Crystal Field Sites Research Articles

Spectroscopy of multisites Cr3+ and Nd3+ in yttrium aluminium garnet

Cr3+ and Nd3+ non-equivalent crystal field sites were observed. By use of a high-resolution dye laser,2E and4A2 splittings, zero-phonon4A2 →2E,2T1,4T2 transitions and vibronic levels of2E doublets of Cr3+ non-equivalent centres were recorded. Energy transfers from Cr3+ multisites to Nd3+ were studied by site-selective spectroscopy and fluorescence decays.

Excited‐state electronic Raman scattering of alexandrite

AbstractRaman scattering is used to investigate the electronic excited‐states of alexandrite at low temperature (13 K). The Raman spectra of Cr3+ ions in two non‐equivalent types of crystal field sites, one having mirror symmetry Cs the other having inversion symmetry C1, are reported. With λe = 514.5 nm excitation, the Raman transitions of the metastable state 2A′ (Rm1) → 2A″(Rm2) and 2A′(Rm1) → 2A″(2T1) of Cr3+ in the Cs site are 36 cm−1 and 600 cm−1, respectively; with λe = 488.0 nm excitation, the Raman transition between 2Ag(R11) and 2Ag(R12) of Cr3+ in the C1 site is 116 cm−1.

Crystallography, spectroscopic analysis, and lasing properties of Nd^3+:Ba_2ZnGe_2O_7

Nd3+:Ba2ZnGe2O7 (BZAG) is an attractive material to be pumped by laser diodes because its fluorescence lifetime is 305 μsec and it has a broad absorption centered at 806 nm. In addition, Nd3+:BZAG has a broad fluorescence spectrum and offers the potential for tunable laser output wavelengths centered at 1.05 and 1.34 μm. Time-resolved, site-selection spectroscopy measurements revealed ten different crystal-field sites for Nd3+ in BZAG, but strong energy transfer between nonequivalent ions at room temperature results in emission from only one major site. Diode-array side pumping of 5-mm-diameter rods produced optical slope efficiencies of 22.8%. Crystal structure, Judd–Ofelt analysis, and laser performance are discussed.

Crystal field effects on the infrared spectra of phones I and II of crystalline CH 4

The results of our investigations of the infrared spectra of crystalline films of 12CH 4 and 13CH 4 are presented. The observed fine structures of the ν 3 and ν 4 fundamentals in the i.r. spectra of phase II (below 20 K) are principally accounted for in terms of crystal field (site and factor group) splittings as are those of the overtones and combinations. The appearance of ν 1 and ν 2 in the i.r. spectra and the splittings of the degeneracies of ν 3 and ν 4 are suggestive of partial disorder (phase II) rather than free rotation of the molecular units in the James and Keenan model of crystalline CH 4.

On the coordination environment of rare earth ions in oxide glasses: Calcium titanosilicate and sodium aluminosilicate glasses

Careful analysis of fluorescence line narrowed (FLN) 5 D 0 → 7F 1,2 spectra of Eu 3+ -doped sodium aluminosilicate (ALGL) and calcium titanosilicate (SPGL) glasses excited via the 5D 0 ← 7F 0 transition reveals the presence of an underlying broadband background. The intensity of this background relative to the FLN signal increases at higher excitation energies and probably arises because of phonon assisted absorption to lower energy crystal field sites. If the background is taken into account when deconvoluting the spectra, crystal field fits using a C 2v Hamiltonian without J-mixing are superior to others previously reported. Based on the similarity of all reported Eu 3+ FLN spectra and a clustering of points on the major crystal field parameter ratios plot ( B 22/B 20 vs B 44/ B 40) it is suggested that Eu 3+ forms quasi-molecular complexes with the glass network in oxide glasses. The framework for a model describing the coordination sphere of rare earth ions in oxide glasses is developed. The framework is used to explain qualitative features of Eu 3+ FLN spectra, including the asymmetry of the 5 D 0 ← 7F 0 absorption, the functional dependence of the 5D 0 → 7F 1 linewidths on excitation wavelength, and the dependence of the degree of clustering of points in the B 22/ B 20 vs. B 44/ B 40 plot on the ratio of non-bridging oxygens in the glass to Eu 3+ ions.

ENERGY TRANSFER OF Eu3+ EXCITED STATES IN SBN CRYSTAL

Eu3+ ions at different crystal-field sites in SBN crystal are selectively pumped. From the time-resolved fluorescence spectra of 5D0→7F1, the energy transfer rates between ions in non-equivalent sites are evaluated. The dependence of fluorescence peak-frequency on the excitation wavenumber indicates the continuous distortion of local crystal field.

Laser-induced grating spectroscopy of alexandrite crystals.

Four-wave-mixing techniques were used to establish and probe population gratings of ${\mathrm{Cr}}^{3+}$ ions in alexandrite crystals at temperatures between 10 and 300 K. The results were interpreted in terms of the interaction of the laser radiation with a two-level atomic system. They provide information about the characteristics of four-wave-mixing signals for this physical situation as well as being useful in characterizing the properties of energy transfer and dephasing within the ensemble of ${\mathrm{Cr}}^{3+}$ ions. The patterns of the transient four-wave-mixing signals are consistent with a model based on the pumping dynamics of ions in the mirror and inversion crystal-field sites. The variation of the signal intensity with laser power is strongly affected by beam depletion. The characteristics of exciton migration among ${\mathrm{Cr}}^{3+}$ ions in mirror sites were determined from the results of measuring the variation of the signal decay rate with grating spacing. The temperature dependences of the ion-ion interaction rate, the exciton-phonon scattering rate, and the diffusion coefficient were determined. These are found to be essentially the same for pumping into the $^{4}\mathrm{T}_{2}$ and $^{2}\mathrm{E}$ levels, but the effects of scattering from a grating of ions in inversion sites is much stronger for $^{4}\mathrm{T}_{2}$ pumping. The dephasing times for the atomic system were found from analyzing the variation of the signal intensity with grating spacing. For pumping into the $^{4}\mathrm{T}_{2}$ level the dephasing is dominated by radiationless decay processes. A model is presented for the decay channel that provides a theoretical explanation for the decay process which is consistent with the measured temperature and frequency dependences of the results as well as their variation with crystal-field strength. For pumping into the $^{2}\mathrm{E}$ level the dephasing is dominated by dephasing processes associated with the inhomogeneous linewidth of the transition.

Preparation, structure, and spectroscopic properties of Nd3+:{La1-xLux}3

Single crystals of lanthanum lutetium gallium garnet (LaLuGaG) were grown by the Czochralski pulling technique. X-ray diffraction and elemental analysis performed on these samples indicate that these garnets do not form with simple stoichiometry described as {La${\mathrm{}}}_{3}$[Lu${]}_{2}$${\mathrm{Ga}}_{3}$${\mathrm{O}}_{1}$2 but with increased Lu concentration in the dodecahedral site and Ga occupancy in the octahedral site. Optical absorption and fluorescence spectra confirm these results, showing inhomogeneous broadening of the spectral lines of ${\mathrm{Nd}}^{3+}$. Various laser gain measurements were performed on {${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Lu}}_{\mathit{x}}$${\mathrm{}}}_{3}$[${\mathrm{Lu}}_{1}$-y${\mathrm{Ga}}_{\mathit{y}}$${]}_{2}$${\mathrm{Ga}}_{3}$${\mathrm{O}}_{12}$ crystals containing 4.3 and 1.3 at. % ${\mathrm{Nd}}^{3+}$ to determine the usefulness of this material as a laser. No optical gain was observed. Time-resolved, site-selection spectroscopy measurements were performed to determine the effects of ion-ion interaction, and show the presence of very weak energy transfer between ions in nonequivalent crystal-field sites. Two-photon excitation spectroscopy measurements demonstrate the presence of very strong two-photon--absorption transitions, which prevents lasing in this garnet.

Inhomogeneous broadening and energy transfer in KNdP4O12:Pr3+

The nature of the inhomogeneous broadening in Pr3+-doped KNdP4O12 crystals is examined by the method of site-selection time-resolved spectroscopy. The presence of Pr3+ ions in two different crystal-field sites, suggested by the observed splitting of the 3P0 state, is confirmed by the selectively excited 3P0 to 3F4 fluorescence spectra. The process of quenching of the 4F3/2 neodymium fluorescence in the presence of Pr3+ traps is studied using time-resolved fluorescence measurements. At low temperature the time dependence of the Nd3+ fluorescence intensity in 1% Pr3+-doped KNdP4O12 is well described by the Inokuti-Hirayama model with the nearest-neighbour transfer rate of 2*105 s-1. The ion-ion transfer rates are also directly measured for various Nd-Nd and Nd-Pr pair satellite lines. Using these results the shape of the 4F3/2 fluorescence decay is predicted in the framework of a discrete host structure model and compared with room temperature data. The process of hole burning at the middle of the Nd excitation profile in KNd0.9Pr0.1P4O12 is explained by the phonon assisted cooperative interaction between Nd and Pr ions.

Optical spectroscopy of Cr 3+ and Rh 2+ in sodium-beta [formula omitted]alumina

Ion exchange techniques were used to substitute Cr 3+ and Rh 2+ ions for the sodium ions present in sodium-beta″-alumina single crystals. The optical absorption and fluorescence spectra were obtained and the fluorescence lifetimes measured for both types of samples. In each case the emission consisted of two bands having significantly different decay times. The temperature dependences of the fluorescence intensities and lifetimes were measured for these bands. The results are interpreted in terms of the energy level structures of these ions in different crystal field sites in this host lattice and the diffusion of ions in the conduction plane which changes these sites.

Fluorescence quenching in KNdxGd1-xP4O12 crystals studied using an isolated ion-pair interaction.

The quenching of fluorescence from the $^{4}\mathrm{F}_{3/2}$ excited state of ${\mathrm{Nd}}^{3+}$ in ${\mathrm{KNd}}_{\mathrm{x}}$${\mathrm{Gd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{P}}_{4}$${\mathrm{O}}_{12}$ (abbreviated KNGP) crystals is investigated. A rich satellite structure of the 1.6-K excitation and absorption spectra is observed and related to the neodymium pairs. Single ${\mathrm{Nd}}^{3+}$ ions in different crystal-field sites were also detected. The ion-ion interaction is directly analyzed by lifetime measurements of Nd-pair satellites in weakly concentrated samples. With the use of the discrete host structure model and the measured microscopic interaction parameters, the shape of the fluorescence decay in strongly concentrated crystals is predicted. Comparison of the theoretically evaluated cross-relaxation rate between two neighboring ${\mathrm{Nd}}^{3+}$ ions and the quenching rates of the satellites indicates that the ions are coupled by the electric dipole interaction. The $^{4}\mathrm{F}_{3/2}$\ensuremath{\rightarrow}${\mathrm{}}^{4}$${\mathrm{I}}_{9/2}$ fluorescence spectra are measured as a function of excitation wavelength and time delay after the excitation pulse. These laser site-selection time-resolved spectroscopy results indicate that at low temperature there is no fast spectral diffusion among the ${\mathrm{Nd}}^{3+}$ ions in KNGP.

Spectroscopic properties of alexandrite crystals.

Details of the optical-spectroscopic properties of alexandrite (${\mathrm{BeAl}}_{2}$${\mathrm{O}}_{4}$:${\mathrm{Cr}}^{3+}$) crystals were studied by different laser-spectroscopy techniques. The temperature dependences of the fluorescence lifetimes and widths of the zero-phonon lines were found to be quite different for ${\mathrm{Cr}}^{3+}$ ions in the mirror and inversion crystal-field sites. The results indicate that direct phonon-absorption processes dominate both thermal line broadening and lifetime quenching for ions in the mirror sites while phonon-scattering processes dominate the line broadening of inversion-site ions and leave their lifetime independent of temperature. Tunable-dye-laser site-selection methods were used to obtain the excitation spectra of the ${\mathrm{Cr}}^{3+}$ ions in inversion sites at low temperature and to identify six types of exchange-coupled pairs of ${\mathrm{Cr}}^{3+}$ ions in the lattice. Time-resolved site-selection spectroscopy was used to monitor the energy transfer between ${\mathrm{Cr}}^{3+}$ ions in mirror and inversion sites at both low and high temperature. Finally, high-power, picosecond pulse excitation was used to produce two-photon absorption, and the resulting emission spectrum was found to exhibit a new fluorescence band in the 400-nm spectral region.

Energy transfer between Eu 2+ ions in RbMgF 3 crystals

Time-resolved spectroscopy after pulsed nitrogen laser excitation was used to characterize the energy transfer between Eu 2+ ions in different crystal field sites in RbMgF 3 crystals. The results are consistent with electric dipole—dipole interaction and indicate that the Eu 2+ ions are forming clusters in this host. As temperature is raised, the upper crystal field components of the metastable states of the ions become thermally populated and this changes the characteristics of the transfer process.

Broad band fluorescence of transition metal doped garnets and tungstates

We give a survey on the laser relevant spectroscopic properties of Cr 3+ and Mn doped garnets and tungstates. From absorption, fluorescence, excitation and lifetime measurements between 4K and 300K the energy level schemes of the active ions are derived. It is shown, that Cr 3+ exhibits broad band fluorescence in crystals with rather low crystal field and/or low site symmetry. This is demonstrated by the different garnets, by the tungstates Al 2(WO 4) 3 and Sc 2(WO 4) 3 and by Cr: ZnWO 4. In the latter compound the maximum of the fluorescence is shifted to about 1μm due to the acentric Cr 3+ site. Mn is incorporated in YAG as Mn 2+, Mn 3+ or Mn 4+ depending on the growth conditions. From the spectroscopic data Mn 3+ is found to be a promising ion for short wavelength tunable laser applications.

Studies of ion pair interaction in KNd xGd 1-xP 4O 12 crystals using site selection time resolved spectroscopy

High resolution pulsed dye lasers were used to investigate the fluorescence of Nd 3+ions in KNd xGd 1−xP 4O 12 crystals. It was found that the low temperature excitation and absorption spectra exhibit the satellite structure. After selective excitation of the satellites the fluorescence spectra and lifetimes of the 4F 3/2 state were recorded. These allowed to distinguish the single neodymium ions in perturbated crystal field sites as well as pairs of ions. Then, in the framework of the discrete host structure model using the measured quenching rates of the satellites the shape of the fluorescence decay was analyzed.

Site-selection spectroscopy of Eu3+-doped germinate glass

A tunable dye laser was used to selectively excite Eu3+ ions in nonequivalent types of crystal field sites in a germinate glass host. The results demonstrate the existence of three major types of sites in this host. In addition, time-resolved spectroscopy results show the presence of energy transfer between ions in these different types of sites.

Energy transfer between Eu3+ ions in LiNbO3, CaWO4, and EuxY1−xP5O14 crystals

Narrow-band laser excitation is used to selectively pump Eu3+ ions in different types of crystal field sites in four host crystals LiNbO3, CaWO4, EuP5O14, and Eu0.01Y0.99P5O14. Energy transfer between ions in nonequivalent sites is characterized using time-resolved spectroscopy techniques and phenomenological rate parameter models. In all four cases, the energy transfer is found to occur by a single-step process through a weak electric dipole–dipole interaction. However, the different spatial distributions of the Eu3+ ions in these hosts produce differences in the energy transfer characteristics.

Energy transfer in CaWO4: Sm3+

The interaction between Sm3+ ions in CaWO4 crystals has been investigated using high resolution dye laser spectroscopy techniques. The results are found to be different from those obtained using either broadband or intermediate resolution laser excitation sources. At low temperatures, both concentration quenching through cross-relaxation and resonant energy transfer processes are found to take place by exchange interaction. At high temperatures, energy transfer between Sm3+ ions in nonequivalent crystal field sites takes place by a two-phonon diffusion process with a diffusion coefficient of the order of 4.9×10−10 cm2 s−1.

Time-resolved site-selection spectroscopy studies of NdAl 3(BO 3) 4 crystals

A pulsed, tunable dye laser was used to selectively excite Nd 3+ ions in nonequivalent crystal field sites in NdAl 3(BO 3) 4 crystals and energy transfer between ions in different types of sites was studied by monitoring the time evolution of the fluorescence spectrum. The results show that the energy transfer rate varies as t -1 2 and increases with temperature. The predictions of various models of phonon-assisted energy transfer are compared to the results.

Time-resolved site-selection spectroscopy investigation of Y3(Al1−xGax)5O12 : Nd3+ crystals

We have used selective laser excitation techniques to investigate the spectroscopic properties of Nd3+ ions in Y3(Al1−xGax)5O12 crystals and found the mixed crystal nature of the host produced both distinct structure and inhomogeneous broadening in the Nd3+ spectra. We employed time-resolved spectroscopy measurements to characterize energy transfer between Nd ions in nonequivalent crystal field sites and the results are consistent with the migration of energy among Nd ions in the dominant type of site with trapping occurring at Nd ions in defect sites. The migration and trapping processes require thermal activation and are consistent with a two-phonon assisted energy transfer mechanism. The efficiency of energy transfer is greater in mixed crystals than in unmixed garnet hosts, and it is shown that this is associated with the increase in the transition matrix elements in the mixed crystals.