Drude Dispersion in the Transmission Line Modeling of Bulk Absorbers at Sub-mm Wave Frequencies: A tool for absorber optimization

Semiconductor (SC)-based bulk absorbers operating in the (sub-) THz range are discussed. The conductivities of the bulk media are described by the Drude model for electron gas where the electron density is controlled. The Drude model predicts the existence of two frequencies of interest: one associated with the scattering time of the electrons and a second associated with the plasma frequency. The dimensions of the absorbers for a specific frequency range can be minimized by tuning the doping levels. Eventually, the maximum ohmic absorption from a bulk material is achieved when the real part of the characteristic impedance of the absorber is matched to the one of the surrounding medium and the imaginary part of the characteristic impedance is high so that the power entering the material is actually transformed in heat. Using a classic transmission line representation, a matching layer is introduced to further increase the absorption capabilities of an SC slab. Measurements using a time-domain spectroscopy (TDS) system show the increased accuracy of the Drude model compared to the quasi-static approximation of the conductivity. The transmission line representation in combination with the Drude model proves to be a simple and accurate tool for integrated antenna front-end design and absorber optimization.