Strengthening CsI crystals for optical applications

Cesium iodide (CsI) is a very useful optical material combining a low refractive index with broad spectral transparency. Unfortunately, the mechanical weakness of CsI crystals severely limits practical applicability of the material. Nearly all uses require mechanical strengthening. The strengthening treatment cannot, of course, degrade desirable single crystal optical properties. Previous work at Honeywell has involved strengthening of alkali halides having the rocksalt structure (KCl, NaCl, etc.). Processed materials have been strengthened by as much as an order of magnitude without measurable optical degradation. Direct application of these processing methods to the dissimilar structure of CsI is impossible due to the limited number of slip systems active in the material. This investigation was, therefore, concerned with development of CsI processing methods that would effectively strengthen the material without introducing cracks or other optically degrading defects. This paper describes successful uniaxial and plane strain processing methods. Detailed process parameters and strengthened CsI material properties are included