MILLIMETER-SUBMILLIMETER WAVELENGTH FILTER SYSTEM

We describe the design, fabrication, measurement, and performance of a set of cryogenic millimeter-submillimeter wavelength filters used in a balloonborne bolometric radiometer. The set contains single resonant mesh grids used as dichroic beam splitters, resonant meshes in a double quarter-wave configuration, a commercial inductive grid filter, and high-frequency blocking filters. The resultant system has passbands at lambda = 1.73, 1.05, 0.61, 0.44 mm with delta lambda/lambda = 0.23, 0.23, 0.12, 0.06. Limits on high-frequency leakage are deduced from laboratory measurements and from the analysis of flight data. The filter set response to three different sources of radiation is presented to show the method and limitations of our characterization. The key element of the filter system is a resonant periodic array of cross-shaped holes etched in thin aluminum. We give an empirical scaling law for the resonant wavelength as a function of structure parameters for aluminum on 25-mu m-thick Mylar. Plots of the transmittance for normally incident radiation and the transmittance and reflectance for a 45 degrees incident radiation are presented.