In this work, we consider the prospects of development of the visible, and IR laser-diode pumped lasers based on TR³⁺-doped double-fluoride crystals. On the basis of estimates of the probabilities of competing non-radiative energy-transfer processes obtained from the experiments and theoretical calculations, the conclusions are drawn on the efficiency of up-conversion pumping and selfquenching of the upper TR³⁺ states excited by laser-diode emission. The effect of the host composition, dopant concentration, and temperature on the efficiency of up-conversion processes is demonstrated on the example of the YLF:Nd, YLF:Er, BaY₂F₈:Er, and BaY₂F₈:Er,Yb crystals. The transfer microparameters for most important cross-relaxation transitions are determined and the conclusions about interaction mechanisms are drawn.

This paper is devoted to the donor (chromium ion) excitation-energy decay caused by interaction with the acceptors (rare earth ions) in garnet crystals, as analyzed within the approximation of a static nonradiative energy transfer.

The subject of anomalous energy transfer in solid-state materials is reviewed. Two models are presented to describe deviations to the standard Inokuti-Hirayama model of energy transfer between ions in solid-state materials. We've considered the non-random distribution of donors and acceptors, and the spectral discrimination of sub-populations of donors and acceptors; all of these affect the observed time-dependent concentrations of excited donors and acceptors. Experimental evidence for the above in solid-state materials has been presented.

The investigations of the temperature dependence of intensity and decay time of the photoluminescence of Erbium in silicon show that the luminescence quenching is caused by a nonradiative energy back transfer from the excited Erbium. The electroluminescence decay is studied by applying different bias conditions during the decay. In a two-beam experiment the PL decay is monitored for different background excitation laser powers. The observed changes of the decay time are a strong evidence for the impurity auger effect as an efficient luminescence quenching mechanism of erbium in silicon. It is also shown that the fast initial luminescence decay component is related to the quenching by excess carriers. The power dependence, the decay profiles with two components and the two-beam experiment can be simulated by solving a set of rate equations considering the formation of excitons, the decrease of pumping efficiency by exciton auger recombination and the decrease of radiation efficiency by the impurity auger effect with free electrons.

Possibilities of the excited state absorption (ESA) spectroscopy as the method for investigation of two physical phenomena are discussed. The first is charge transfer (ChT) process in doped ionic crystals. The second is the study of phonon spectra of doped ionic crystals by measuring narrow non-phonon lines (I-lines) in ESA spectra. ChT bands can be measured much more easily in ESA than in ground state absorption (GSA) spectra. Temperature dependence of ChT bands of Cr³⁺ ion doped crystals suggests that ChT process is mainly of a donor nature e.g., [(3d yields 4s) or 3d yields (oxygen orbital)]. ChT band positions of Ti³⁺:Al₂O₃ and Cr³⁺:Li₂Ge₇O₁₅ crystals measured by ESA spectroscopy and two-step photoionization methods are approximately the same. Phonon repetitions of I-lines in the ESA spectra of doped crystals are in very good agreement with phonon spectra of crystal hosts. It allows us to assume the ESA spectroscopy as a method for determination of phonon spectra of doped ionic crystals.

In the optical spectrum of the KZnF₃ crystal doped with chromium ions, the absorption lines of Cr²⁺-Cr³⁺ ion pairs with dynamic electron transfer between chromium ions via the intermediate fluorine ion (double 180 degree exchange) are observed. The value of the transfer integral for the e_g-electron was directly measured for the first time using the temperature dependence of the integral intensity of the absorption lines.

Epitaxial fluoride layers (CaF₂, SrF₂ and CdF₂) were grown and doped with rare- earth ions by means of molecular beam epitaxy. This technique allows creation of new fluoride heterostructures (including superlattices) with high level and controlled profile of doping. Photoluminescence spectra of divalent ions (Eu²⁺ and Sm²⁺) were used to study strains and stress relaxation in the films. In the structures grown by MBE one can observe effects which are not actual in bulk crystals. They are crossing of 4f⁶ and 4f⁵5d levels of Sm²⁺ ions in CaF₂ layers with tensile strain as high as 2%, new shape of inhomogeneously broadened Sm²⁺ zero phonon emission line in very thin CaF₂ layers arising from modification of strain fields of defects near the surface, bleaching of photoluminescence of RE ions in thin CaF₂:Sm²⁺ layers on Si(111) and in CdF₂-CaF₂:Eu²⁺ superlattices. Luminescence of trivalent ions (Nd³⁺ and Er³⁺) in CaF₂ films is quenched at much higher concentration of the dopants than in bulk crystals. It makes these films very attractive for potential optoelectronic applications.

Bleaching effects in photoluminescence (PL) of rare earth Sm²⁺ ions in CaF₂:Sm/Si(111) heterostructures and Eu²⁺ ions in [CdF₂/CaF₂:EU]_n/Si(111) superlattices have been observed for the first time. The PL was excited by He-Ne ((lambda) equals 633 nm) and N₂ ((lambda) $equals 337 nm) lasers respectively. It was found that illumination of wavelength (lambda) <EQ 366 nm results in recovery of Sm²⁺ PL in the bleached spot. A mechanism based on electron transfer between Si and these ions through CaF₂ conduction band is suggested to explain the phenomenon.

A study of photoluminescence, photo-EPR, and of photoconductivity was carried out on SrTiO₃ and Sr_1-xCa_xTiO₃ (x equals 0.007) crystals containing a small amount of Cr. As a result, the location of Cr electronic levels in the band gap was found, Cr³⁺ being a donor located at 1.85 eV below the CB bottom and Cr⁴⁺ being an acceptor at 1.4 eV above the VB top. In accordance with this scheme, it was shown that: (1) Photons with energies exceeding 1.85 eV excite Cr³⁺ R luminescence in the course of the events; h(nu) _exc + Cr³⁺ yields e + Cr⁴⁺ yields Cr³⁺(²E) + phonons yields Cr³⁺(⁴A₂) + h(nu) _R. (2) Excitation with photon energies above 1.4 eV converts Cr⁴⁺ to Cr$3+):h(nu) _exc + Cr⁴⁺ yields h + Cr³⁺. (3) A band with low energy threshold at 2.1 eV is observed both in excitation spectra of R luminescence and of photoconductivity. The specific character of photoluminescence excitation manifests itself in a number of interesting observations: (i) Strong increase of steady-state R luminescence intensity under IR irradiation additional to excitation in the visible. (ii) IR induced transient R luminescence caused by previous irradiation in the visible. (iii) Very long tails on the time scale of 1000s in R luminescence decay with weak dependence on temperature from 2 K to 78 K.

The results of investigation of photochromic effect in Bi₁₂SiO₂₀ crystals doped with V, Cr, Mn, Fe and Cu ions are presented. The optical absorption and photoinduced spectra were investigated in the 0.3 - 3 micrometer range at T equals 80 - 300 K. The contribution of the inherent defects in the form of Bi³⁺ and Bi⁵⁺ ions in Si-lattice sites to photochromic effect was defined. The mechanisms of the photochromic effect are proposed.

An influence of vibronic coupling on optical spectra of small-radius impurity center in crystals is considered. The consideration is carried out with allowance for the multi-mode nature of impurity-crystal vibrational spectrum. The participation of various-frequency modes in formation of vibronic spectra is analyzed.

Adiabatic potential of K₂ZnF₄:Cu²⁺ impurity system is calculated within the frames of pair potential approximation and shell model. The Jahn-Teller multi-body energy contribution is approximated by the lower branch of [CuF₆] cluster adiabatic potential. It is shown that Q_(theta )- and Q_(epsilon )-displacements of the distorted octahedron of F ions surrounding Cu impurity ion may be assumed as the normal vibrational modes. Thus K₂ZnF₄:Cu²⁺ vibronic states energy spectrum calculation reduces to the solution of the (Exe)-problem with the adiabatic potential, which contains not only the main terms (1/2(DOT)K₀(DOT)(rho) ²-V_e(DOT)(rho) ) but also relatively small corrections (-1/2(DOT)(Delta) K(DOT)(rho) ²cos(2(phi) )+F(DOT)(rho) _DOTcos(phi ) treated by perturbation theory. g-tensor components temperature dependences calculated by averaging over vibronic levels are consistent with the experimental data. Simulation of hydrostatic pressure influence on the K₂ZnF₄:Cu²⁺ properties predicts essential changes in g-tensor components values and temperature dependences.

Usually, broad-band spectra of transition-metal ions are explained as a result of vibrational quanta's emission or absorption. In this paper optical spectra are interpreted as a result of oscillator position detecting by absorption or emission of light quantum. The basis of coherent oscillator states is then used in averaging procedure to obtain spectrum lineshape. Such formalism is useful in high-temperature limit and excitation by ultrashort laser pulses. Parameters of effective dipole momentum to calculate optical transition strength for any configuration of crystal lattice are introduced. Model calculations are performed for Ti³⁺. Two-hump structure in optical absorption band is reproduced without taking into account Jahn-Teller effect. The existence of absorption band in the far-infrared region is demonstrated. Simple procedure to calculate non-radiative transitions probability is developed.

The experimental and theoretical study of electron spin-lattice relaxation of Yb³⁺ ions in phosphate glasses was performed. Measurements were made with electron spin echo method. Some dependencies of the relaxation on the temperature, concentration, and sample dimension are supposed to be caused by localized vibrational states.

In this contribution we analyze the electron-lattice coupling in a case of localized orbital triplet states using the emission, absorption and the excited state absorption (ESA) spectra of the d degree(s) complexes in CaWO₄ and CaMoO₄ crystals. We consider coupling to a fully symmetrical one-dimensional mode as well as to (epsilon) and (tau) ₂ Jahn-Teller (J-T) modes. Our method allows us to estimate the contribution of the J-T effect and coupling to the full symmetry breathing mode to the overall electron-lattice coupling energy. We also estimate anharmonicity of the lattice vibrations related to the spin-orbit coupling between the components of the luminescing triplet.

The propagation of nonequilibrium terahertz acoustic phonons in dense (alpha) -Al₂O₃ ceramics with grain size approximately 10 micrometer is studied. Phonons are generated by the heat pulse technique and optical detection of 0.87 THz (29 cm^-1) phonons via the Cr³⁺ impurity ions fluorescence is used. The diffusive propagation of phonons from heater to optical detector is observed. Experimental results show that the mean free path of phonons is about 150 micrometers and thus is significantly larger than the grain size. These results are compared with computer calculations (by M. B. Melnikov) considering refraction and reflection of plane acoustic waves at the interfaces of disoriented grains in acoustic mismatch approximation. The dynamics of terahertz phonons generated at LHT by laser excitation in sol-gel technology grown monolithic transparent highly porous (gamma) - Al₂O₃ samples was studied using the phonon-induced anti-Stokes FLN spectra of Cr³⁺ impurity ions (²E - ⁴A₂ lines). The generated phonon distribution besides long-living (approximately tens of ms) thermalized Planckian distribution contains essentially nonequilibrium vibrational excitations peak around (omega) approximately equals 0.6 THz (20 cm^-1) with high occupation numbers n_(omega ) and decay time approximately 1 ms. Discussed are the possible nature of these vibrations with frequencies (omega) which are consistent with acoustic size-resonance modes of (gamma) - Al₂O₃ nanometer scale spherical microcrystals which form the material and the possible role of these vibrations in spectral dynamics of optically generated phonons.

The resonant vibronic effect in high-resolution luminescence spectra of europium compounds is investigated. The cases of weak and strong nonadiabatic vibronic mixing between the ⁷F₂-electronic state of Eu³⁺ and near-resonant vibronic satellites of the ⁷F₀-electronic state are considered. The intensity redistribution and line shifts in the region of the e⁵D₀-⁷F₂ transition are observed. The conditions of resonance disappear in the deuterated compounds when vibration frequencies are appreciably decreased. Creation of new vibronic states is witnessed by emergence of their satellites due to lattice vibrations in vibronic sideband of ⁵D₀-⁷F₂ transition.

Electronic transitions between the Stark components of ²F_7/2 and ²F_5/2 terms in 4f¹³ configuration are observed in the CaF₂:Tm²⁺ and CaF₂:Tm²⁺ absorption and luminescence spectra. The transitions are magnetic-dipole. Theoretical analysis in LSJ representation allows us to determine the relation between electronic transition probabilities. Experimental relations between observed transition probabilities are estimated using vibronic sideband integral intensity of the (Gamma) ₇ $ARLR (Gamma) ₇' transition for normalization. The theoretical results vary from the experimental ones. Analysis of the possible reasons for the discrepancy has been performed. Suppression of the (Gamma) ₇ $ARLR (Gamma) ₇ transition is associated with the resonant reabsorption of radiation. Suppression of (Gamma) ₇ yields (Gamma) ₈' transition in absorption is caused by intensity transfer to vibronic transitions due to the interaction of Tm²⁺ ions with the even (Gamma) _3g and (Gamma) _5g vibrations. The degree of this suppression was evaluated in the framework of electrostatic approximation. Results of the calculation are in agreement with the experimental ones.

The physical mechanisms of x-ray storage phosphors are discussed. This includes questions like the nature of the traps involved, the storage and reading process, the time constants involved, the thermal stability of storage, and the sensitivity. Finally some questions related to film preparation and image quality are investigated and the potential of the 'image plates' for various applications are outlined.

There is a strong interest in the development of inorganic scintillators with a fast response and a high light yield to be used in fundamental research, medical diagnostics, and a number of industrial applications. One of the main lines of research concerns scintillators based on luminescence from the allowed 5d - 4f transitions of lanthanide ions in appropriately chosen host lattices. A review is presented.

In this work we present the x-ray absorption spectroscopy study of Ni_cMg_1-cO (0.01