Al-doped CLBO crystals were grown from 0.5 mol% Al₂O₃- and 10 mol% LiF-added stoichiometric melt by the solution-stirring top-seeded solution growth method. The addition of a fluoride component was used to decrease the solution viscosity. High-quality single crystal with the size of 79×36×22 mm³ was obtained by the LiF flux solution. The concentration of Al incorporated into the crystal was about 10 wt ppm. On the other hand, the water impurities in a CLBO sample significantly degrade its UV optical properties and the bulk LIDT [2]. Therefore, prior to experiment, the water impurities in Al-doped and undoped CLBO samples were reduced by post-growth heat treatment at 150 ^oC in ambient and dry atmosphere.

We evaluated UV-induced degradation in CLBO. Figure 1 shows a schematic of the experimental setup for measuring degradation induced by UV pulses. UV beam (wavelength: 266 nm; peak power density at the focal point: 56 MW/cm²) was tightly focused at the center of CLBO sample to observe the degradation behavior in a short time. The transmitted power through the aperture after the sample was measured to detect the UV-induced degradation that occurred near the focal point in CLBO. Figure 2 shows a typical result obtained for undoped and Al-doped CLBO samples. Pattern distortion occurred after UV illumination for a while, and the transmitted power then decreased gradually. As is the case with the bulk LIDT [2], the UV-induced degradation resistance was significantly improved by eliminating water impurities from the crystal. The more important result is that Al-doping of CLBO can slow down UV-induced degradation. We will discuss about properties of Al-doped crystals and the degradation phenomena in detail.

Fig. 1. Schematic of the experimental setup for measuring degradation induced by UV pulses.

Fig. 2. Top: 266 nm transmitted power degradation through CLBO sample at 150 ^oC and aperture. (a) Undoped CLBO before dehydration, (b) undoped CLBO after dehydration, and (c) Al-doped CLBO after dehydration. Bottom: (d) Initial and (e) distorted transmitted UV beam through CLBO. The images were observed on fluorescent paper in the far-field.

Rererences

[1] K. Takachiho, M. Yoshimura, Y. Fukushima, Y. Takahashi, M. Imade, T. Sasaki, and Y. Mori, Appl. Phys. Express 6 (2013) 022701.

[2] T. Kawamura, M. Yoshimura, Y. Honda, M. Nishioka, Y. Shimizu, Y. Kitaoka, Y. Mori, and T. Sasaki, Appl. Opt. 48 (2009) 1658.

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