Optical properties of Zitex in the infrared to submillimeter

The results of measurements of the refractive index and power attenuation coefficient of Zitex at 290, 77, and 4 K in the spectral region from 1 to 1000 mum are presented. Zitex is a porous Teflon sheet with a filling factor of similar to50% and is manufactured in several varieties as a filter paper. Zitex is found to be an effective IR block, with thin (200-mum) sheets transmitting less than 1% in the 1-50-mum range while attenuating less than or similar to10% at wavelengths longer than 200 mum. Some variation in the cutoff wavelength is seen, tending to be a shorter-wavelength cutoff for a smaller pore size. In addition, the thermal conductivity of Zitex at cryogenic temperatures has been measured and is found to be roughly one half that of bulk Teflon. Finally, its dielectric constant has been measured in the submillimeter as n = 1.20, resulting in extremely low dielectric reflection losses. As a result, Zitex is particularly useful as an IR blocking filter in low-noise heterodyne receivers; in the millimeter-wave range (lambda greater than or similar to 850 mum or nu less than or equal to 350 GHz) the attenuation of alpha less than or equal to 0.01 cm(-1) for a 3.5-mm thickness filter of Zitex G125 would raise receiver noise temperatures by <1 K. (C) 2003 Optical Society of America.