Anomalous dispersion measurement in terahertz frequency region by photomixing

Photomixing and coherent homodyne detection have been used to measure the absorption and dispersion of dielectric media. The signal to noise ratio was sufficient to allow the dispersion and absorption characteristics of a polytetrafluoroethylen (Teflon) sample to be extracted up to a frequency of $1.6\mathrm{THz}$ . Due to the relatively narrow spectral linewidth of terahertz source, estimated to be in order of $5\mathrm{MHz}$ , the anomalous dispersion in the vicinity of a rotational transition of carbonyl sulfide is clearly observed, with a refractive index variation of about ${10}^{-4}$ being measured at frequencies within $40\mathrm{MHz}$ of the transition.