The paper presents research on the temperature dependence of the spectroscopic properties of chromium ions in Cr2+-only doped and Cr2+,Fe2+ co-doped Zn0.7Mn0.3Se solid solution crystals within the temperature range from 77 to 380 K. It is shown that the absorption and luminescence spectra maxima of Cr2+ ions in the co-doped sample shift almost linearly towards higher energies compared to Cr2+-only doped sample across the entire investigated temperature range. The lifetime of Cr2+ ions in the co-doped sample remains practically independent of temperature up to 200 K, while in the Cr2+-only doped Zn0.7Mn0.3Se crystal it remains temperatureindependent up to 250 K. Differences between the temperature dependences of Zn0.7Mn0.3Se:Cr2+ and Zn0.7Mn0.3Se:Cr2+,Fe2+ samples can be attributed to the influence of Fe2+ ions presence, which affect the local vibrations at the position of Cr2+ ion.

Radiation with a wavelength of ~4 µm was used for quasi-collinear excitation of the Fe:ZnSe laser. The highest output energy was achieved at the optimal crystal temperature of 230 K with the generated radiation wavelength of ~4.5 µm.

Temperature dependences of spectroscopic (absorption, fluorescence, lifetime) and laser output properties (energy, oscillation spectra) of the Fe 2+ :ZnSe laser under ~4 μm radiation pumping are presented. Wide laser oscillation spectrum of up to ~4.9 μm is demonstrated.

The spectral and laser output characteristics of Cr:ZnSe single crystals optically polished at four different lengths of 2, 3, 5, and 10 mm combined with or without antireflection (AR) coatings were investigated under pulse and continuous wave (CW) laser diode excitation at the wavelength of 1.7 μm. The absorption and fluorescence spectra of Cr2+ ions were measured at room temperature. Optimization of laser system efficiency was performed step-by-step in dependence on the active length of the crystal sample, output coupler reflectivity, the duration of the pump pulse, and the repetition rate. For a given 1 : 1 transferring optics, a 3-mm thick AR-coated Cr:ZnSe crystal was chosen as optimal in pulse mode of operation: The 10

The influence of $\mathrm{Fe}^{2+}$ co-doping on the spectroscopic properties of $\mathrm{Cr}^{2+}$ ions in $\mathrm{Zn}_{1-x} \mathrm{Mn}_{\mathrm{x}} \mathrm{Se}$ and Zn 1 ${ }_{\mathrm{x}} \mathrm{Mg}_{\mathrm{x}} \mathrm{Se}(\mathrm{x}=0.3$) crystals at room temperature is demonstrated. The absorption and fluorescence peak wavelengths shift to higher energies in the Cr -Fe co-doped crystals. The fluorescence decay curves in the Cr -Fe co-doped crystals have a non-exponential shape with the same lifetimes at the far stage as the crystals doped only with $\mathrm{Cr}^{2+}$.

The effect of magnesium co-doping on growth conditions, crystal composition, optical and spectral properties of manganese or chromium-doped Ca3(VO4)2 (CVO) crystal was studied. The single crystals were grown by the Czochralski method. Effective segregation coefficients of manganese and chromium in the CVO matrix with magnesium co-doping were calculated. A redistribution of the absorption lines intensities was observed in the absorption spectra of CVO:Mn:Mg and CVO:Cr:Mg. Low (15 K) temperature fluorescence measurements confirmed that co-doping CVO crystal with Mg ions leads to the redistribution of Mn ions concentration in different valence states, but does not cause changes in their spectral properties. In the presence of magnesium, additional annealing of the materials resulted in a slower reduction of the valence state of doping ions and a noticeable change in crystal coloration. All investigated impurities have practically no effect on the specific ferroelectric domain structure of CVO.

The room-temperature spectroscopic properties of Cr2+ ions in Zn0.7Mn0.3Se and Zn0.7Mg0.3Se crystals co-doped with Cr2+ and Fe2+ ions were investigated and compared to Cr2+-only-doped ones. The “blue” shift of both absorption and fluorescence lines of Cr2+ ions in positions with nearby Fe2+ ions was demonstrated to be 350cm-1 and 250cm-1 for Zn0.7Mn0.3Se and Zn0.7Mg0.3Se crystals, respectively. This shift was shown to have a serious influence on the Cr2+-Fe2+ energy transfer process due to the worse energy resonance between Cr2+ and Fe2+ ions.

The spectroscopic properties of cobalt ions in Ca 3 (VO 4 ) 2 crystals doped via thermal diffusion and during Czochralski growth process are studied. The spectroscopic properties of M1 and M2 Co 2+ optical centers are presented. A narrow fluorescence line at 1170 nm is attributed to trivalent Coions.

Zn0.85Mn0.15Se crystals co-doped with Cr2+ and Fe2+ ions are investigated. Concentration quenching of Cr2+ and Fe2+ ions with an increase in Cr2+ ion concentration is demonstrated. The parameters of Cr2+ -> Fe2+ ions nonradiative energy transfer are determined. It is shown that for the given Fe2+ doping level of -4 x 1018 cm- 3 increasing the concentration of Cr2+ ions above 9.5 x 1018 cm-3 is inefficient.

New high optical quality calcium orthovanadate Ca-3(VO4) (2) crystal doped by chromium ions was synthesized. Polyvalent character of doping chromium ions in as-grown crystal was revealed. Crystal annealing at 1000 degrees C seems to result in increase of chromium ions in four plus state. Spectroscopic properties of chromium ions in Ca-3(VO4)(2) crystal were investigated.

The absorption cross-section of Cr2+ ions in a range of cubic Zn1–xMnxSe (x = 0–0.3) solid solutions was determined using nonlinear transmission measurements. The maximum absorption cross-section of about 1.04 × 10–18 cm2 was determined and shown to be practically independent of the Mn content (x) in the solid solution.

The spectroscopic properties of Cr2+ ions in a range of cubic Zn1-xMnxSe (x = 0-0.4) solid solutions were investigated. An almost linear decrease in 5E level energy was observed, resulting in both absorption and fluorescence maxima shifts of approximately 30 cm-1 per 10 % of Mn. The temperature dependence of Cr2+ ion decay time was studied, and an increase in nonradiative quenching rate was revealed when Mn concentration (x) increased. The position of the Cr2+ ions zero-phonon line (about 4871 cm-1) in the Zn1-xMnxSe (x = 0.4) crystal was determined.

The absorption cross-section of Cr 2+ ions in a range of cubic Zn 1– x Mn x Se ( x = 0–0.3) solid solutions was determined using nonlinear transmission measurements. The maximum absorption cross-section of about 1.04 × 10 –18 cm 2 was determined and shown to be practically independent of the Mn content ( x ) in the solid solution.

Activation of calcium orthovanadate crystals by cobalt ions was carried out by introducing cobalt oxide into the melt during Czochralski growth process or during high-temperature annealing of nominally pure Ca3(VO4)2 single-crystal using Co3O4 powder as a solid-phase diffusant. The maximum concentration of the Co ions in the matrix is achieved in the case of the diffusion doping process. For the concentration series of Ca3(VO4)2:nCo crystals, the local structure was investigated and the crystal chemical composition was determined. Polarized absorption spectra of doped crystals are obtained. The influence of the atmosphere and the annealing temper-ature on the redistribution of the intensities of the characteristic absorption bands in the visible and near-IR spectral regions is shown. Luminescence parameters of the doped crystals were studied.

In this work, the high-temperature diffusion doping method was used for introduction of active cobalt ions into calcium orthovanadate Ca3(VO4)2crystals. Experimental samples were made from a nominally pure CVO single crystal obtained by the Czochralski method. The high-temperature diffusion conditions have been optimized to obtain doped crystals of optical quality during annealing in open and closed zones. Diffusion coefficients of cobalt ions (D) were calculated for various conditions: annealing time 24 - 48 hours; temperature range 1150-1300°C; diffusants - oxide compounds of calcium, cobalt and vanadium: Co3O4, Ca10Co0.5(VO4)7 and Ca3(VO4)2:2wt.%Co3O4; diffusion direction is parallel or perpendicular to the CVO crystal optical axis. The calculated values of the diffusion coefficient varied between 2.09·10-8—1.58·10-7 cm2/s. The activation energy of the diffusion process was determined to be 2.58±0.5 and 2.63±0.5 eV for the [001] and [100] directions, respectively. The maximum cobalt concentration in doped CVO crystals was 2·1020 cm–3. The absorption spectrum of diffusion-doped Ca3(VO4)2:Co samples demonstrates the presence of absorption bands characteristic for Co2+ and Co3+ ions. It was shown that the intensity ratio of the characteristic absorption bands varies depending on the crystal doping method. The optical anisotropy of the crystal increases with dopant concentration increase.

The spectral-kinetic characteristics of Nd3+ optical centers (A, B, C centers) in Sr3LaNb3O12 crystals are studied by selective laser spectroscopy technique. It is found that the luminescence decay of A centers (donors) at the first stage is quenched by an ordered ensemble of acceptors (B centers) while the second stage is disordered and follows the Fo spacing diaeresis rster law. The macro-(gamma(N-1) = 55 cm(-1/2), gamma(N-2) = 95 cm(-1/2)) and microparameters (C-DA = 4.5.10(-40) cm(6)/s) of the ion-ion (Nd3+-Nd3+) interaction are calculated using the static quenching theory. The closest distance between neodymium ions involved into the ion-ion (A and B optical centers) interaction is calculated to be R-min. = 0.45 nm, and the boundary time at which the ordered luminescence quenching stage changes for disordered one is determined as t(bound). similar to 19 mu s. It is shown that the A centers luminescence quenching mechanism is the dipo-le-dipole (S = 6) one. The Nd3+ luminescence build-up and decay kinetics in ensemble of acceptor centers (B centers) in case of unequal donor and acceptor lifetimes (tau(D) < tau(A)) are determined.

Temperature dependences of Cr2+ spectral and laser characteristics of Cr:ZnSe and Cr:Zn1-xMnxSe (x≈0.05, 0.1, 0.2, and 0.3) single crystals are compared and reported for ~1.73 μm laser excitation. Slope efficiency of ~44% at 78 K with pulse duration of ~140 ns was demonstrated.

Optical characteristics of new highly dispersed Bi3+Ge4O12:Pr powders are studied. An efficient excitation energy transfer from BGO to Pr3+ leading to a decrease of HDP_BGO fluorescence decay time from 160 to 35 ns is found.

Calcium orthovanadate Ca-3(VO4)(2) crystals doped with manganese ions have been fabricated by high-temperature diffusion doping. Nominally pure CVO plates were prepared from the Czochralski grown single crystal. They were used as an effective matrix for diffusion impurity ions from solid phase (Mn2O3 powder) to solid phase (CVO single crystal). The high-temperature diffusion conditions were optimized to obtain the doped crystals of optical quality under using open and closed annealing zones. Diffusion coefficients of manganese ions were calculated for different conditions (temperature interval 1150-1300 degrees C, annealing time 24 h, 48 h, directions [001], [100]) of diffusion doping of CVO samples. The diffusion coefficient D varied from (3.7 +/- 0.6) . 10 8 cm(2)s(-1) at 1150 C up to (1.3 +/- 0.2) . 10(-7) cm(2)s(-1) at 1300 C. The diffusion activation energy was determined to be 1.7 +/- 0.4 eV and 3.3 +/- 0.4 eV for the processes based on Fick's laws during [100] and [001] direction, respectively. The ultimate solubility of manganese in CVO crystals at T = 1300 degrees C was 3 . 10(20) cm(-3). The absorption spectrum of diffusion doped Ca-3(VO4)(2):Mn samples is characterized by a combination of absorption bands specific for Mn2+, Mn3+ and Mn5+ ions. It was indicated that Mn5+ concentration in the diffusion doped CVO:Mn is higher than in CZ-grown Mn-doped materials.

Novel Zn1-xMgxSe (x approximate to 0.2 and x approximate to 0.3) single crystals co-doped with Cr2+ and Fe2+ ions were investigated. The excitation was performed either via the energy transfer from Cr2+ to Fe2+ ions with Q-switched Er:YLF laser at the wavelength of similar to 1.73 mu m or with direct Fe2+ ions excitation with Q-switched Er:YAG laser system at similar to 2.94 mu m, for comparison. The temperature dependences of spectroscopic characteristics were measured for both types of excitation. The crystal with a higher amount of magnesium (x approximate to 0.3) has broader absorption bands for both doping ions and their maxima are shifted towards longer wavelengths. The fluorescence spectra were similar for both types of the excitation. The difference caused by the excitation process was observed in fluorescence decay time measurements. For the direct Fe2+ ions pumping the fluorescence decay time was shorter for all measured temperatures in comparison with the excitation via the Cr2+ -> Fe2+ energy transfer. The same difference was observed in measured delay times between the pump and fluorescence signals. The Fe2+ ions laser oscillation spectra at 78 K were centered around the same wavelengths for both types of excitation: similar to 4.5 mu m and similar to 4.85 mu m for magnesium contents of x approximate to 0.2 and x approximate to 0.3, respectively.