Monoclinic layers of KY1-xYbx (WO4)(2) with a thickness of up to 330 mu m were grown on KY(WO4)(2) substrates by liquid phase epitaxy (LPE) using K2W2O7 solvent. The layers grown from solutions of up to 7.5 at.% substitution of Y by Yb generally had flat surfaces on {010} and {110} crystal faces with no macroscopic defects at the layer/substrate interface. Already at 10 at.% of Yb substituting Y, some defects at the interface tend to appear and increase for higher concentrations. The layers grown on {310} and {(1) over bar 11} faces show more growth instabilities even at low Yb concentrations. The thermal evolutions of the lattice mismatches between the epilayer and the substrate for {010}, {110}, {310} and {(1) over bar 11} faces were calculated to range from 0.55% at 1273 K to -0.31% at 298 K. The optical spectroscopic studies carried out with these samples agree well with Yb-doped KYW bulk crystals, the measured absorption and calculated (by the reciprocity method) emission cross-sections at 981 nm with E parallel to N-m being sigma(abs) = 12.5 x 10(-20) cm(2) and sigma(em) = 15.8 x 10(-20) cm(2), respectively.

Epitaxial layers of KLu 1-x Yb x (WO 4 )2/KLu(W04)2 with different Yb concentrations were grown by liquid-phase epitaxy. The quality of the epitaxial layers principally depends on the Yb concentration and the crystal face. In solutions with up to 50 mol % of substitution of Lu 2 O 3 by Yb 2 O 3 , defect-free layers can be grown and the highest quality face is (010). The layers grown on the (-111) face tend to have a higher density of morphologies. The optical spectroscopy of Yb 3+ in thin layers coincides with that obtained in doped bulk crystals.

Epitaxial layers of KLu1-xYbx(WO4)(2)/KLu(WO4)(2) with different Yb concentrations were grown by liquid-phase epitaxy. The quality of the epitaxial layers principally depends on the Yb concentration and the crystal face. In solutions with up to 50 mol % of substitution of Lu2O3 by Yb2O3, defect-free layers can be grown and the highest quality face is (010). The layers grown on the (- 111) face tend to have a higher density of morphologies. The optical spectroscopy of Yb3+ in thin layers coincides with that obtained in doped bulk crystals.

Epitaxial layers of up to 50% Yb-doped monoclinic KLu(WO4)2 could be successfully grown on passive KLu(WO4)2 substrates. These composite samples were characterized and continuous-wave and mode-locked laser operation was achieved with Ti:sapphire and diodelaser pumping. A 10% Yb-doped epitaxy provided an output power exceeding 500 mW at 1030 nm and a maximum slope efficiency of 66% with Ti:sapphire laser pumping. A 50% Yb-doped epitaxy exhibited serious thermal problems without special cooling and rather limited cw performance. Quasi-cw operation provided in this case an average output power of 43 mW at 1032 nm for a 10% duty cycle. More than 100 mW cw could be generated at 1030 nm also with diode-pumping of the 10% Yb-doped KLu(WO4)2 epitaxy. Pulses as short as 114 fs were generated at 1030 nm with this same sample under Ti:sapphire laser pumping in a laser mode-locked by a saturable absorber mirror.

The crystal structure of monoclinic KLu(WO4)2(KLuW) crystals was determined at room temperature by using single-crystal X-ray diffraction data. The unit-cell parameters werea= 10.576 (7),b= 10.214 (7),c= 7.487 (2) Å, β = 130.68 (4)°, withZ= 4, in space groupC2/c. The unit-cell parameters of KLu1−xYbx(WO4)2were determined in relation to Yb concentration. Vickers micro-indentations were used to study the microhardness of KLuW. The linear thermal expansion tensor was determined and the principal axis with maximum thermal expansion (\alpha_{33}' = 16.72 × 10−6 K−1), X_3', was located 13.51° from thecaxis. The room-temperature optical tensor was studied in the near-infrared (NIR) and visible range. The principal optical axis with maximum refractive index (ng= 2.113),Ng, was located 18.5° from thecaxis at 632.8 nm. Undoped and ytterbium-doped KLuW crystals were grown by the TSSG (top-seeded-solution growth) slow-cooling method. The crystals show {110}, {\bar{1}11}, {010} and {310} faces that basically constitue the habit of the KLuW crystals.

Epitaxial monoclinic double tungstate composites based on the strongly anisotropic KLu(WO/sub 4/)/sub 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-/spl mu/m-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.

Epitaxial growth of highly Yb-doped films on passive substrates is especially interesting for the strongly anisotropic monoclinic double tungstate hosts where the extremely large cross sections would permit the utilization of the thin disk laser concept with very thin (<100 μm) active layers which can be face cooled and pumped in a single pass. In this paper femtosecond mode-locked operation with the same 10% Yb-doped KLuW/KLuW sample is demonstrated. The 100-μm thick Yb:KLuW layer was grown by liquid phase epitaxy on the [010] face, normal to the Np-principal optical axis, of a 1.1-mm thick KLuW substrate.

Epitaxial growth of highly Yb-doped films on passive substrates is especially interesting for the strongly anisotropic monoclinic double tungstate hosts where the extremely large cross sections would permit the utilization of the thin disk laser concept with very thin (<100 μm) active layers which can be face cooled and pumped in a single pass. Recently we demonstrated efficient laser operation of Yb-doped epitaxial composites on the basis of the monoclinic KLu(WO4)2 (KLuW) crystal [1]. The results were superior in comparison to those we previously achieved using KY(WO4)2 [2] due to the closer lattice parameters of KLu(WO4)2 and KYb(WO4)2 [1]. Here we demonstrate femtosecond mode-locked operation with the same 10% Yb-doped KLuW/KLuW sample [1]. The 100-μm thick Yb:KLuW layer was grown by liquid phase epitaxy on the (010) face, normal to the Np-principal optical axis, of a 1.1-mm thick KLuW substrate. The sample was oriented for polarization parallel to the Nm-optical axis and positioned under Brewster angle between two focusing mirrors (RC=-10 cm) folding an astigmatically compensated Z-shaped cavity. The laser was longitudinally pumped through one of the folding mirrors using an f=6.28 cm lens by a tunable cw Ti:sapphire laser focused to a beam waist of about 30 μm in the position of the epitaxial crystal. One arm contained an additional focusing mirror (RC=-10 cm) to increase the intensity on the semiconductor saturable absorber mirror (SAM) used for passive mode locking which terminated the cavity. Two SF10 Brewster prisms with a separation of 31 cm were inserted in the other arm, containing the plane output coupler, for dispersion compensation. The cavity length corresponded to a repetition rate of 100 MHz.

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.

Epitaxial layers of Yb:KLu(WO/sub 4/)/sub 2/ were grown on KLu(WO/sub 4/)/sub 2/ substrates and highly efficient cw (66% slope efficiency and 415 mW output power) and passively mode-locked (114 fs pulse duration) operation at 1030 nm is demonstrated.

Mode locking based on an epitaxial composite of the monoclinic double tungstate crystal Yb:KLu(WO4)2 is realized. A 100 microm thin Yb:KLu(WO4)2 layer grown on a KLu(WO4)2 substrate is used as an active medium in a laser passively mode locked by a semiconductor saturable absorber. Pulse durations of 114 fs have been achieved for an average power of 31 mW at 1030 nm. Results in the femtosecond and picosecond regimes of the Yb:KLu(WO4)2/KLu(WO4)2 laser are presented. The great potential of Yb-doped tungstate composite structures as active elements for mode-locked laser systems is demonstrated.

Bulk and epitaxial composites of Yb:KLu(WO4)2 were grown and characterized. CW-lasing @ 1 µm was demonstrated achieving conversion efficiencies of 50% and output powers of 1 W for the bulk and 25.5% and 0.5 W for the composite Yb:KLu(WO4)2.

High quality epitaxial double tungstate crystals were grown and continuous-wave laser operation at 1030 nm was demonstrated for the first time with a 25-/spl mu/m-thin Yb:KYW layer on a KYW substrate.

Epitaxial monoclinic double tungstate laser crystals were grown with high crystalline quality. Based on these Yb-doped composites, laser operation was demonstrated. Continuous-wave laser emission of a Yb:KYW∕KYW crystal was achieved at 1030nm. The 25-μm-thin Yb:KYW layer was pumped at wavelengths near 980nm by a Ti:sapphire laser. A maximum output power of 40mW was obtained at room temperature.

We successfully grew langbeinite-type structures containing Ti4+ and Ln3+ ions as M2Ti2-xLnx(PO4)3 crystals (M = K+ or Rb+ and Ln = Er3+ or Yb3+) by high-temperature-solution methods. We took accurate composition measurements of these crystals, paying special attention to the lanthanide contents. We resolved the structure of the Rb2Ti1.01Er0.99(PO4)3 crystal and studied the evolution of this structure with the temperature. We found that this compound decomposed at 1323 K. We made a preliminary study to check the nonlinear optical properties of these materials, showing that the second-harmonic generation efficiency of the Rb2Ti1.01Er0.99(PO4)3 was at least similar to that of potassium dihydrogen phosphate.

In recent years, Yb3+ has attracted much attention as an activating ion because of its small quantum defect for laser emission from 2F5/2 to 2F7/2 at ~1.03 µm, which provides high efficiency and reduced heat generation. A promising material for Yb3+ lasers is KYb(WO4)2 (KYbW) [1]. It can be grown from high-temperature solutions [2]. A suitable substrate material for the growth of single-crystalline layers with thicknesses in the range of the absorption length of ~13 µm at 981 nm is KY(WO4)2 (KYW). We demonstrate the liquid phase epitaxy (LPE) of KYbW layers at start temperatures as low as 520°C from the chloride solvent KCl-NaCl-CsCl. This temperature is favorable in order to decrease the thermal stresses due to the differences in the thermal expansion coefficients of substrate and layer. Moreover, the choice of [010]-oriented KYW substrates bypasses the large difference in the thermal expansion coefficient along the [010] direction. Our spectroscopic investigations show that the fluorescence lifetime of ~250 µs measured in our LPE-grown KYbW layers is dominated by radiative decay and is very similar to that measured in top-seeded-solution-grown bulk samples [2]. Fast energy migration among the Yb3+ ions and energy transfer to small amounts of Tm3+ and Er3+ ions present in the YbCl3 reagent lead to visible upconversion luminescence in the layers under 981-nm excitation. [1] P. Klopp, U. Griebner, V. Petrov, X. Mateos, M.A. Bursukova, M.C. Pujol, R. Sole, J. Gavalda, M. Aguilo, F. Guell, J. Massons, T. Kirilov, F. Diaz, Appl. Phys. B 2002, 74, 185 [2] M.C. Pujol, M.A. Bursukova, F. Guell, X. Mateos, R. Sole, J. Gavalda, M. Aguilo, J. Massons, F. Diaz, P. Klopp, U. Griebner, V. Petrov, Phys. Rev. B 2002, 65, 165121

In recent years, Yb3+ has attracted much attention as an activating ion because of its small quantum defect for laser emission from 2F5/2 to 2F7/2 at ~1.03 µm [1], which provides high efficiency and reduced heat generation. Of high practical interest is the thin-disk laser concept [2], which possesses a tremendous advantage over rod lasers because of its axial-cooling approach and consequent weak thermal lensing and good beam quality. A promising material for Yb3+ thin-disk lasers is KYb(WO4)2 (KYbW) [3]. It can be grown from high-temperature solutions [4]. Nevertheless, the growth of high-quality, single-crystalline layers with thickness in the range of the absorption length of ~13 µm at 981 nm has as yet not been reported. A suitable substrate material is KY(WO4)2 (KYW), but the relatively large differences in the thermal expansion coefficients between KYW and KYbW along the [100], [001], and especially [010] directions [5] favor low temperatures for the hetero-epitaxial growth. For the first time, we demonstrate liquid phase epitaxy (LPE) of KYbW layers. The layers were grown at start temperatures as low as 520°C, which is favorable in order to decrease the thermal stresses due to the differences in the thermal expansion coefficients of substrate and layer. Moreover, the choice of [010]-oriented substrates bypasses the large difference in the thermal expansion coefficient along the [010] direction. KY1-xYbx(WO4)2 layers with varying x = 0.03-1.00 were grown by LPE. The chloride solvent consisted of the eutectic composition [6] 24.4 mol.% KCl, 30.4 mol.% NaCl, and 42.2 mol.% CsCl. The growth temperature spanned the range from 580 to 500°C and the cooling rate was 0.67-1.00 Kh-1. Crack-free, transparent KYbW layers were grown on (010) substrates. Spectroscopic investigations have shown that the lifetime of ~250 µs measured in our LPE-grown KYbW layers is dominated by radiative decay and is very similar to that measured in top-seeded-solution-grown bulk samples [4]. Fast energy migration among the Yb3+ ions and energy transfer to small amounts of Tm3+ and Er3+ ions present in the YbCl3 reagent lead to visible upconversion luminescence in the layers under 981-nm excitation. [1] T.Y. Fan, IEEE J. Quantum Electron. 29, 1457 (1993). [2] A. Giesen, H. Hugel, A. Voss, K. Wittig, U. Brauch, H. Opower, Appl. Phys. B 58, 365 (1994). [3] P. Klopp, U. Griebner, V. Petrov, X. Mateos, M.A. Bursukova, M.C. Pujol, R. Sole, J. Gavalda, M. Aguilo, F. Guell, J. Massons, T. Kirilov, F. Diaz, Appl. Phys. B 74, 185 (2002). [4] M.C. Pujol, M.A. Bursukova, F. Guell, X. Mateos, R. Sole, J. Gavalda, M. Aguilo, J. Massons, F. Diaz, P. Klopp, U. Griebner, V. Petrov, Phys. Rev. B 65, 165121 (2002). [5] M.C. Pujol, X. Mateos, R. Sole, J. Massons, J. Gavalda, F. Diaz, M. Aguilo, Mater. Sci. Forum 378-381, 710 (2001). [6] D. Ehrentraut, M. Pollnau, S. Kuck, Appl. Phys. B 75, 59 (2002).