Novel Techniques and Instrumentation for Material Characterization from Microwave to THz Frequencies

Abstract

This dissertation focuses on the investigation of microwave and terahertz (THz) nanomaterials, devices, components and systems. The work done in this dissertation is categorized into three main parts: theoretical modeling and experimental characterization of THz photoconductive antennas (PCAs) in the far-field time-domain spectroscopy (TDS) configuration, demonstration and performance study of an emitter array based THz near-field imaging system, and the intrinsic property characterization of carbon-based nanomaterials. First, the important applications of THz technology and the critical elements involved in the generation of THz pulsed signal, including femtosecond (fs) laser, photoconductive material and PCA structure itself, are introduced. The theoretical modeling approaches of THz PCA used in the literature are compared and the 3D full-wave model is chosen and implemented using the finite-difference time-domain (FDTD) algorithm. Two exemplary PCAs, stripline and dipole, are simulated using this full-wave model and compared. Second, the effects of the photoconductive materials (with different carrier lifetime and mobility) and the antenna structures on the far-field THz radiation properties are systematically examined in terms of pump laser power dependence and DC bias voltage dependence via an in-house built THz-TDS system. The polarization and cancellation effects are additionally investigated experimentally to better understand the PCA radiation mechanism. The THz bandwidth improvement with regard to the dispersion of the critical optical component is illustrated and the refractive indices of several dielectric materials are coherently characterized afterwards with this new TDS setup. Third, a PCA array based THz near-field imaging system in emission mode is proposed incorporating the Hadamard multiplexing method to achieve system signal-to-noise ratio (SNR) improvement. The system performance is represented in terms of spatial resolution. Simulation estimations based on the HFSS time-domain solver combined with the FDTD algorithm show reasonable agreement with the experiments. Furthermore, intrinsic microwave properties of single-walled carbon nanotube (SWCNT) and graphene are measured with the aid of two different coplanar waveguide (CPW) transmission-line test fixtures, extracted with de-embedding algorithms and expressed by lumped-element equivalent circuit models. In addition, the nonlinearity (power dependence) of graphene material in terms of its second- and third-order harmonics is reported. The third-order harmonic is specially considered to estimate the intermodulation distortion (IMD) towards graphene antenna applications.