Here laser studies of Ce<sup>3+</sup>:LiLu<sub>x</sub>Me<sub>1-x</sub> F<sub>4</sub> (Me=Y<sup>3+</sup>,Yb<sup>3+</sup>) mixed crystals homologous to well-known Ce<sup>3+</sup>:LiYF<sub>4</sub>and Ce<sup>3+</sup>:LiLuF<sub>4</sub> UV crystalline active media are reported. Optical spectroscopy and X-ray diffraction studies have
proved scheelite structure of the investigated mixture crystals. Advantages of LiLuYF<sub>4</sub>:Ce<sup>3+</sup>+Yb<sup>3+</sup> mixture crystals were
detected. Among them an opportunity to wider tuning range of laser oscillation in comparison to LiLuF<sub>4</sub>:Ce<sup>3+</sup>+Yb<sup>3+</sup>
active medium by means of varying YF<sub>3</sub>/LuF<sub>3</sub> content relation. As it was shown from laser experiments mutual shift of
5d-4f emitting transition of Ce<sup>3+</sup> ions and color centers absorption band due to lattice parameter change gives an
optimum for YF<sub>3</sub>/LuF<sub>3</sub> content relation corresponding to minimal overlap and maximal amplification band. EPR and
optical spectroscopy revealed the optimum ratio of Y<sup>3+</sup> ions and Lu<sup>3+</sup> in solid solutions of LiF-LuF<sub>3</sub>-YF<sub>3</sub>, where the
segregation coefficient Ce<sup>3+</sup> of ions is increased 3-5 times compared with crystals LiYF<sub>4</sub> and LiLuF<sub>4</sub>.
Here we report on pump-probe studies of KY<sub>3</sub>F<sub>10</sub> and CaF<sub>2</sub> doped with Ce<sup>3+</sup> and Yb<sup>3+</sup> ions. The crystals
Ce<sup>3+</sup>:KY<sub>3</sub>F<sub>10</sub> and Ce<sup>3+</sup>:CaF<sub>2</sub> show attractive spectral characteristics for tunable UV lasers application but have poor
photochemical stability. Their properties under intense UV pumping are affected by excited state absorption and color
This work was aimed at dynamic processes investigation induced by laser radiation of UV spectral range in
KY<sub>3</sub>F<sub>10</sub> and CaF<sub>2</sub> doped with Ce<sup>3+</sup> and Yb<sup>3+</sup> ions.
Optical gain was observed on CaF<sub>2</sub>:<sup>3+</sup>+Yb<sup>3+</sup> in the range 325-335 nm for the first time. It proves Yb<sup>3+</sup> ions
coactivation antisolarant effect elaborated by us earlier. It is shown that in investigated crystals under a pump radiation,
resonant to 4f-5d Ce<sup>3+</sup> ions transitions several types of color centers (CC) are formed. Set of these CC types are
determine the absorption at expected stimulated emission spectral range and magnitude of the induced loses is dependent
on many factors. As a result of competition between CC formation, free charges recombination and CC photodestruction
this balance can be shifted towards either rising or dropping losses at 5d-4f Ce<sup>3+</sup> ions luminescence spectral range.
We have discovered that total intracavity losses, being dependent on color centers amount, go down for higher pump
energies for LiCaAlF<sub>6</sub>: Ce<sup>3+</sup>. This dependence is explained by the formation by the pump radiation and destruction of
color centers due to laser radiation. The more energy remains in the lasing cavity, the lower the color centers absorption.
Such dependencies were investigated for active medium crystals grown by different methods. Influence of growth
conditions on active media characteristics is discussed.
As a result we have shown for the first time that the crystal LiCaAlF<sub>6</sub>:Ce<sup>3+</sup> internal losses depend on the pump energy.
Method has been worked out to determine the intracavity losses of the laser, which allows evaluation of prospects of its
practical use in the most correct way.
Here spectral-kinetic properties of Ce<sup>3+</sup> ions and UV pumping induced color centers in KY<sub>3</sub>F<sub>I0</sub> (KYF) crystals are reported and discussed. The efficiency of crystal-chemical approach of UV pumping induced solarization processes inhibition in these crystals was shown. Laser oscillation was achieved on the Ce:KYF crystals which were considered earlier as unpromising as active medium due to its photochemical instability.
Opportunities of laser tests as a powerful and unique research tool of active medium fundamental properties directly in real laser operation conditions for variable intracavity losses are discussed. New technique of laser experimental data interpretation was developed and applied to the analysis of laser tests results for the series of Ce<sup>3+</sup>:LiYb<sub>x</sub>Lu<sub>1-x</sub>F<sub>4</sub> single crystals. Contribution coefficient into total pump-induced intracavity losses of ESA from upper laser level of Ce<sup>3+</sup> ions and CC absorption at the laser wavelength was estimated. Basic parameters relevant to laser action performance were defined: Ce<sup>3+</sup> ions ionization cross-section by pumping radiation and Ce<sup>3+</sup> ion ESA at the laser wavelength, CC absorption (ionization) cross-section at the both pumping and laser radiation wavelength, life-times of transient and long-living color centers, stimulated emission cross-section and relative values of pump-induced free-charges trapping rate by host lattice defects and their corresponding recombination rate.
The spectral-kinetic characteristics of Ce<sup>3+</sup> ions doped Na<sub>4</sub>Y<sub>6-x</sub>Yb<sub>x</sub>F<sub>22</sub> (x=0-0.05) single crystals were studied. Ce<sup>3+</sup> ions 5d-4f interconfigurational fluorescence quantum yield versus Yb<sup>3+</sup> ion codopant concentration was measured. Pump-induced color center absorption spectra were studied and the efficiency of Yb<sup>3+</sup> ions codoping antisolarant crystal-chemical technique applied to Na<sub>4</sub>Y<sub>6</sub>F<sub>22</sub>:Ce<sup>3+</sup> was demonstrated. The optimal Yb<sup>3+</sup> ions content from the point of view of effective tunable laser action was estimated. The obtained results allow proposing Na<sub>4</sub>Y<sub>6-x</sub>Yb<sub>x</sub>F<sub>22</sub> as a new prospective photochemically stabilized material for UV/VUV application.
Here the photochemical properties of CaF<sub>2</sub> crystal doped by Ce<sup>3+</sup> and Yb<sup>3+</sup> ions under pumping with ultraviolet (UV) irradiation resonant to the Ce<sup>3+</sup> ion interconfigurational 5d-4f transitions were studied and significant codoping effect on the crystal solarization suppression was observed. The photodynamic processes model occurring in the active medium and which taking into account the color center formation and processes of enhanced free charges recombination via valence change of variety of impurity centers was proposed. This model allows to explain both electronic and hole types color centers suppression in Ce:CaF<sub>2</sub> sampels codoped by Yb<sup>3+</sup> ions. It was shown that Yb<sup>3+</sup> ions in the crystal act as centers of electron and hole recombination with probability dependent on the extra charge compensation type. Therefore the proposed antisolarant mechanism of the coactivation effect satisfactorily describes photochemical properties of fluorite and other crystal hosts.
All known solid-state UV/VUV active media based on the interconfigurational 5d-4f transitions of trivalent rare earth ions in wide-band gap fluoride crystals are solarizable under intense UV/VUV pumping. Active media- specific pumping-induced color centers, on one hand, would absorb the laser radiation and reduce the laser efficiency. On the other hand they also get bleached by the pump laser radiation so that the current value of total intracavity losses is governed by dynamic equilibrium and depends on some active medium microparameters as well as the pump flux and is cavity specific. Presented here are the model of dynamic processes in a UV solarizable active medium under pumping conditions and the new technique of laser experimental data analysis for variable intracavity losses based on this model. It is shown that properly set laser experiment enables us to obtain the necessary active medium microparameters, including the pump excited state absorption and color center absorption of laser radiation cross- sections, photoelectron trapping rate by host lattice defects and their recombination rate. The results of this technique application to the laser experiment analysis for the series of Ce<SUP>3+</SUP>:LiYb<SUB>x</SUB>Lu<SUB>1-x</SUB>F<SUB>4</SUB> single crystals are presented.