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In this work, we investigated mechanisms of the energy transfer in Tm : KY(WO4)2, Tm : KLu(WO4)2 and Tm:NaBi(MoO4)2 crystals. Room-temperature absorption and emission spectra were used to determine microparameters of energy migration among thulium ions in the 3H4 and 3F4 excited states in the frames of Förster – Dexter theory. Parameters of cross-relaxation 3H4 + 3H6 → 3F4 + 3F4 and energy migration were obtained via analysis of luminescence decay 3H4 → 3F4 with a hopping model. The parameters describing excitation migration between thulium ions in 3H4 state obtained by two methods were in good agreement. It has been shown that the dipole-dipole mechanism of interaction is responsible for the efficient cross-relaxation process in the crystals under study. The results indicate that the energy migration between 3H4 enhances the cross-relaxation at thulium content more than ∼1.3–1.5 at. % in these laser materials. The obtained values of the migration parameters CDD exceed the values of the cross-relaxation parameters CDA, and the energy transfer in these materials can be described with the hopping model. An efficient cross-relaxation process leads to the relatively high efficiencies of the systems based on these crystals under pumping at 0.8 µm. The dominant process of energy migration between thulium ions in 3F4 excited state makes tungstate and molybdate crystals good candidates for the Ho3+ co-activation for laser generation at 2.1 µm. Parameters obtained in this study can be used for mathematical modeling of laser characteristics.

Lasing and spectroscopic properties of an LiY0.3Lu0.7F4:Pr3+ crystal grown by the Bridgman–Stockbarger method were investigated in detail. Absorption and luminescence spectra were recorded using polarized light. Lifetimes of excited states 3PJ were determined. Absorption and emission transition intensities were calculated according to f–f-transition intensity theory taking into account interconfiguration interaction. Stimulated emission transverse cross sections were determined using the Fuchtbauer–Ladenburg method. Pumping by an InGaN laser diode produced continuous-wave laser generation at 522.6 nm (3P1 → 3H5) in Pr-doped mixed fluoride crystals with a maximum output power of 0.38 W and a slope efficiency of 19%.

We present a compact diode-pumped Tm:KYW laser passively Q-switched with polycrystalline Cr:ZnSe saturable absorber. The laser operates at around 1910 nm and 1940 nm and produces pulses with duration as short as 10 ns and energy up to 40 μJ.

In this work we report the spectroscopic properties of Tm3+:NaBi(MoO4)2 crystals with the dopant concentrations of 0.7 at.% and 3 at.%. The energy levels of the Tm3+ in the NaBi(MoO4)2 host were determined from polarized optical absorption and photoluminescence spectra measured at 77.4 K. Radiative properties of the crystals were calculated in context of Judd-Ofelt theory. Raman spectra of the crystal were studied. The concentration dependences of emission decay times of 3H4 and 3F4 levels were analyzed. The potential of the crystal for building tunable and ultrafast pulse lasers is shown on the base of cross sections and gain coefficient in the range of 1.9 μm.

A Sm3+:KY(WO4)2 crystal was grown by the modified Czochralski technique. Polarized absorption and fluorescence spectra, as well as a fluorescence decay curve, were recorded at room temperature. Radiative properties such as emission probabilities, branching ratios and radiative lifetimes were investigated within the framework of the Judd-Ofelt theory as well as the theory of f-f transition intensities which takes into account the influence of the excited configurations. Emission cross section spectra were determined. 4G5/2 fluorescence decay was analyzed within the framework of the Inokuti-Hirayama model. The spectroscopic properties of Sm:KYW crystal were compared with those of other Sm3+-doped materials.

A Tb3+:YAB crystal was grown using a high-temperature solution growth on dipped seeds technique. Polarized ground state absorption and fluorescence spectra from the 5D4 level, as well as a fluorescence decay curves of 5D4 and 5D3 levels, were recorded at room temperature. Radiative properties such as emission probabilities, branching ratios, and radiative lifetime were investigated within the theory of 4f-4f transition intensity in the case of the intermediate configuration interaction.

Ca9Nd(VO4)7 single crystal was grown by the Czochralski method in inert atmosphere. The crystal structure and chemical composition were studied. Polarized absorption and luminescence spectra were investigated in details. It was found that the Ca9Nd(VO4)7 crystals belongs to self-activated laser materials with a weak concentration quenching of luminescence. Judd-Ofelt analysis was performed. The emission cross-sections spectra for 4F3/2 → 4I9/2, 4I11/2, 4I13/2 transitions were determined. For the first time Raman spectra of the Ca9Nd(VO4)7 single crystal were recorded and interpreted.

Pure and Nd-doped Ca 10 Li(VO 4 ) 7 single crystals were grown by the Czochralski method. The structure of Ca 10 Li(VO 4 ) 7 single crystal was refined starting from a model of Ca 10 K(VO 4 ) 7 using the powder diffraction data collected at a laboratory high-resolution diffractometer. The defect structure of the single crystal was studied with the use of both, high-resolution diffraction using a laboratory instrument and X-ray topographic techniques employing synchrotron radiation at the Hasylab laboratory (Hamburg). Polarized absorption and luminescence spectra of Nd-doped Ca 10 Li(VO 4 ) 7 crystal were investigated in details. The laser oscillation was obtained under flash lamp pumping and the slope efficiency of 0.87% was achieved in the free-running mode. Preliminary examination of lasing properties points that Ca 10 Li(VO 4 ) 7 :Nd crystal can be a highly efficient solid state laser medium. Crystals of this kind are expected to be suitable for application as efficient non-linear optics materials.

We report on a comprehensive spectroscopic and laser characterization of monoclinic Yb,Tm:KLu(WO4)2 crystals. Stimulated-emission cross-section spectra corresponding to the 3F4 → 3H6 transition of Tm3+ ions are determined. The radiative lifetime of the 3F4 state of Tm3+ ions is 0.82 ms. The maximum Yb3+ → Tm3+ energy transfer efficiency is 83.9% for 5 at.% Yb – 8 at.% Tm doping. The fractional heat loading for Yb,Tm:KLu(WO4)2 is 0.45 ± 0.05. Using a hemispherical cavity and 5 at.% Yb – 6 at.% Tm doped crystal, a maximum CW power of 227 mW is achieved at 1.983–2.011 μm with a maximum slope efficiency η = 14%. In the microchip laser set-up, the highest slope efficiency is 20% for a 5 at.% Yb– 8 at.% Tm doped crystal with a maximum output power of 201 mW at 1.99–2.007 μm. Operation of Yb,Tm:KLu(WO4)2 as a vibronic laser emitting at 2.081–2.093 μm is also demonstrated.

Principal component analysis of temperature dependences of fluorescence spectra of Yb3+:Y3Al5O12 and Yb3+:CaF2 is used for determining the Stark structure of Yb3+ energy levels. The results obtained are in a good agreement with reference data determined by the conventional methods of absorption and time-resolved fluorescence spectroscopy at low temperatures.

We study the nature of the intensity parameter Ω2 and the associated line strength S of the 4I9/2 → 4G5/2 hypersensitive transition in the series of Nd3+ doped Ca9Ln(VO4)7 and Ca10A(VO4)7 double calcium orthovanadate crystals. It has been shown that the covalence of neodymium – ligand bonds has a weak effect on Ω2 and S. They are mainly determined by the symmetry of the local environment of the neodymium ions, which lowers from yttrium ion to lanthanum ion for Ca9Y/La/Nd(VO4)7 crystals and from potassium ion to lithium ion in a series of Ca10K/Li(VO4)7 crystals.

Up to 15 mW of continuous wave deep red laser output at 702 nm are demonstrated using an Eu-(7.6 at.%):LiYF 4 crystal under pumping with a frequency doubled Ti:sapphire laser at 393.5 nm.

We report on yellow Tb3+:LiLuF 4 solid-state lasers. Under 2.1 W of 2ω-OPSL-pumping at 486.2 nm the laser delivers 0.45 W of cw output at a wavelength of 587.5 nm and a slope efficiency of 23 %.

Trivalent praseodymium doped materials are the most established gain media for solid-state lasers with direct visible emission. The progress in blue InGaN laser diodes (LD) and frequency doubled optically pumped semiconductor lasers (2ω-OPSL) enables efficient pumping of Pr3+-doped materials and efficient laser operation was realized in the cyan, green, orange, red and deep red parts of the optical spectrum [1]. The high gain and corresponding short upper state lifetime of several of these transitions should allow for comparably high pulse energies at high repetition rates e.g. in microchip configuration. This motivates a thorough investigation of the Q-switched operation of Pr:LiYF4 (Pr:YLF) and suitable modulators for this purpose. Previously, passively Q-switched diode-pumped Pr:YLF lasers were realized by using semiconductor saturable absorber mirrors [2] and Cr4+:YAG crystals [3]. Co2+: MALO crystals are well-known saturable absorbers for Q-switched eye-safe 1.5–1.6 μm lasers [4]. Here we report for the first time to our knowledge on a passively Q-switched Pr:YLF laser with Co2+:MgAl 2 O 4 (Co:MALO) as a saturable absorber.

A Tb3+:KYW crystal was grown by the modified Czochralski technique. Polarized ground state absorption and fluorescence spectra, as well as a fluorescence decay curve, were recorded at room temperature. Radiative properties such as emission probabilities, branching ratios, and radiative lifetime were investigated within the theory of 4f–4f transition intensity in the case of a strong configuration interaction. The limitations of visible laser operation of Tb3+-doped double tungstate crystals are discussed.

An investigation of Tm-Ho energy transfer in Tm(5at.%),Ho(0.4at.%):KYW single crystal by two independent techiques was performed. Based on fluorescence dynamics measurements, energy transfer parameters Psub71/suband Psub28/subfor direct (Tm→Ho) and back (Ho→Tm) transfers, respectively, as well as equilibrium constant Θ were evaluated. The obtained results were supported by calculation of microscopic interaction parameters according to the Förster-Dexter theory for a dipole-dipole interaction. Diode-pumped continuous-wave operation of Tm,Ho:KYW microchip laser was demonstrated, for the first time to our knowledge. Maximum output power of 77 mW at 2070 nm was achieved at the fundamental TEMsub00/submode.

We present, to the best of our knowledge, the most efficient solid-state laser directly emitting in the yellow spectral range. Without any nonlinear conversion steps, a Tb3+:LiLuF4 laser operates at a wavelength of 587.5 nm with an output power of 0.5 W and a record-high slope efficiency of 25% with respect to the absorbed pump power at 486.2 nm. Despite the detrimental influence of 4f-excited state absorption, this efficiency is comparable to those obtained by complex nonlinear methods to generate yellow laser emission. Our approach, in combination with the progress in laser diodes at the required pump wavelength, paves the way for the development of cost-efficient, robust, and easily manageable diode-pumped yellow laser sources.

Eu3+-doped LiYF4 is reexamined as a laser material for the visible spectral region. Polarized absorption and emission cross sections as well as the fluorescence lifetime are determined. Branching ratios and radiative lifetime are calculated within the theory of 4f-4f transition intensities, which takes into account the influence of an excited configuration of the opposite parity 4fN-15d. Continuous-wave laser operation at 702 nm is demonstrated with a maximum output power of 15 mW and a slope efficiency of 4.6% under pumping with a frequency-doubled Ti:sapphire laser at 393.5 nm.

A mixed fluoride crystal of LiLu0.65Y0.35F4 doped with Nd3+ ions was grown by the Bridgman–Stockbarger method. Polarized absorption and luminescence spectra as well as luminescence lifetime were measured at room temperature. Emission probabilities, branching ratios and radiative lifetime were studied within the Judd–Ofelt theory and the emission cross section spectra were calculated. Efficient continuous wave laser operation was demonstrated with the crystal. A maximum output power of 7.7 W and slope efficiency of 60% were achieved at 1047 nm for the TEM00 mode.

Energy transfer parameters of Tm(5at.%),Ho(0.4at.%):KYW single crystal were investigated. A diode-pumped CW Tm,Ho:KYW microchip laser was realized. Maximum output power of 77 mW at 2070 nm was achieved in the fundamental TEM00 mode.

We report on a 2ω-OPSL-pumped passively Q-switched Pr3+:LiYF4 laser operating at the wavelengths of 523, 607, and 640 nm. For this, we utilized a Co2+-doped MgAl2O4 (MALO) crystal as a saturable absorber in the visible range for the first time, to the best of our knowledge. In the green spectral region, the pulse duration was 210 ns with a pulse energy of 3.6 μJ and a repetition rate of 125 kHz. The minimum pulse duration of 87 ns at the highest pulse energy of 8.6 μJ was obtained at a repetition rate of 110 kHz at 607 nm. The highest Q-switched average laser power of 1.4 W was obtained at an absorbed continuous-wave pump power of 3.3 W with a slope efficiency of 47% at 640 nm. Absorption saturation measurements with Co:MALO were performed, and ground and excited state absorption cross sections in the visible spectral range were determined.

Pure and Yb-doped Ca 9 Y(VO 4 ) 7 (CYVO) single crystals were grown using the Czochralski method. Their crystal structure was determined using powder diffraction and the defect structure was characterized using high-resolution diffraction. The undoped crystal is found to be composed of a transparent and an opaque parts. In the latter, yttrium-rich inclusions were detected. Spectral-kinetic characteristics of Yb3 -doped crystals is reported.

Fluoride crystals are the most common host materials for visible lasers due to large energy band gap and low phonon energy. Among the rare-earth ions, the Pr3+ is very attractive ion for visible solid-state lasers. Efficient laser operation in Pr-doped crystals was obtained around 480 nm, 523 nm, 607 nm, 640 nm, 698 nm and 720 nm [1]. Mixed fluorides LiY x Lu x-1 F 4 crystals posses higher optical quality and require less expensive lutetium fluoride during growth in contrast to LiYF 4 and LiLuF 4 , respectively. Moreover, in the case of Ce3+ and Nd3+ ions, the mixed fluorides have higher segregation coefficient comparing to both LiYF 4 and LiLuF 4 crystals [2, 3]. The optimal cationic composition is x = 0.6–0.7.