CBF is a potential nonlinear material for third harmonic generation. CBF single crystals were grown using LiF as a flux. 300 mW average power at 343 nm by frequency tripling were obtained. d_eff were estimated as from conversion efficiency measurements. We present here studies of new flux to growth CBF such as: LiF-B₂O₃, LiF-NaF, LiF-CaF₂, CaF₂, NaF. Diffferentiel thermal analysis (DTA), spontaneous crystallization and crystal growth using these flux will be presented.

1. Introduction

The calcium fluoroborate, Ca₅(BO₃)₃F is an interesting and new nonlinear optical material for UV applications. This material crystallizes in the same acentric space group Cm (Z=2) as YCa₄O(BO₃)₃ (YCOB) but with extented properties towards UV. CBF can be grown by flux method, using LiF as a flux [1][2]. Its structure is built from [BO₃]^3- anionic groups, which are the most suitable for the design of nonlinear crystals aimed to have UV applications. CBF can be considered as derived from YCOB since Y³⁺ is replaced by Ca²⁺ and O^2- is substituted by F^- to preserve the electroneutrality of its structure. The CBF crystals are chemically stable and are not hygroscopic. The UV absorption edge of CBF is around 190nm. The nonlinear optical properties was determined by Xu et al [3].

     2.  Third harmonic generation in CBF (λ=343 nm)

      Fig. 1: CBF single crystal grown using LiF as a flux

3.  Study of new flux

The usual flux to growth CBF is LiF. The prominent drawback of LiF is its volatility. Moreover, single crystals of CBF often contain inclusions of LiF. That is why an optimization of the flux has to be done. We have studied the influence of LiF-B₂O₃, LiF-NaF, LiF-CaF₂, CaF₂, NaF on thermal properties and growth of CBF.  The crystallization temperature of CBF for the ratio 20 CBF / 80 LiF (% mol) is around 1000 °C. We studied the system 20 CBF - (80 - x) LiF - x B₂O_3. The addition of B₂O₃ to LiF allows to reduce the crystallization temperature of CBF to 950 °C for the composition 20 CBF - 75 LiF - 5 B₂O₃. This is interesting concerning the volatility of the melt. With NaF as a flux, CBF crystallize at 1000 °C too but X-ray diffraction analysis reveals Ca₃B₂O₆ phase. The ratio CBF/NaF has to be adjusted to see if NaF can be a suitable flux for CBF. The DTA study of the system 20 CBF – (80-x) LiF – x CaF2 (% mol) is represented on Figure 2.      

Fig. 2: differential thermal analysis cooling curves of the system 20 CBF - (80-x) LiF - x CaF₂

The use of CaF₂ is interesting because the thermal properties of the system are not changed while reducing the quantity of LiF. The other advantage is that CaF₂ is one of the reagent in the synthesis of CBF so it does not disturb the system by introducing foreign elements. Spontaneous crystallization and growth of single crystals using flux mentioned above will be presented at the conference.

[1]G. Chen et al, Journal of Crystal Growth, 292, 449–453 (2006).

[2]Xu et al, Crystal Growth & Design, 9, 2235-2239 (2009)

[3]Xu et al, Optics Express, 16, (2008)

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