| Terahertz (THz) waves lie between the microwave and infrared region of the electromagnetic spectrum. Until recently this is one of the least explored areas of the electromagnetic spectrum. Now it is gaining attention within the scientific research community due to its important applications in areas such as security scanning, biomedical imaging, material characterization, ultra-fast electronics and communication. This in turn drives an increasing need for improved THz optical components such as filters, antireflection coatings, and polarization modulators.;Vanadium dioxide (VO2) is an ideal insulator-metal phase change material due to the low transition temperature (∼ 340 K) and ultrafast transition speed (∼ fs), and the phase transition can be stimulated by thermal, optical, and electrical methods. Therefore, VO2 has been intensively studied for active and ultrafast devices in infrared and THz frequency range. In the thesis, I study the phase transition properties of VO2 thin film deposited on c-sapphire substrates by temperature dependent transmitted THz-TDS, and then investigate the potential applications of VO 2 wire-grid gratings for active THz polarization modulators.;Poly (3, 4-ethylenedioxythiophene)/Poly (4-styrenesulfonate) (PEDOT/PSS) is a conductive polymer that is commonly used in organic optoelectronics. Its optical and electrical properties can be significantly changed by doping with organic solvents. In this thesis, transmitted THz time-domain spectroscopy (THz-TDS), as a non-contact method, combined with Drude-Smith model is applied to study the conductivity enhancement mechanisms of dimethylsulfoxide (DMSO) or ethylene glycol (EG) doped PEDOT/PSS. We find that the conductivity of 6% DMSO doped PEDOT/PSS is greatly increased comparing to the pristine PEDOT/PSS, and it is nearly real and frequency independent at 0.3-2.5 THz range. Based on this special material property of 6% DMSO doped PEDOT/PSS and equivalent transmission line circuit theory, I fabricate variable step THz neutral density filters by controlling the thickness of 6% DMSO doped PEDOT/PSS thin film. In addition, I investigate the applications of 6% DMSO doped PEDOT/PSS thin films in THz antireflection coatings for quartz and silicon substrates based on impedance matching effects. The internal reflections at the substrate-air interface can be efficiently suppressed by ∼ 39 nm and ∼ 101 nm thick 6% DMSO doped PEDOT/PSS films on quartz and silicon substrate, respectively. |
