Ronald R. Willey, Willey Optical, Consultants, Melbourne, FL
Southwell recently reported designs for very narrow band filters with narrow reflectance, broad absorptance, but no transmittance (NBR); and other filters with narrow band absorptance, broad reflectance, but no transmittance (NBA). The surprising index versus thickness patterns for these filters is very similar to narrow bandpass (NBP) filters and general reflectors. When single thin layers of noble metals, on the order of 10 nm, are inserted into the otherwise dielectric material designs, they have dramatic effects. The metals with the right properties of index of refraction for this work turn out to be the bulk values of iridium and silver. However, when these films are deposited in very thin layers, the properties are not at all like the bulk values. It is therefore necessary to deposit test samples of the candidate materials in the approximate thicknesses anticipated to be used in the designs. Final designs can then be approached using realistic values for the indices of refraction. It appears that a new specialty area has been opened in the field of optical thin film design.