Structural, spectroscopic, and tunable laser properties ofYb3+-dopedNaGd(W

Abstract

Single crystals of Yb 3+ -doped NaGd͑WO 4 ͒ 2 with up to 20 mol % ytterbium content have been grown by the Czochralski technique in air or in N 2 +O 2 atmosphere and cooled to room temperature at different rates ͑4-250°C/h͒. Only the noncentrosymmetric tetragonal space group I4 accounts for all reflections observed in the single crystal x-ray diffraction analysis. The distortion of this symmetry with respect to the centrosymmetric tetragonal space group I4 1 / a is much lower for crystals cooled at a fast rate. Na + , Gd 3+ , and Yb 3+ ions share the two nonequivalent 2b and 2d sites of the I4 structure, but Yb 3+ ͑and Gd 3+ ͒ ions are found preferentially in the 2b site. Optical spectroscopy at low ͑5 K͒ temperature provides additional evidence of the existence of these two sites contributing to the line broadening. The comparison with the 2 F 7/2 ͑n͒ and 2 F 5/2 ͑nЈ͒ Stark energy levels calculated using the crystallographic Yb-O bond distances allows to correlate the experimental optical bands with the 2b and 2d sites. As a novel uniaxial laser host for Yb 3+ , NaGd͑WO 4 ͒ 2 is characterized also with respect to its transparency, band-edge, refractive indices, and main optical phonons. Continuous-wave Yb 3+ -laser operation is studied at room temperature both under Ti:sapphire and diode laser pumping. A maximum slope efficiency of 77% with respect to the absorbed power is achieved for the polarization by Ti:sapphire laser pumping in a three-mirror cavity with Brewster geometry. The emission is tunable in the 1014-1079 nm spectral range with an intracavity Lyot filter. Passive mode locking of this laser produces 120 fs long pulses at 1037.5 nm with an average power of 360 mW at Ϸ97 MHz repetition rate. Using uncoated samples of Yb: NaGd͑WO 4 ͒ 2 at normal incidence in simple two-mirror cavities, output powers as high as 1.45 W and slope efficiencies as high as 51% are achieved with different diode laser pump sources.

Facile hydrothermal crystallization of NaLn(WO4)2 (Ln=La-Lu, and Y), phase/morphology evolution, and photoluminescence

“…The NaLn(WO 4 ) 2 compound has the scheelite-type tetragonal structure (space group: I − 4), in which there are two different lattice sites occupied by the [Na 2-group has the S 4 site symmetry [24][25][26]. In this structure, tungsten is coordinated by four oxygen atoms at a tetrahedral site and the [La 3+ /Na + ] site is coordinated eightfold by oxygen [27,28], as shown in Figure S1.…”

High-power ultrafast thin disk laser oscillators and their potential for sub-100-femtosecond pulse generation

“…These problems are usually particularly severe for gain materials with a large emission bandwidth necessary for femtosecond pulse generation. Many of these materials have a disordered crystal structure, and the broad emission spectrum takes its origin in different lattice environments for the laser active ions [23]. However, this disordered lattice restricts the propagation of phonons, which often leads to a significantly lowered thermal conductivity.…”

“…Thus, the flux method has to be used [23], which is a technique with very slow growth rates. Consequently, large crystals of high optical quality are harder to obtain than with the Czochralski or heat exchanger method.…”

Optical spectroscopic study of Eu^3+ crystal field sites in Na_3La_9O_3(BO_3)_8 crystal

“…Thus, the assignment of each A, B, C or D site to a specific site in the crystal structure must be led by the symmetry-related characteristics of the optical centers resolved in the FLN spectra. Though Eu 3+ ion usually substitutes lanthanide cations in most of lanthanide-based compounds, in some mixed oxides, containing monovalent cations, these ions may have the same, or nearly the same, oxygen coordination than the one at the lanthanide site giving rise to some structural disorder [16] which facilitates the occupancy of these sites by the RE ions if charge compensation is allowed; therefore, we start this correlation with the inspection of the symmetry characteristics of their oxygen environments.…”