Growth of transparent SrB₄O₇ crystal fiber by the μ-PD method

Abstract

We grew transparent SrB4O7 (SBO) crystal fibers by the micro-pulling-down (μ-PD) method at pulling rates (VP) of 0.01-0.40 mm/min. We observed the SBO crystals by optical microscopy. The SBO crystal fiber at VP of 0.40 mm/min was cloudy due to the incorporation of voids. In contrast, transparent single-crystal fibers were obtained at VP of less than 0.02 mm/min. This is the first time that transparent SBO crystal fibers have been grown with a length of several centimeters. These results demonstrate that it is crucial to grow SBO crystals at a low VP to obtain transparent crystal fibers. In addition, we considered the generation mechanism of voids in the SBO crystals by using the relation between the growth rate of SBO crystals and the diffusion rate of voids. Among these noble borate crystals, SrB4O7 (SBO) has attracted much attention as a nonlinear crystal in the DUV region because of its superior transparency in the vacuum UV region, high radiation damage threshold, and high nonlinearity [4][5][6]. This crystal belongs to the Pnm21 space group and has unit cell parameters a = 4.4255 Å, b = 10.709 Å, C = 4.231 Å, and Z = 2 [7]. Crystal growth of SBO has been achieved by the Czochralski (CZ) method [8], whereas Fan et al. [9] reported crystal growth by the Kyropoulos method. The nonlinear properties in the DUV region include the wavelength conversion of femtosecond second-harmonic generation pulses down to 125 nm using an SBO crystal [10].One of most serious and common problems in borate crystal growth is macroscopic opaque defects, which make the crystal cloudy [11]. Cloudiness in SBO borate crystals grown by the CZ method has also been reported and it was revealed that reducing the growth rate produced transparent SBO crystals without cloudiness [12].We have examined growth of SBO crystal fibers by the micro-pulling-down (μ-PD) method because crystal fibers can be easily used as nonlinear devices for lasers. In this paper, we investigated growth conditions to suppress the appearance of cloudiness during growth, and transparent SBO crystal fibers were grown by the μ-PD method. The generation mechanism of cloudiness in crystals is also discussed.

Effects of the Application of Electric Fields on the Growth of SrB4O7 Crystals by the Micro-Pulling-Down Method

“…The crystal fibers were the same as those used in our previous study and are transparent with smooth surfaces (Figure 2A). 26 Since the Mohs hardness of SBO is 9, the preparation of test pieces was difficult. Hence, some small scratches remain on the surfaces of the crystals (Figure 2B).…”

“…Figure shows the hot zone of the μ-PD furnace. Electrodes were set in the μ-PD furnace used in our previous studies (Figure A). − A platinum crucible with a rectangular nozzle (5 mm × 1 mm × 2 mm (height)), which has a 0.4 mm cylindrical hole at the center of the base, was used (Figure B,C). To keep the temperature of the melt constant, the platinum crucible was surrounded by a refractory.…”

“…The stoichiometric composition was selected as the growth condition. These parameters were the optimized values in our previous studies to obtain high-quality SBO crystals. − During the growth, various voltages E in the range of 0–2000 V/cm were applied to growth fronts of seed crystals through Pt electrodes. Since the position of the Pt wire, which fixes the seed crystals, moves downward during the growth, the efficiency of the electric fields would decrease as the seed crystals are pulled down.…”

“…The temperature gradient range of the μ-PD method is 1–2 units larger than that of a general melt growth method such as the CZ method, since the hot zone is small. − When they are grown under a high-temperature gradient, SBO crystals with a homogeneous composition are made possible. We have in fact optimized the growth conditions and succeeded in obtaining transparent SBO single crystals. − The growth by the μ-PD method could be a suitable method for the uniform application of an electric field as well. We anticipate that, since the motion of molecules in a melt is faster than that in a solid, the alternating application of an electric field during the growth can control the orientation of the domains.…”

Growth of SrB4O7 Crystal Fibers along the c-Axis by Micro-Pulling-Down Method

“…The µ-PD furnace used in this study was the same as in our previous studies [23][24][25][26]. Figure 1A-C show a hot zone of the µ-PD furnace.…”