Growth and properties of KBe 2 BO 3 F 2 crystal
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Chuangtian Chen, Yicheng Wu, and Zuyan Xu Beijing Center for Crystal R&D, Academy of Sciences, Beijing 100080, People’s Republic of China (Received 18 January 2003; accepted 21 July 2003)
Single crystals of KBeBO3F (KBBF) up to 2 mm in thickness were grown by using a spontaneous nucleation method. The flux was composed of KF and B2O3, and the optimal ratio for growing KBBF single crystal thicker than 1 mm is suggested to be (1.0–1.5):(5.0):(0.7–1.2) (KBBF:KF:B2O3) in molar ratio. The ideal morphology of KBBF is two sets of tabular facets and a set of rhombic facets. Surface morphology was characterized both by optical microscopy and atomic force microscopy. Transmission spectra showed that high transmission was maintained up to 170 nm. Based on refractive index measurements, phase matching curves for types I and II were determined. Using a prism coupling method, the effective fourth harmonic generation for an Nd:YAB laser was realized in an 0.6-mm-thick KBBF sample. The output energy of the 266-nm beam was 0.282 mJ, and the conversion efficiency was 11.8%. A 183.6-nm laser over 1 mW was obtained from a Ti:sapphire second harmonic generation beam in an 0.8-mm KBBF sample. The conversion efficiency reached 2.3%. A 179.4-nm laser was also observed from a 358.7-nm laser by directly frequency doubling. This is the shortest wavelength obtained through nonlinear optical processes in a solid-state laser.
I. INTRODUCTION
Since the report on the first borate ultraviolet (UV) laser crystal KB5O8 · 4H2O (KB5),1 borate crystals have been found to be attractive for use in the deep ultraviolet (DUV) region with a relatively high laser damage threshold. KB5 is not a good UV crystal due to its small nonlinear optical (NLO) coefficient and water-solubility properties. After the development of –BaB2O2 (BBO)2 crystal, several other borate UV crystals such as LiB3O5 (LBO),3 KBe2O3F (KBBF),4 and Sr2Be2BO7 (SBBO)5 were fabricated by Chen’s group. Among these, KBBF is an important DUV crystal that has a UV absorption edge as low as 155 nm. Until now, this is the only crystal known to generate UV radiation below 200 nm. KBBF was first synthesized by Soviet scientists in 1968.6 In 1970, Solv’eva et al.7 determined its structure as space group C2. In 1987, Chen’s group synthesized the crystal and discovered its strong NLO a)
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J. Mater. Res., Vol. 18, No. 10, Oct 2003 Downloaded: 11 Mar 2015
effect by using the powder second harmonic generation (SHG) test. Until 1993, single crystals up to 5 × 5 × 0.3 mm3 were grown by using a flux method.8 The structure was determined again in 1995, and it was found9 that the real structure of KBBF belongs to a trigonal space group R32. Some basic properties of KBBF have been reported based on these small single crystals. KBBF is one crystal that is very difficult to grow. In 2000, T. Sosaki et al. in a review paper10 mentioned that KBBF crystals suitable for laser experiments are still less than
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