Far infrared filters for the Galileo-Jupiter and other missions

Abstract

Extrapolation of PbTe/II-VI multilayer interference-filter technique from 20 to beyond 40 microm is described and PbTe transparency reviewed; improvements below 20 microm are reported. A composite filter cutting on steeply at 40 microm is described that uses absorptive films of ZnS and As(2)S(3), thin quartz, and supplementary multilayer interference. Absorptive filters are described containing the II-VI compounds since these are found transparent at wavelengths shorter and longer than their reststrahl.

Cooled optical filters forQ-band infrared astronomy (15-40 μm)

“…The availability of thin-film deposition layer materials with low stress and transparent optical properties in the Q-band region is limited, particularly where achieving highest antireflective transmission at the longest wavelength is paramount. The deposition of IV-VI lead telluride (PbTe) condenses in a polycrystalline face-centered cubic NaCl-type structure with an exceptionally high refractive index, which has been reported by us on former occasions [21] and provides many useful features. The unusual characteristics of the lead chalcogenides, such as a narrow bandgap with a positive temperature coefficient, high dielectric constant and large carrier mobility make it unique amongst polar semiconductors, and also valued as a thermoelectric material.…”

Performance Estimation of the Mid-Infrared Camera and Spectrometer Aboard SPICA

“…Longer wavelength BPFs require a thicker layer, which is di±cult to fabricate given the limited availability of the materials. Very few¯lters have been fabricated for bandwidths longer than 27 m (Seeley et al, 1981). On the basis of the above-mentioned reports, we assumed BPF transmissions of 80%, 70%, and 30% in the wavelength regions c < 17 m, 17 m c < 26 m and 26 m c , respectively.…”

Coating Materials

“…Absorption can also be influenced by the oxygen partial pressure (Hawkins et al 2000, Seeley et al 1979, 1980, Binet al 1997. High purity 5N materials are recommended with impurity levels of about 1 ppm.…”

“…Only ZnS molecules after recombination stick to the substrate (Goldfinger eta!. PbTe shows the highest refractive index of all evaporation materials (Hawkins et al 2000;Seeley et al 1979Seeley et al , 1980. ZnSe is used mainly for IR applications, e.g.…”