TTG-CVD based ZnS material preparation

Infrared thermal imagers are widely used in security, night vision and infrared temperature measurement with the advantages of target recognition under all-weather conditions. However, the imaging quality is restricted by the quality of ZnS infrared optical lens. The annealed ZnS bulk was fabricated through Three-Temperature zone Gradient Chemical Vapor Deposition (TTG-CVD) furnace. The ZnS bulk with cubic sphalerite structure was characterized by X-ray diffraction. No hexagonal wurtzite structure was detected. It indicates that ZnS bulk possessing optically isotropic property, which can meet the design requirements in lens. The average transmittance of ZnS bulk was measured as 71.6% in the long-wavelength infrared band of 8-12 μm. The refractive index uniformity of ZnS bulk was measured as 1.94 × 10⁻⁵ at 1.06 μm. As encouraged by the above optical parameters, ZnS infrared optical lens was further produced by adopting optically cold-mechanical process and single-point diamond turning techniques. When the spatial frequency of ZnS infrared optical lens was 20 lp/mm, the modulation transfer function (MTF) of the half or 0.707 field of view was close to the diffraction limit. The root mean square value (RMS) of the diffuse speckle caused by aberration in the central field and 0.707 field of view was less than 20 μm in the pixel size. Meanwhile, the system distortion was less than 1% in the infrared imaging system. It shows TTG-CVD based ZnS crystal is promising for infrared applications.