Cryogenic infrared multilayer filters: the origin of low-temperature shift of the pass-band edge

The design, manufacture and optimum performance of a cut-on IR for the 8-14 micrometers is discussed. The filter is composed of PbTe/ZnS symmetrical stacks deposited on tow sides of a ZnSe(ZnS) substrate. The refractive index of PbTe was found to 5.6 and that of ZnS 2.15. In order to obtain the required pass/stop band profile the principal filter was composed of ten basic periods. The transparent in the mean IR-range multilayer coatings are stable of thermal cycling from 300 up to 8 K with steady optical characteristics in this temperature range. The average transmittance of filters was 75 percent in the desired spectral regions and was constant in the temperature interval 300-8 K. The effect of cooling on the position of the pass band edge of the interference multilayer thin-film coating is experimentally revealed. A model is suggested which explains the origin of low- temperature shift of the pass-band edge within the scope of variation in the optical thickness of a layer with the high refractive index. The result of the carried out numerical analysis are in well agreement with the experimental data and may be used for cryocorrection of the interference IR multilayer thin-film.