GROWTH OF β BaB2O4 SINGLE GRYSTALS AND SHG EFFICIENCY MEASUREMENTS

We report here the growth and characterization of non linear single crystals p BaB204 (BBO) which is an interesting material for second harmonic generation. Single crystals were grown from NqO-B20, solutions using the top seeded solution growth method. The experimental set up included a Nd:YAG, Q switched, focalised with a cylindrical lens on the samples. Efficiency up to 40 % were obtained using 6 mm long crystal. 1. Introduction p BaB204 (BBO) is an interesting material for second harmonic generation (SHG) (1). It exhibits high damage threshold and a wide transparency range (2). BaB,O, is known to exist in two phases : a high temperature a phase and a low temperature 13 phase with a transition temperature of about 925" C (3). a BaB20, melts congruently a t 1096O C but the growth of 6 phase requires a growth temperature below 925O C in a solvent. The real success in growing f3 BaB20, came with discovery of a suitable solvent. The workers of Fqjian Institute have studied all of the pratical solvent for this material : BaCI,, BaF,, Li,O, NaBO, and Na20 (4). They reported that N%O yields the best results in terms of crystal size and defect density. 2.1 Preparation of charges Several melt compositions were investigated in the system BBO-Na,O-B20,. In order to obtain the desired low temperature p phase, growth must be carried out from solution below 925" C. With Na,O-B20, as solvent, we find that p BaB20, can be crystallized a t about 900" C from solutions with typical composition of 80 % mole BBO 16 % mole N%O 4 % mole B,O,. Charges were weighed out and fractionnal amounts were placed in pure platinium crucible to be melted in a synthesis furnace a t 1025O C. 2.2 Top seeded solution growth apparatus High thermal gradient furnace was used in order to initiate a good growth from the seeds and to minimize the defects density in the crystals. At the melt surface the axial temperature gradient was about 50" C/cm. BBO seeds were held rigidly using a platinum holder. Before seeding the melt was heated a t about 1000" C during several hours. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19917202 JOURNAL DE PHYSIQUE 1V 2.3 Growth conditions Crystal growth experiments were performed with the use of seeds oriented along the y-axis. BBO seeds having a rotation rate of 10-25 rpm were introduced carefully in the melt in order to avoid nucleation, a t about 900' C to minimize solution viscosities. Typically we used a cooling rate of 0.025" C/hour and a pulling rate of 1 mm/day.