Optical spectroscopy of Cr3+ ions in LiF single crystals

This paper reports on the luminescence spectrum, its excitation-wavelength dependence and decaytime of Cr³⁺:LiF crystals. In contrast to Al₂O₃ and MgO, where the Cr³⁺ luminescence at low temperature is due to the R-line and its vibronic sideband, luminescence from Cr³⁺:LiF is assigned to the broad-band ⁴ T ₂⁴ A ₂ transition. A periodic structure in the vibronic sidebands of this transition occurs with energy separations of ca. 60 meV. The linear polarisation of the vibronic sidebands shows that their radiative transition takes place from the lowest excited level of the ⁴ T ₂ state, i.e. ⁴ B ₁, into the ⁴ A ₂ ground state of Cr³⁺ ions in orthorhombic symmetry sites. The crystal-field parameters calculated from the experimental data, and luminescence decay-time of the two polarized configurations of the ⁴ B ₁(⁴ T ₂)⁴ A ₂ transition are also reported.     

Study of sensitization of Cr4+ ions by Yb3+ ions in yttrium-aluminum garnet crystals

The possibility of using ytterbium ions as sensitizers of luminescence of tetravalent chromium in yttrium-aluminum garnet single crystals is studied. It is found that average values of the microparameters of electronic energy transfer between ytterbium and chromium ions are C _DA=7.05×10^?36 cm⁶/s and C _DD=4.55×10^?43 cm⁶/s.     

Nonlinear theory of multiphonon relaxation of excited rare-earth ions in laser crystals

A general formula for the probability of multiphonon nonradiative transitions between J multiplets of 4f ^N states of trivalent rare-earth ions in a crystal derived for a nonlinear mechanism is discussed. A technique is developed for calculating the quantities involved in this formula. Particular attention is given to calculating the spectral density J ^(p)(Ω), which is the Fourier transform (at the transition frequency) of the pth power of the correlation function K(t) of host ion displacements. Based on the central limit theorem from probability theory, an approximation is proposed for the spectral density J ^(p)(Ω). The theoretical values of nonradiative multiphonon transition rates are compared with experimental ones.     

Local structure of the trigonal defect center for Cr3+ ions in KMgF3 crystals

Abstract

The zero-field splitting D, the anisotropic g-factors g _∥, Δg(=g _∥ ? g _⊥) and the first excited state splitting Δ(² E) for the trigonal Cr³⁺–V_K center in KMgF₃: Cr³⁺ crystals have been studied from Macfarlane's high-order perturbation formulas. From the studies, the local structure of the trigonal center is obtained. The local lattice distortions (i.e., the displacement directions of the ions in the center) are consistent with the expectation based on the electrostatic interaction.     

Interaction of excited holmium and thulium ions in yttrium scandium gallium garnet crystals

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 52, No. 4, pp. 598–602, April, 1990.     

Radiative and non-radiative decays from the excited state of Ti3+ ions in oxide crystals

The fluorescence spectra of Ti³⁺ in Y₃Al₅O₁₂ (YAG), Al₂O₃ (sapphire), YAlO₃ (YAP) observed at 10 K are composed of zero-phonon lines accompanied by the broad vibronic sidebands. The temperature dependence of the fluorescence lifetime and of the total intensity of the broadband measured in YAG and Al₂O₃ indicate that the radiative decay times from the excited states are nearly constant in the range 10–300 K. This demonstrates that the broadband radiative emissions in Ti³⁺:YAG and Ti³⁺:Al₂O₃ are due to magnetic dipole transitions or to electric dipole transitions induced by static odd-parity distortion, respectively. The decrease of the fluorescence lifetime with increasing temperature in Ti³⁺:YAG and Ti³⁺:Al₂O₃ is due to non-radiative decay from the excited state which occurs through phonon-assisted tunnelling between the excited and ground states. The radiative decay of Ti³⁺:YAP is enhanced with increasing temperature, indicating that radiative decay rate contains a term associated with odd-parity phonons. Nevertheless, a non-radiative decay rate of 3.6 × 10⁴ s^–1 observed in the temperature range 10–300 K is due to excited state absorption, which depopulates the excited state and quenches the fluorescence at the laser wavelength.     

Local structure and the electron paramagnetic resonance parameters for the Cr3+ ions at K+-vacancy site in Cr3+:KZnF3 laser crystal

The quantitative relationship between the electron paramagnetic resonance (EPR) parameters D,g_∥,g_⊥ and the local structure parameters of Cr³⁺ ion in KZnF₃ crystals is established. The local structure for Cr³⁺ paramagnetic center in KZnF₃:Cr³⁺ crystal has been determined from EPR parameters of Cr³⁺ ion. This work shows that the trigonal crystal field of Cr³⁺ ion in KZnF₃ crystals comes from following two origins: (1) the nearest-neighbor K⁺ vacancy caused by the charge compensation in the [1 1 1]-axis direction; and (2) the lattice distortions of the nearest-neighbor fluorine coordination caused by the K⁺ vacancy and the differences in mass, charge, and radius between Cr³⁺ ion and Zn²⁺ ion. The unified calculation of the EPR zero-field splitting and g factors, taking into account the K⁺ vacancy and the lattice distortions, has been carried out on the basis of the complete diagonalization procedure and the superposition crystal-field model, all calculation results are in excellent agreement with the experimental data. Although the main source of the trigonal crystal field comes from the K⁺ vacancy caused by the charge compensation, the contribution of the lattice distortion cannot be neglected.     

Near-infrared laser operation of Cr3+ centers in chromium-doped LiInGeO4 and LiScGeO4 crystals

Laser action has been demonstrated in two new tunable Cr3+-doped crystals. Absorption, emission, and excitation measurements for various crystal orientations have been performed for chromium-doped LiInGeO4 and LiScGeO4 single crystals. From the spectroscopic measurements, two optical active centers were identified: tetrahedrally coordinated Cr4+ and octahedrally coordinated Cr3+. Gain-switched, tunable laser operation of Cr3+ centers has been demonstrated in both crystals. An ultrawide tuning range from 1150 to 1480 nm for LiInGeO4 and 1220-1380 nm for LiScGeO4 has been achieved. To our knowledge this is the widest bandwidth and the most near-infrared shifted wavelength range for lasing ever demonstrated for Cr3+ ions.     

Role of monovalent alkali ions in the Yb3+ centers of CaF2 laser crystals

Yb³⁺ and M⁺ monovalent alkali ions (M⁺ = Li⁺, Na⁺, K⁺)-co-doped CaF₂ cubic laser crystals were grown by the micro-pulling-down method (μ-PD) under CF₄ atmosphere. Structural and spectroscopic characterizations of Yb³⁺ in substitution of Ca²⁺ (absorption, emission and decay curves) were carried out to study the effect of M⁺ ions as charge compensators.     

Spectrum and energy levels of Cr3+ ions and exchange-coupled Cr3+ pairs in lanthanum aluminate

From the optical spectra the energy levels of the Cr³⁺ ion in LaAlO₃ were determined. In the cubic field approximation we obtained with the parametersDq=1750cm^-1,B=661 cm^-1, andC=2908 cm^-1 a good agreement between the calculated and measured energies. For higher Cr³⁺ concentrations the fluorescence spectrum of the exchange-coupled first nearest (1N) Cr³⁺ pairs was separated from the vibronic fluorescence spectrum of the single ions by selective excitation. From the experimental data the energy level scheme of the 1N pairs is constructed for the states ¦⁴ A ₂·⁴ A ₂〉 and ¦⁴ A ₂·² E〉. In the ground state the pairs are ordered antiferromagnetically due to an isotropic exchange interaction with an exchange integralJ=(?68 ± 1) cm^-1. From the fluorescence decay the intrinsic lifetimes of the fluorescent states of the single ions, the 1N and the 2N pairs were determined to be equal to (86± 4) msec, (1.75± 0.25) msec, and (31 ± 5) msec respectively. The fluorescence decay shows an energy transfer between the single ions and the pairs.     

Highly efficient cooperative energy transfer from Ho3+ and Tm3+ ions to Ce3+ ions in crystals

A phenomenon of highly efficient cooperative energy transfer from Ho³⁺ and Tm³⁺ ions to two-particle (2Ce³⁺) cooperative acceptors in crystals of solid solutions of La_1?x Ce _x F₃ is revealed. The rates of cooperative energy transfer in Ho³⁺→2Ce³⁺, Tm³⁺→2Ce³⁺, and Tb³⁺→ 2Yb³⁺ systems are measured, as well as their dependence on the magnitude of the matrix elements of donor transition.     

Photoluminescence of LiNbO3 crystals doped with Er3+ and Yb3+ ions

Results of cooperative phenomena investigations in the impurity subsystem of lithium niobate crystals doped with Er³⁺ and co-doped with Yb³⁺ impurity ions under continuous wave and pulsed excitation at 975 nm and 1064 nm wavelengths are presented. Dependences of some spectroscopic characteristics on the intensity of laser pumping are studied. Based on the pair centers model the analysis of the cooperative luminescence behavior in LiNbO₃:Yb³⁺+Er³⁺ crystals is performed.     

Spectroscopic characteristics of the Nd3+ ions in garnet crystals

The article deals with the research of oscillator strengths and the Ω_t (t=2,4,6) intensity parameters of Nd³⁺ doped garnet crystals. It has been found that the absorption intensity of the ⁴I_9/2→⁴G_5/2, ²G_7/2 hypersensitive transition in the Ca₃(NbGa)₅O₁₂ crystals doped with Nd³⁺ is considerably higher than the corresponding values in the following garnet crystals: Y₃Al₅O₁₂:Nd, (GdY)₃(ScAl)₅O₁₂:Nd, Gd₃(ScAl)₅O₁₂:Nd and (GdСa)₃(GaZr)₅O₁₂:Nd. It has been established also that the Ω₂ intensity parameter of the Ca₃(NbGa)₅O₁₂:Nd crystals is one order of magnitude higher compared to the same parameter for the other garnet crystals. These effects in the Ca₃(NbGa)₅O₁₂:Nd crystals are explained by the presence of the Nd³⁺ ions optical sites with a symmetry of local environment C₂, C_2v, C₁.