In recent years, research of ReAl₃(BO₃)₄(Re=Y, Gd) crystals as ultraviolet nonlinear optical crystal has attracted more and more attention. [1, 2] reported that YAl₃(BO₃)₄(YAB) crystal presents good nonlinear optical properties, especially for the fourth harmonic generation of Nd-based lasers at 266 nm. However, it is difficult to grow high-quality YAB single crystals due to high growth temperature and large viscosity. In addition, impurity ions such as Fe3+ etc. in YAB lead intense non-intrinsic absorption around 266nm wavelength. In this study, using top-seeded solution growth method, a high-quality YAB crystal with the dimensions of 20×20×18mm3 was obtained from B₂O₃-Li₂O flux system (Fig. 1). The fourth harmonic generation of a frequency doubled Nd:YAG laser, from 532 to 266 nm, was carried out with four different lengths of YAB crystal elements which show obvious UV absorption. The results show that the conversion efficiency from 532 to 266 nm is inversely proportional to the length of the crystal elements. Therefore, UV absorption of YAB crystal seriously affects its fourth harmonic laser output properties. To overcome the shortcoming, we grew an eliminating the UV absorption of YAB crystal using a special equipment in oxygen-free atmosphere (the UV transmittance spectrum shown in Fig. 2). Further investigations in the fourth harmonic laser output of free UV absorption of YAB crystal are in progress.

GdAl₃(BO₃)₄(GAB) crystal belong to the same structure type as YAB. We explore a metal oxide-B₂O₃-fluoride flux system for GAB crystal growth. Base on lower growth temperature and viscosity of the system, it is easier to grow large-size GAB crystals. In this paper, GAB crystals grown successfully by top-seeded solution growth method. The transmittance spectrum indicates that the UV cut-off edge of GAB crystal is 175nm. Powder SHG test on ground crystals reveals that GAB is phase-matchable material with SHG intensity approximately as large as 3.5 times that of KDP standard. The refractive indices of GAB crystals were precisely measured. The SHG phase-matching angles were calculated based on the Sellmeier equations. The shortest wavelength that can be achieved by type-I phase-matching is down to 248nm.

These results show that ReAl₃ (BO₃)₄ (Re=Y, Gd) are promising fourth harmonic generation materials.

Fig. 1. YAB crystals grown with the dimensions of 20×20×18mm³    

   Fig. 2. UV transmittance spectrum of YAB crystal grown in non-oxygen atmosphere

References:

[1] Xuesong Yu, YinchaoYue, Zhang-gui Hu, J.Crystal Growth. 312 (2010) 3029-3033.

[2] Qiang Liu, Ning Ye, etc. Optics Letters. 36(2011) 2653-2655.

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