Single crystal growth and effect of cleavage micro-striations on the crystallinity and optical properties of mercurous halide single crystals

Abstract

Mercurous halide crystals, which crystallize in a one-dimensional chain-like structure, have attracted great attention as polarizing and acousto-optic devices in the long wave infrared range for their large birefringence, a wide transparent range, and a large acousto-optic figure of merit. However, it is difficult to obtain large-size single crystals with high quality due to the severe layered cleavage nature of crystals. Additionally, crystal processing still remains a challenge owing to the soft brittleness and cleavage nature. To date, high-quality, large-size mercury halide crystals have been successfully obtained by our group. Herein, micro-striations in the (001) surface of the mercury halide crystals caused by the cleavage along the 〈110〉 orientation in mechanical polishing were reported for the first time. The crystal micro-striations act as diffraction gratings, producing “diffraction spots” when illuminated by light. These cleavage striations seriously deteriorate the crystal crystallinity, enlarging the full width at half maximum (FWHM) of the X-ray rocking curves for the polished surfaces. But the striations on the crystal surface can be effectively attenuated by optimizing the polishing process. The discovery and effective suppression of micro-striations in mercurous halide crystals lay a solid foundation for the fabrication of polarizing and acousto-optic devices with high performance in the long-waved infrared range.