The subject of the study is Cr4+ doped Li2ZnGeO4. The choice of this material is based on the structure of the compound, built of lithium, zinc and germanium tetrahedra, guaranteeing successful doping with Cr4+. The isostructural Li2ZnSiO4 shows a broad emission spectrum and, unlike many other silicates, has the necessary long lifetime. Li2ZnGeO4:Cr4+ glass-ceramic was synthesized for the first time. A systematic investigation of the Li2O-ZnO-GeO2-B2O3 system was carried out to identify the optimal initial glass composition and thermal treatment conditions required for obtaining the desired glass-ceramic. The results of nucleation and crystallization of nanophases in this multicomponent system are presented and discussed. Characterization of the synthesized glass-ceramics was performed using XRD, DSC, TEM, and complementary techniques. The resulting Li2ZnGeO4:Cr4+ glass-ceramic contains nanoparticles with an average size of approximately 20 nm and a concentration of about 45 wt% within the glass matrix, while maintaining good transparency. Optical characterization revealed that the Li2ZnGeO4:Cr4+ glass-ceramic exhibits broad emission in the range of 1000-1500 nm, with a maximum at 1220 nm, corresponding to a single Cr4+ emission center. The measured luminescence lifetime ranges from 79 to 95 & micro;s. The optimized preparation conditions and the favorable optical properties make this glass-ceramic a promising candidate for a different specific applications, required additional specific research.

We present the synthesis and optical characterization of Li2CaGeO4: Cr4+ glass-ceramic. A systematic investigation was made to choice the optimal initial glass composition from the Li2O-CaO-GeO2-B2O3 system and the optimal thermal treatment conditions to obtain the glass-ceramic. The glass-ceramic possess several characteristics such as, i) 18-24 nm size of the nanoparticles, ii) about 60 wt % Li2CaGeO4: Cr4+ nanoparticles concentration in the glass, and iii) good transparency. The optical measurements and calculations show that Cr4+ experiences a medium-strength crystal field (Dq/B = 1.56) near the cross-point of the energy levels 3T2 and 1E (Dq/B = 1.7). Dq/B values for different compounds are compared and discussed. Intense transition of excited electrons between the 3T2 and 1E levels is observed. As a final result, wide optical emissions from 1100 to 1500 nm, centred at 1215-1250 nm and lifetime around 40 mu s were obtained. This lifetime achieved for Li2CaGeO4: Cr4+ glass-ceramic is significantly longer than the values for the well-known Cr4+ doped forsterite and garnet (about 5 mu s). It is comparable with the lifetime of some silicate glass-ceramic, but their production is more complicated because of the higher temperature for the initial glass preparation.

Spectral and mechanical strength properties of Yb 3+ ,Li + :ZnWO 4 crystals were investigated, diode-pumped ZnWO 4 laser with an output power of 2,41 W was demonstrated.

${T m}^{3+}, \mathbf{L i}^{+}: \mathbf{Z n W O}_{4}$ single-crystal was grown by the Czochralski (Cz) method. The actual concentrations of the dopants were measured. Dispersions and temperature coefficients of the refractive indices (RI), as well as the mechanical strength characteristics of the crystal were measured. Spectroscopic studies of the crystal were carried out. The first laser operation above 2 $\mu \mathrm{m}$ at this crystal was obtained.

Calcium niobium gallium garnet (CNGG) crystals doped with thulium (Tm 3+ ) ions possess disordered structure leading to inhomogeneously broadened emission bands which makes them attractive for generation of ultrashort pulses at ~2 μm [1] . The actual composition of CNGG deviates from the stoichiometry and cationic vacancies are present to ensure charge compensation. They can be eliminated by codoping with univalent alkali cations (Li + , Na + ). The related calcium tantalum gallium garnet (CTGG) shows better thermal properties than CNGG [2] . Here, we report on the growth, spectroscopy and first laser action in a Tm 3+ ,Li + -codoped CTGG (Tm:CLTGG) crystal.

Monoclinic Yb 3+ ,Li + :ZnWO 4 crystals were grown by Czochralski and their structure is refined. The spectroscopic properties of Yb 3+ ions were studied with polarized light and the scheme of Stark splitting of Yb 3+ manifolds in these crystals was determined. The Raman spectra were measured. A diode-pumped 2.90 W laser action at 1059 nm in the crystals was demonstrated.

Epitaxial monoclinic double tungstate composites based on the strongly anisotropic KLu(WO4)(2) (KLuW) were grown with high crystalline quality and laser operation of ytterbium was demonstrated for the first time. Highly efficient CW laser emission of an Yb:KLuW-KLuW crystal was achieved near 1030 nm. The 100-mum-thick Yb:KLuW layer was pumped at wavelengths near 980 nm by a tapered diode laser as well as by a Ti:sapphire laser. More than 500 mW of CW output power and slope efficiencies up to 66% were obtained at room temperature without cooling.

Tm:KGd(WO 4 ) 2 is studied as a three-level laser on the 3 F 4 3 H 6 transition and a tunable source in the 2-m spectral range, operating at room temperature.An overall tunability extending from 1790 to 2042 nm is achieved with maximum output powers of 400 mW for an absorbed pump power of 1 W. Various doping levels, pump wavelengths and polarization configurations are compared and the advantages of the monoclinic double tungstates over other Tm-hosts are outlined.

The strongly anisotropic monoclinic double tungstate crystal KLu(WO/sub 4/)/sub 2/ was doped with Yb/sup 3+/ ions and shown to be a highly efficient active laser medium in the 1-/spl mu/m spectral range for continuous-wave room-temperature operation, very suitable for pumping with InGaAs laser diodes.

We grew good-optical-quality KGd(WO/sub 4/)/sub 2/ single crystals doped with erbium and ytterbium ions at several concentrations of dopants using the top-seeded-solution growth slow-cooling method (TSSG). We performed the spectroscopic characterization of this material related to the 1.5-mm infrared emission of erbium which is interesting for laser applications. To do this, we carried out polarized optical absorption at room temperature (RT) and at low temperature (6 K) and performed luminescence studies of the emission and lifetime. We obtained the 1.5-mm emission of erbium after selective laser pump excitation of the ytterbium ion and energy transfer between the two ions. The maximum emission cross section for 1.5 mm was about 2.56/spl times/10/sup -20/ cm/sup 2/ for the polarization of light with the electric field parallel to the N/sub m/ principal optical direction. This value was higher than for other erbium-doped materials with application in solid-state lasers such as LiYF/sub 4/:Er(YLF:Er), Y/sub 3/Al/sub 5/O/sub 12/:Er(YAG:Er), YAlO/sub 3/:Er, and Al/sub 2/O/sub 3/:Er.

The strongly anisotropic monoclinic double tungstate crystal KLu(WO4)(2) was doped with Yb3+ ions and shown to be a highly efficient active laser medium in the I-Mm spectral range for continuous-wave room-temperature operation, very suitable for pumping with InGaAs laser diodes.