Temperature dependence of the index of refraction of TOPAS cyclic olefin copolymer in the terahertz range

Past measurements on the cyclic olefin copolymer TOPAS reveal a spectrally-flat refractive index within the terahertz range, giving the material exciting potential applications in optical apparatuses. Here we use THz spectroscopy to expand the understood frequency range, and as a probe to extract information regarding the temperature dependence of the refractive index, in which we find that the index of refraction increases with decreasing temperature. In hopes to understand this counterintuitive behavior, we report the temperature dependence of the reflectivity due to vibrational modes within the C-H band via FTIR. Doing so, we find a similarly reversed behavior in which the excitation energy of some of the vibrational modes seems to increase with temperature, providing a puzzling departure from expected behaviors.