2 emission lifetimes of these two structural centers (1.48 ms for C2 and 8.4 ms for C3i at 300 K) as well as the emission quenching are quite distinct, whereas Sm3+ ions in C2 centers are involved both in intra-center C2 → C2 cross-relaxation on intermediate levels and inter-center C2 ↔ C3i energy transfers (that imply only Stark levels of the ground 6H5/2 and metastable 4G5/2 manifolds), Sm3+ ions in C3i sites can participate only in the last processes. The spectroscopic data determined in this work (energy level structures, absorption and emission cross-sections, de-excitation processes parameters) suggest the prospect of Sm-doped Y2O3 ceramics as phosphor or four-level laser active materials on the highest intensity line at 608 nm in the 4G5/2 → 6H7/2 transition under GaN diode laser (~405 nm) pumping or as suppressors of amplified one-micron spontaneous emission for the Nd:Y2O3 lasers.

Strontium lanthanum hexa-aluminates crystals (ASL) doped with Sm3+ with formula Sr1-xSmyLax-yMgxAl12-xO19 are investigated for lasers operating in visible spectral range. The crystal with compositional parameters x = 0.2 and y = 0.05 have been grown by the Czochralski pulling technique. High resolution optical absorption and emission spectra at 10K and room temperature were recorded. Preliminary investigations of spectroscopic properties of Sm3+ doped ASL crystal were carried out.

The spectroscopic characteristics of Dy3+ in YAG (Y3Al5O12) transparent ceramics prepared by solid state synthesis were investigated, with special attention to the visible (yellow, blue) emissions that could be pumped directly by the new developed blue–violet or near UV laser diodes. The absorption and emission spectra, recorded at different temperatures from 10 to 300 K, were analyzed and relevant new parameters, improved energy level scheme, temperature effects, intensity parameters based on Judd-Ofelt theory and radiative transition rates, branching ratios, etc., were estimated and the data are compared with previous reports on garnet single crystals. The potential of the Dy3+: YAG transparent ceramics for yellow or blue laser emission is discussed.

This work is focused on spectral investigations of Tm3+ doped Sc2O3 transparent ceramic as potential material for diode-pumped solid-state laser emitting around 2μm. In the context of the Judd–Ofelt (J–O) theory a series of spectroscopic parameters such as J–O intensity parameters, oscillator strengths, radiative transitions probabilities, and radiative lifetimes as well as branching ratios are evaluated. The gain cross-sections which lead to an estimation of the probable operating laser wavelength for the 3F4→3H6 Tm3+ laser transition were also calculated.

The characteristics of Nd→Yb energy transfer in (Nd, Yb):Y3Al5O12 (YAG) transparent ceramics, obtained from steady state and dynamic spectral studies on different single doped and codoped samples, prepared by solid state synthesis, are presented. The Yb3+ emission in codoped (Nd, Yb):YAG samples under cw excitation of the donors (Nd3+) revealed efficient energy transfer to Yb3+ (at low temperatures too), due to the overlap of the Nd3+ F43/2→I49/2 emission and Yb3+ F27/2→F25/2 absorption. From the global decays of the Nd3+ F43/2 level in single and codoped samples, the transfer efficiencies were estimated; they increase with Yb3+ content reaching ∼93% for 1 at. % Nd, 5 at. % Yb:YAG sample. The mechanisms and parameters of the Nd→Yb energy transfer were inferred from the Nd3+ decays in the codoped samples, after the separation of the self-quenching inside the donors (Nd3+) from the transfer to acceptors (Yb3+). It was proved that the Nd→Yb energy is resonant and dominated by dipole-dipole interactions. Due to the large transfer efficiencies, (Nd, Yb):YAG ceramic could be a promising material for Yb3+ emission under pumping in strong Nd3+ absorption lines.

In the attempt to obtain new efficient laser systems, the Nd → Yb energy transfer characteristics in Y2O3 transparent ceramics were investigated. The spectral characteristics that assure the efficient resonant Nd3+ → Yb3+ energy transfer are presented, especially the overlap integral of the Nd3+ 4F3/2 → 4I9/2 emission with the Yb3+ 2F7/2 → 2F5/2 absorption in (Nd, Yb):Y2O3. The static spectral data reveal that the Nd3+ → Yb3+ energy transfer shows: a strong dependence on Yb3+ content (less on Nd3+), a small variation with temperature and the implication of both Yb3+ sites in transfer, though only Nd3+ in C2 are active. From the analysis of Nd3+ 4F3/2 emission decays in double doped samples it is concluded that the Nd → Yb transfer for Nd ∼1 at.% is of direct type, through the dipole–dipole interaction with global energy transfer microparameter ∼CDA,Nd–Yb ≈ 1 × 10−38 cm6 s−1 and transfer efficiencies up to 98% for rather low (∼5 at.%) Yb3+ content.

ABSTRACT This article presents new data on multicenter structure of the optical spectra and of emission dynamics of the doping ions in various types of crystalline and polycrystalline cubic laser materials (garnets, sesquioxides). It is inferred that the satellite structure of the active ions is induced by crystal field perturbations inside the near neighbor associations of the active ion with specific defects of the host lattice (P-type satellites independent on doping concentration) or with other active ions (M-type satellites dependent on doping concentration). Such satellite structures are described for various rare earth ions; including Nd3+, Pr3+, Er3+. In case of the garnet crystals, both types of satellites are present, and the satellites P can be connected with departures from stoichiometry; in garnet ceramics, they are much less intense. In contrast, the structures of M satellites are similar in garnet crystals and ceramics, indicating similar distribution of the doping ions in the host matrix. The luminescence spectra of these materials are influenced by the emission quantum efficiency of the various centers. In the case of sensitized materials, the mutual crystal field perturbations could modify the absorption spectra on both the sensitizer ions. In the case of materials with charge compensation, the electric charge disordering determined by the process of compensation strongly influences the optical spectra of the doping ions and their distribution. The relation between the global spectroscopic properties and those measured by microscopic methods is also considered. The connection between the fabrication process, structure, spectroscopic, and laser emission properties is shown and it is inferred that the multicenter structure of the spectra, together with emission dynamics, can be an efficient tool for investigating the microstructure of the laser materials, for optimization of laser properties, or for tailoring new laser materials.