Growth and characterization of Ca5(BO3)3F fiber crystals, a new nonlinear optical material for UV light generation

doi:10.1016/j.optmat.2011.04.012

Optical Materials

Volume 33, Issue 11, September 2011, Pages 1621-1625

https://doi.org/10.1016/j.optmat.2011.04.012 Get rights and content

Abstract

For the first time, Ca₅(BO₃)₃F (CBF) fiber crystals were grown by the micro-pulling down technique from LiF-based flux. Regular and transparent fibers were obtained with good optical quality. Characterization by Raman spectroscopy and EDX microprobe showed fibers compositionally homogeneous. The specific heat capacity of the fibers was measured indicating a laser damage threshold slightly lower, at room temperature, than for crystals belonging to the related rare earth calcium oxoborate family.

Highlights

► CBF fiber crystals are grown for the first time by the micro-pulling down technique. ► Influence of the couple growth rate/capillary diameter on the crystal quality. ► How to grow regular and transparent fibers. ► First specific heat capacity measurement.

Introduction

Compact all solid-state lasers emitting in the ultraviolet range are very desirable for numerous applications such as materials processing, photolithography, micromachining, detection of gaseous pollutants, biology or medicine because excimer lasers are somewhat cumbersome with a high operational cost. However, no direct UV laser effect can be produced in any crystal. Therefore, the only way to achieve it is by frequency conversion (doubling and/or frequency summing).

Because of a broad range of transparency, high damage threshold and moderate birefringence, crystals of the borate family are interesting nonlinear optical materials allowing the generation of a UV laser light at 355 or 266 nm either by summing an IR wavelength at 1064 nm and a doubled frequency at 532 nm (third harmonic generation) or by doubling twice a 1064 nm wavelength (fourth harmonic generation). Among the available crystals, one can cite: β-BaB₂O₄ (BBO), LiB₃O₅ (LBO), CsB₃O₅ (CBO) and CsLiB₆O₁₀ (CLBO) [1], [2], [3], [4]. Each crystal has advantages, but also drawbacks. One of them is the hygroscopy and some deliquescence on the contrary to Ca₅(BO₃)₃F (CBF) which is also chemically stable. CBF derives from the RECa₄O(BO₃)₃ (RE: rare earth metal) crystals, that are promising nonlinear optical materials [5], with a similar structure [6]. In CBF, RE³⁺ is replaced by Ca²⁺ and O²⁻ is substituted by F⁻ for electro-neutrality reasons. Because CBF is not congruently melting, bulk single crystals cannot be grown from the stoichiometric melt by the Czochrlaski technique. The growth of CBF bulk crystals requires top seeded solution growth or flux method using LiF as the flux [7], [8], [9]. The crystals obtained by these methods showed promising optical properties for frequency conversion [7], [8], [10]. Further, the growth of good quality single-crystal fibers could be very advantageous for the miniaturization and integration of devices. Indeed, crystal fibers offer several advantages towards bulk crystals: shape favorable to light propagation, dimensions avoiding more or less tedious cutting and polishing steps, growth speed, better crystal quality [11].

Thus, in this paper, we report, for the first time, on the growth and characterization of CBF fiber crystals by the micro-pulling down technique [12].

Section snippets

Experimental methods

The CBF powders were prepared according to the procedure previously described [7].

Crystal growth experiments were performed by the micro-pulling down technique using the resistive heating apparatus recently installed in our laboratory. Typical dimensions of the used Pt crucibles were: 10 × 5 × 3 mm³. The nozzle was 2 mm length and from 0.7 to 1 mm internal diameter (external diameter: 0.9–1.1 mm). The seed was cut from a CBF crystal grown by the flux method from the CBF–LiF system. It was oriented

Crystal growth and characterization

Very few papers report on the use of the flux method to grow fiber crystals by micro-pulling down technique [13], [14], [15], [16]. Nevertheless, in a first attempt, we chose to grow fibers by micro-pulling down using the flux technique. The flux had the composition: 70% CBF, 22.5% LiF and 7.5% B₂O₃ to prevent excessive LiF evaporation [17]. Several fibers were successfully produced with a capillary of 0.7 mm internal diameter. The fibers had lengths between 10 and 40 mm and a nearly circular

Conclusions

For the first time, CBF fiber crystals have been grown by the micro-pulling down technique from a LiF-based flux. The obtained fibers appear to be compositionally homogeneous and, under well defined conditions for the capillary size, they exhibit good transparency and optical quality. Further work requires now to optimize growth conditions (capillary size, pulling rate, etc.) and pulling fibers with the good crystallographic orientation to fulfill phase matching conditions (φ angle with X axis

Acknowledgements

The authors are indebted to the Department of Advanced Materials and Structures (DAMS) of the Public Research Center Henri Tudor in Esch on Alzette (Luxembourg) for having performed SEM and EDS microprobe measurements and to Dr. Christelle Vagner of Laboratoire Chimie Provence, UMR CNRS 6264, Aix-Marseille I, II and III University in Marseille (France) for specific heat measurements.

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Growth and characterization of Ca5(BO3)3F fiber crystals, a new nonlinear optical material for UV light generation

Abstract

Highlights

Introduction

Section snippets

Experimental methods

Crystal growth and characterization

Conclusions

Acknowledgements

J. Cryst. Growth

J. Cryst. Growth

J. Cryst. Growth

J. Cryst. Growth

J. Cryst. Growth

J. Cryst. Growth

J. Cryst. Growth

J. Alloys Compd.

Opt. Mater.

J. Opt. Soc. Am. B

Growth and characterization of Ca₅(BO₃)₃F fiber crystals, a new nonlinear optical material for UV light generation