Polarization Imaging for crystallographic orientation of large mercurous halide crystals

Polarization Imaging is a useful technique to optically determine the orientation of optic axis of birefringent crystals by examining the interference patterns produced in convergent polarized light by the crystal. We developed a polariscope, also known as a conoscope to characterize large mercurous bromide (Hg2Br2) crystals. Such crystals have large birefringence and they are transparent from 0.35 to 30 micron. They are very useful in designing Acousto-Optic Tunable Filters (AOTFs) for multi-spectral and hyperspectral imaging applications, especially in the strategic Long Wavelength Infrared (LWIR) atmospheric window covering 8 to 12 mm. Fabrication of an efficient LWIR AOTF in Hg2Br2 crystal requires knowledge of precise crystallographic orientation of the crystal. We have grown 2-inch in diameter and 2-inch long Hg2Br2 crystals, by vapor phase technique. The Laue x-ray diffraction technique is difficult in the case of this material, especially for large as grown crystals, due to absorption and x-ray induced fluorescence. Conoscopy is a good technique to verify optic and other axes directions and is complimentary to the x-ray diffraction method used for precise crystallographic orientation. We are reporting here, use of a combination of conoscopy, x-ray diffraction, and the birefringent property of Hg2Br2 to identify the optic and other axes directions in such crystals.