Simple and complex rare-earth borates have been actively studied for the last few decades as possible active and nonlinear-optical media or as both in one crystal. Among the great number of published works we want to mention [1, 2], which consider the growth and study of yttrium-lanthanum-scandium borate with the chemical formula Y_0.57La_0.72Sc_2.71(BO₃)₄ – (LYSB). The authors report that they have found the composition at which stabilization of  huntite – like noncentrosymmetrical structure with space group R32 takes place. They had grown relatively large crystals from which samples for the analysis of quality and second-harmonic generation were made.  The major problem they faced was the impossibility to grow a crystal of this composition with a high optical quality [3]. Therefore, its practical application is doubtful.

In the Institute of Geology and Mineralogy, Siberian Branch of the RAS, a procedure of solid-phase synthesis was used to synthesize compounds of similar composition and with identical X-ray patterns, replacing La cations by Ce, Pr, or Nd [4]. Spontaneous crystals containing Pr & Nd were grown using LiBO₂-LiF flux as a solvent. We did not succeed in growing Ce-bearing spontaneous crystals. In this paper we report on the progress in the growth of bulk Nd-bearing (NdYSB) crystals and study of their optical and nonlinear-optical properties. We can suppose that the chemical composition of NdYSB is similar to LYSB formula: Y_0.57Nd_0.72Sc_2.71(BO₃)₄.

To grow bulk crystals, we used the eutectic composition of the  LiBO₂–LiF system with 59 mol.% LiBO₂ and eutectic temperature T=725ºC. The search for a flux was carried out among the studied diagrams of melting ability of systems based on borates and fluorides of alkali metals. The selected eutectic composition has a low viscosity, as LiF promotes the rupture of spatial bonds between borate groupings.  

A crystal weighing 5.3 g was grown by the Kyropoulos procedure. The temperatuure interval of growth was 17 degrees, beginning from 900°C. The crystal distinctly displays the faces of rhombohedron and hexagonal prism. The grown crystals have a perfect cleavage on rhombohedron. The hardness of crystals is about 6 on the Mohs scale.

A 3.05 mm thick plate was cut and polished from the grown crystal with orientation 31 degrees to the optical axis on the assumption that the SHG (second harmonic generation) angle must be close to the angle of phase matching for LYSB  (for which θ= 33.48 degrees, φ=0) [2]. The plate has transparent windows which are big enough for analyzing the optical quality and efficiency of SHG. The optical quality of the studied sample was higher than that of LYSB crystal. It was confirmed by the Schlieren method.

The SHG experiments evidence that the θ angle for type I phase matching is 34.8°. In continuous mode with a focused Gaussian beam we obtained P(ω)/P(2ω) = 1.9W/810 nW and 2W/460 nW for 43μm and 92μm waist size, respectively. That corresponds to calculated d_eff I (pm/V) 0.24 and 0.23, respectively without taking the absorption into account. In pulsed mode (repetition rate 1kHz, pulse width 90ns), we obtained 0.6W/614μW and 0.56W/310μW for the same beam waist sizes. The calculated transformation efficiency of the sample was found to be rather low, about 0.1%. At the same time, the qualitative pattern of transformation evidences the opposite: we observed intense green spots at the output.

NdYSB transmission measurements were conducted on the Perkin Elmer Lambda 900 device in the range of wavelength from 190 nm to 3200 nm. It is a typical Nd spectrum with a lot of absorption peaks in the 250-900 nm range. Transmission coefficient at 532 nm was only about 0.06. Transmission measurement on this sample in the experimental conditions of SHG yielded still lower transmission coefficient at 532 nm – 0.009.

As a next step we have decreased the thickness of example from 3.05mm to 0.88mm and repeated SHG experiment by 43μm waist size. In continuous mode we obtained P(ω)/P(2ω) = 2.75W/3.42μW. That corresponds to calculated d_eff I = 0.91 pm/V without taking the absorption into account. This value increases to 2.5 pm/V if we take into account the 21cm^-1 at 532nm and 0.2cm^-1 at 1064nm absorption calculated with the differential transmission spectrum of two samples of 0.88 and 1.76mm.

Measurements have also been performed in a pulsed mode. For the maximum incident power 0.82W corresponding to the density of 156MW/cm² we have obtained about 0.5% efficiency. Apparently higher transformation efficiency can be achieved at a higher incident power. We can also predict an increase in the efficiency by thinning the sample thickness.

The laser properties of this crystal is under investigation.

References

1. N.Ye, J.L.Stone-Sundberg, M.A.Hruschka, G.Aka, W.Kong, and D.A.Keszler. Nonlinear optical crystal Y_xLa_ySc_z(BO₃)₄ (x+y+z=4). - Chem. Mater. 17(10), 2687–2692 (2005).
2. N.Ye, Y.Zhang, W.Chen, D.A.Keszler, and G.Aka. Growth of nonlinear optical crystal Y_0.57La_0.72Sc_2.71(BO₃)₄. - J. Cryst. Growth 292(2), 464–467 (2006).
3. M.Bourezzou, A.Maillard, R.Maillard, Ph.Villeval, G.Aka, J.Lejay, P.Loiseau, and D.Rytz. Crystal defects revealed by Schlieren photography and chemical etching in nonlinear single crystal LYSB – Optical materials express, 2011, Vol. 1, No. 8, pp. 1569-1576.
4. A.E.Kokh, N.G.Kononova, M.V.Fedorova, P.P.Fedorov, and M.N.Mayakova. New nonlinear optical crystals of the M_1-xSc_3-yY_x+y(BO₃)₄ (M=Ce, Pr, or Nd) family. – Doklady Physics, 2012, Vol. 57, No. 4, pp. 148-150.

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