Wavelength-independent anti-interference coating for the far-infrared

• Published in: Infrared physics 1987-09, Vol.27 (5), p.327-333
• Main Authors: McKnight, S.W., Stewart, K.P., Drew, H.D., Moorjani, K.
• Format: Article
• Language: English
• Subjects: Exact sciences and technology; Fundamental areas of phenomenology (including applications); Lenses, prisms, and mirrors; Optical elements, devices, and systems; Optics; Physics
• ISSN: 0020-0891
• DOI: 10.1016/0020-0891(87)90074-1

The transmission and reflection of radiation at an interface between two dielectrics with a thin conducting film is analysed under conditions appropriate to the far-infrared. When the transmission is from a more dense to a less dense optical medium it is demonstrated that the reflectivity can be made arbitrarily small for a wide range of wavelengths by selecting the appropriate sheet resistance for the conducting film. This property can be exploited to produce a coating that drastically reduces the interference fringes in a flat plane-parallel dielectric substrate or window. The condition depends only on the film resistance which can be monitored precisely during deposition. We have demonstrated this effect by evaporating films of Nichrome on silicon substrates which reduce the interference fringe contrast to less than 1 percent transmittance from 10–100cm −1. Near the anti-interference condition the fringe contrast is shown to be a sensitive probe of the film conductivity. Experiments with a film of amorphous Fe 0.66-B 0.33 suggest a frequency dependence in the FIR conductivity of this material.