Lucian Gheorghe

Herein we present a roadmap for tailoring the crystal growth conditions, near-infrared (NIR) laser emission, and self-frequency doubling (SFD) performances of newly developed Nd-doped La_xGd_ySc_4−x−y(BO₃)₄ (Nd:LGSB) crystals. Three different Nd³⁺ doping concentrations of 2.3 at.%, 3.5 at.%, and 4.6 at.% were investigated. Considering their incongruent melting, special conditions were employed for the growth using the Czochralski technique. Laser emission performances at 1062 nm in the CW regime were evaluated for uncoated crystal samples with different orientations (a-cut, c-cut, and SFD-cut). The highest slope efficiency η_sa = 0.68 was obtained for the 4.6 at.% c-cut Nd:LGSB crystal, with a randomly polarized emission. The a-cut 4.6 at.% Nd:LGSB crystal delivered a linearly polarized beam with a slope efficiency η_sa = 0.63. The SFD-cut 2.3 at.% and 3.5 at.% Nd:LGSB crystals achieved slightly lower efficiencies of ~ 0.56. The SFD capabilities of 2.3 at.% and 3.5 at.% Nd:LGSB crystals were also explored. Green laser emission at ~531 nm was achieved with a diode-to-green conversion efficiency increasing significantly from 0.17% to 1.44%, respectively. These results demonstrate that the Nd-doping concentration, crystal orientation, and sample length of Nd:LGSB crystals, must be carefully selected depending on the specific requirements of the intended application.

Buried depressed-cladding waveguides were fabricated in 0.7-at.% Nd:Ca₃Li_0.275Nb_1.775Ga_2.95O₁₂ (Nd:CLNGG) and 7.28-at.% Yb:CLNGG disordered laser crystals grown by Czochralski method. Circular waveguides with 100 μm diameters were inscribed in both crystals with picosecond (ps) laser pulses at 532 nm of 0.15 μJ energy at 500 kHz repetition rate. A line-by-line writing technique at 1 mm/s scanning speed was used. Laser emission at 1.06 μm (with 0.35 mJ pulse energy) and at 1.03 μm (with 0.16 mJ pulse energy) was obtained from the waveguide inscribed in Nd:CLNGG and Yb:CLNGG, respectively, employing quasi-continuous wave pumping with fiber-coupled diode lasers. The waveguide realized in RE³⁺-doped CLNGG crystals using ps-laser pulses at high repetition rates could provide Q-switched or mode-locked miniaturized lasers for a large number of photonic applications.

Three incongruent melting La_wNd_xGd_yY_zSc_4-w-x-y-z(BO₃)₄ (LGYSB:Nd) crystals with different Y concentrations (z = 0.15, 0.05, and 0.025) have been grown by the Czochralski method for the first time. The LGYSB:Nd-type crystals exhibit an acentric structure similar to that of the natural mineral huntite CaMg₃(CO₃)₄, with space group R32. The composition of the LGYSB:Nd_3 (z = 0.025) crystal grown from the starting melt composition La_0.628Gd_0.547Y_0.025Nd_0.05Sc_2.75(BO₃)₄ was measured by inductively coupled plasma mass spectrometry method, and it was found to be La_0.6794Gd_0.4105Y_0.0178Nd_0.0381Sc_2.8542(BO₃)₄. The transversal spatial distribution of the refractive index in the LGYSB:Nd_3 crystal was investigated. Third-order nonlinear optical susceptibility χ⁽³⁾ of the LGYSB:Nd_3 crystal was determined from third-harmonic generation experiments with ultrashort (fs) laser pulses. The optical transmission spectrum was measured in the range of 200-2000 nm. The absorption cross-section at 808 nm in σ-polarization was determined to be 1.18 × 10^-19 cm² for the LGYSB:Nd_3 crystal. The 10K absorption spectra revealed that the Nd³⁺ ions occupy only La³⁺ cationic sites in the LGYSB host matrix. The emission cross-section at 1064 nm in σ-polarization was determined to be σ_em(σ) = 1.74 × 10^-19 cm² for the LGYSB:Nd_3 crystal. The fluorescence lifetime was found to be τ = 115 μs for all of the LGYSB:Nd crystals. The LGYSB:Nd_3 laser was operated at an emission wavelength of 1062 nm with very high slope efficiencies of η_sa = 0.74 and η_sa = 0.64 in quasi-cw and cw regimes, respectively.

This paper reviews the progress in developing the La_xY_ySc_4-x-y(BO₃)₄-LYSB and Yb-doped La_xY_ySc_4-x-y(BO₃)₄-LYSB:Yb huntite-type crystals grown by the Czochralski method as new candidates for the next generation of nonlinear optical (NLO) and/or laser crystals. Considering the incongruent melting of these crystals, the initial compositions of the melt and the pulling and rotation rates were optimized. Additionally, a special thermal setup was engineered to grow LYSB-type crystals by the Czochralski crystal growth method. The chemical compositions of the LYSB and LYSB:Yb grown crystals were found to be La_0.78Y_0.32Sc_2.90(BO₃)₄ and La_0.78Y_0.32Yb_0.04Sc_2.86(BO₃)₄, respectively. Therefore, for the LYSB:Yb crystal, the doping concentration of Yb³⁺ ions was considered to be 4 at.% with respect to the nonstoichiometric (La_1-xY_x)_1.25Sc_2.75(BO₃)₄ undoped compounds, i.e., LYSB:Yb (4 at.%). In terms of NLO properties, the obtained results demonstrate that LYSB and LYSB:Yb (4 at.%) crystals possess remarkable properties specific to huntite-type crystals. The main advantage of these crystals consists in the fact that they may be obtained with large dimensions and excellent optical quality by the Czochralski method, which recommends them as a new class of highly efficient crystals for different NLO applications, including second harmonic generation (SHG) of high-power or high-energy laser beams. The laser performances of the LYSB:Yb (4 at.%) crystal prove its favorable intrinsic properties to generate laser emissions in the 1 µm range with high efficiency. The efficient laser emission at ~1028 nm together with good NLO characteristics to convert its own emission into emission at ~514 nm via SHG make the LYSB:Yb (4 at.%) crystal a very promising active medium to be used in self-frequency doubling configuration.

Spectroscopic characteristics of RE³⁺ ions (RE = Sm, Dy, and Pr) doped in partially disordered Ca₃Nb_1.6875Ga_3.1875O₁₂-CNGG and Ca₃Li_0.275Nb_1.775Ga_2.95O₁₂-CLNGG crystals are reviewed in detail to assess their prospects as laser crystals with emission in the visible spectral domain. All investigated crystals were grown using the Czochralski crystal growth technique. High-resolution absorption and emission measurements at different temperatures, as well as emission dynamics measurements, were performed on the grown crystals. The spectroscopic and laser emission characteristics of the obtained crystals were determined based on the Judd-Ofelt parameters. The obtained results indicate that CNGG:RE³⁺ and CLNGG:RE³⁺ (RE = Sm, Dy, and Pr) crystals can be promising materials for lasers in the visible range.