In the last years a number of investigations devoted to the creation of a regular polar twin structure in nonferroelectric crystals have appeared [1, 2]. The abilities of the structures for obtaining deep UV region optical radiation by the second harmonic generation (SHG) with quasi-phase-matching (QPM) in quartz and Li₂B₄O₇ have been shown. Orthorhombic α-SrB₄O₇ crystals with a short fundamental edge close to 120 nm, high damage resistivity and chemical stability, as well as sufficiently high optical nonlinearity [3, 4, 5] have been shown to be attractive candidates for the same role [6,7].

The arrangement of a polar twin structure in Czochralski grown α-SrB₄O₇ crystals has been investigated by the preparation of several cross-sections and subsequent etching in a nitric acid solution.

The crystals have been growing mainly with the seeds oriented in [100] direction (space group being Pmn2₁). The formation of the twin structure has been observed to be located exceptionally on one side away from the seed in the direction of the polar axis [001]. This direction is conditionally named “minus” direction because the plane (001) on this side is robust for etching unlike the analogous plane from the other side which has the etching rate which is two orders of magnitude higher. It is established that a complicated structure of microtwins appeared in the growth pyramids (101) growing in the “minus direction” (see Fig 1).

The microtwins appear as uneven, wavelike sheets parallel to the plane (010) (see Fig 2). The observations of several crystals show that their average extensions along the axes [100] and [001] are up to 20 μm and 200 μm correspondingly, while their extension along the axis [010] is quite uniform and comprises about 1 μm.

It is also seen that if a microtwin happens to run across the boundary between the growth pyramids (101) and (100), its subsequent shape drastically alters. From its intersection point, this twin expands infinitely into the growth pyramids (100) and (101) along the axes [101] and [100] respectively, forming a plain layer (henceforth “plain twin”) with a constant extension along the axis [010]. Such multiple plain twins compose a laminated structure with twin boundaries parallel to the plane (010). As opposed to the original microtwins, the plain twins can extend along the axis [010] from sub-micron values to several hundreds of microns. The dimensions, planarity and thickness stability of each individual plain twin are the features favorable for organizing QPM conditions for NLO doubling of high aperture beams.

Fig. 1. The arrangement of a polar twin structure in α-SrB₄O₇ crystal. View direction is [010].

Fig. 2. The “minus” (001) cross-section. Boundary between the growth pyramids (101) with the microtwins and (100) with a few plain twins.

Acknowledgements: This study is supported by PSB RAS Project 2.5.2, by Grant of the President of the Russian Federation for the support of leading scientific schools SS-4828.2012.2 and by RFBR Grant 11-02-00596-а.

References

1. S. Kurimura, M. Harada, K. Muramatsu, M. Ueda, M. Adachi, T. Yamada, and T. Ueno. Quartz revisits nonlinear optics: twinned crystal for quasi-phase matching // Optical Materials Express. – 2011. - V. 1, No. 7. – pp. 1367-1375.

2. Kensaku Maeda, Satoshi Uda, Kozo Fujiwara, Jun Nozawa, Haruhiko Koizumi, Shun-ichi Sato, Yuichi Kozawa, and Takahiro Nakamura. Fabrication of quasi-phase-matching structure during paraelectric borate crystal growth // Applied Physics Express – 2013. - No.6. - pp. 015501-1 - 015501-3.

3. Oseledchik Yu.S., Prosvirnin A.L., Pisarevskiy A.I., et al. New nonlinear optic crystals: strontium and lead tetraborates // Optical Materials. – 1995. – V. 4. – pp. 669-674.

4. Petrov V., Noack F., Shen D., Pan F., Shen G., Wang X., Komatsu R., Alex V.. Application of the nonlinear crystal SrB₄O₇ for ultrafast diagnostics converting to wavelengths as short as 125 nm // Optical Letters. – 2003. –V. 29, No. 4. – pp. 373-375

5. Zaitsev A.I., Aleksandrovskii A.S., Zamkov A.V., Sysoev A.M.. Nonlinear optical, piezoelectric, and acoustic properties of SrB₄O₇ // Inorganic Materials. – 2006. – V. 42, No. 12. – pp. 1360-1362

6. Aleksandrovsky A.S., Vyunishev A.M, Zaitsev A.I., and Slabko V. V. Random quasi-phase-matched conversion of broadband radiation in a nonlinear photonic crystal // Phys. Rev. A – 2010. – V. 82. – p. 055806.

7. Aleksandrovsky A.S., Vyunishev A.M., Zaitsev A.I. Applications of random nonlinear photonic crystals based on strontium tetraborate // Crystals. – 2012. - V.2., No.4. – pp.1393 - 1409.

• Legal notice:
  

  Copyright (c) Pielaszek Research, all rights reserved.
  The above materials, including auxiliary resources, are subject to Publisher's copyright and the Author(s) intellectual rights. Without limiting Author(s) rights under respective Copyright Transfer Agreement, no part of the above documents may be reproduced without the express written permission of Pielaszek Research, the Publisher. Express permission from the Author(s) is required to use the above materials for academic purposes, such as lectures or scientific presentations.
  In every case, proper references including Author(s) name(s) and URL of this webpage: https://science24.com/paper/29787 must be provided.

1. Optical birefringence of β-SrB₄O₇ crystals with strontium substitutions
2. Growth, optical and microstructural properties of PbB₄O₇ plate crystals