Spectroscopic and laser investigations of Nd{sup 3+} and Yb{sup 3+} in rare-earth oxyborates

Since 1997 a new class of non-linear crystals have been intensively investigated, the rare-earth and calcium oxyborates RECa{sub 4}O(BO{sub 3}){sub 4} -RECOB (especially with RE{sup 3+} = Y{sup 3+} or Gd{sup 3+}). These crystals can be grown from melt and have large effective non-linear coefficients; they present a large transparence range from 0.2-5 mm, are non-hygroscopic, have a high damage threshold, can be doped with large concentrations of laser active ions, etc. Undoped these crystals are good doublers and doped with Nd{sup 3+} or Yb{sup 3+} they can be used in the self - conversion regime for transformation of the infrared radiation into green, blue or red. A series of fundamental structural, spectroscopic and laser emission characteristics of Nd{sup 3+} or Yb{sup 3+} doped RECOB are still under study. Thus, one important aspect discussed in literature is the degree of disorder of RECOB crystals. The research performed in the frame of CERES project was devoted to the growth of the of RECOB crystals doped with Nd{sup 3+} or Yb{sup 3+}, structural investigations and high-resolution optical spectral investigations and Nd{sup 3+} laser emission. The growth of RECa{sub 4}O(BO){sub 3} (Y{sup 3+} and Gd{sup 3+}) single crystals was performed with an ADL furnace by using Czochralski method. Good optical crystals were obtained on direction; X-ray structural studies were also performed. The low temperatures high-resolution optical spectral measurements on Nd{sup 3+} and Yb{sup 3+} doped GdCOB and YCOB allowed the clarification of some structural characteristics of these crystals. Two problems were studied: the intrinsic structure of undoped crystals and the doping effects. Studies revealed for the first time by optical spectroscopy essential differences between GdCOB and YCOB crystals; they refer to the satellite structure of Yb{sup 3+} or Nd{sup 3+} and inhomogeneous broadening, indicating a disordered structure much larger for YCOB, qualitatively in accord with recent X-ray results. Based on spectral, emission decay and structural investigations, models for minority sites in these crystals as due to non-stoichiometric or inversion charge defects of the type RE{sup 3+} -><- Ca{sup 2+} or RE{sup 3+} in Ca{sup 2+} sites were proposed and their effects in emission investigated. Extended energy level schemes for Yb{sup 3+} and Nd{sup 3+} prevailing (in RE{sup 3+} sites) and minority centers in GdCOB and YCOB were determined. A theoretical parametric crystal field calculation was performed for Nd{sup 3+} by varying free ion and crystal field parameters. A characteristic of the crystal field parameters is the large value of the second order parameters that leads to a strong splitting of metastable {sup 4} F{sub 3/2} Nd{sup 3+} level ({approx}190 cm{sup -1}) that leads to a parasitic effect for Nd{sup 3+} in GdCOB, i. e. the shift of the laser emission from 1.06 mm to 1.09 mm at high pumping. The spectroscopic bases for the improvement of the laser emission parameters in fundamental and self-doubling regime of Nd{sup 3+}: GdCOB and the reduction of heat generation were investigated. The analysis of spectral data shows that the laser emission parameters of Nd{sup 3+} in GdCOB at 1.06 mm can be improved by direct pumping in the emitting level in the band {sup 4}I{sub 9/2}(Z{sub 2}){yields}{sup 4}F{sub 3/2}(R{sub 1}) at 887 nm, instead of using the usual diode pumping in the line {sup 4}I{sub 9/2}(Z{sub 1}){yields}{sup 4}F{sub 5/2}(S{sub 1}) at 811 nm. Experimental investigations confirmed this idea and for fundamental emission at 1.06 mm the slope efficiency increased from {approx} 0.46 to {approx} 0.61 while the threshold decreased from {approx} 115 mW to {approx} 75 mW. The parasitic change of emission wavelength at high powers did not show. These improvements are amplified in self-doubling emission. The emission characteristics under direct pump 887 nm as compared to the conventional pump 811 nm are evidently improved. Thus at 600 mW absorbed power the emitted power in green was 2.16 larger than with conventional pumping. (authors)