No abstract

SrB 4 O 7 (SBO) has recently received much attention as a wavelength-conversion material which can operate into the deepest ultraviolet wavelengths (125 nm). The utilization of SBO for semiconductor lithography and laser ablation requires fully transparent crystals. Here, we show the growth of SBO crystal fibers with near-stoichiometric composition by the μ-pulling-down method. SrB 2x O 3x+1 ceramics sintered with various values of x were evaluated by X-ray diffraction (XRD). The melting point of the SBO ceramics was measured by differential scanning calorimetry (DSC) as 1017 °C, and the heat of fusion was 63.3 J/g. Furthermore, we observed SBO crystal fibers grown with various x values by optical microscopy. As a result, even when the deviation from stoichiometric composition was ± −0.1 mol % SrO (x = 2 ± 0.004), SrB 2 O 4 /SrB 6 O 10 appeared. Also, DSC curves of SBO ceramics exhibited only one melting peak. These results demonstrate that SBO shows no solid-solution width and that the stoichiometric composition is equal to the congruent composition. Furthermore, we succeeded in obtaining transparent SBO crystal fibers by growth with a stoichiometric composition. In contrast, since growth ridges including SrB 2 O 4 /SrB 6 O 10 were formed on surfaces of the SBO single crystals, SBO crystal fibers grown with nonstoichiometric composition were opaque. We could explain the formation of growth ridges via compositional supercooling on rims.

SrB 4 O 7 (SBO) has received much attention as a solid-state ultraviolet laser crystal for a wide variety of applications such as micromachining and spectroscopy. Here we demonstrate the effects of electric fields E on the growth of SBO crystals along the b axis by the μ-pulling-down method to fabricate SBO with periodic twins, which increases the conversion light intensity. During the growth, the direction of electric fields was alternated for 30 min periods. The crystal fibers were evaluated by polarized optical microscopy, X-ray diffraction, and transmission electron microscopy. Without an external voltage, single SBO crystals grew. In contrast, at E ≥ 400 V/cm needles of SrB 2 O 4 appeared inside SBO fibers. At E ≥ 1000 V/cm, new domains of SBO grew from the seed crystals, and interestingly, growth twins were formed perpendicular to growth fronts. Thus, we found that the growth of SBO by the μ-PD method via alternating the application of opposing electric fields exhibits two mechanisms. In contrast, when the electrodes in melt and growth fronts switched to + and −, respectively, growth fronts changed from concave into convex shapes, irrespective of E. We could explain the relation between the two mechanisms and the inversion of surface shapes, using the convection.

SrB 4 O 7 (SBO) shows superior transparency in the vacuum ultraviolet (UV) region, a high radiation damage threshold, and high nonlinearity, and SBO crystals have therefore attracted great interest as materials for low-cost, high-power UV light sources. To use SBO crystals as laser sources, it is essential to grow transparent SBO crystals. Here we show the effects of temperature gradient G on the growth of SBO crystals by the micro-pulling-down method. At low G, SBO crystal fibers were obtained. The density of striations and the size of voids also decreased with decreasing G. These results demonstrate that a low G is essential for growing completely transparent SBO crystal fibers. In addition, we considered the effects of G on the oscillatory growth of SBO crystals in relation to latent heat removal.

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.