Thomas Keating can manufacture TPX (poly 4 methyl pentene-1) for THz components to order. We have purchased the business and manufacturing equipment of Yarsley, who were our suppliers until they decided to stop manufacturing the material. TPX is a useful THz material because it has the lowest refractive index of common substances at 1.46: It is low loss, though not quite as low as HDPE, say. It has the distinct advantage that is transmitting in the visible, allowing alignment of QO systems with visible light (say a HeNe Laser).
(Taken from "THE OPTICAL CONSTANTS OF SOME COMMON LOW LOSS POLYMERS BETWEEN 4 AND 40 CM-1", Jim Birch et al, DES 61, Feb 1981, National Physical Laboratory, Teddington UK.)