The Development of On-Chip THz Time-Domain Spectroscopy

Since the development of efficient THz sources, THz-TDS has been proved to be a promising tool to probe directly the intermolecular modes, rotational motion and intermolecular vibrations of molecules in a variety of chemicals capable of extracting useful spectroscopic information. In this thesis, an on-chip spectroscopy system based on coplanar waveguide (CPW) technology has been designed, optimised and tested, in order to probe spectral features of overlaid polycrystalline materials. As proof of principle, this system was used to recover the THz spectra of α-lactose monohydrate, observing spectral features at ~ 0.53 THz and ~ 1.37 THz. A significant frequency shift in the 1.37 THz feature was observed when the on-chip spectroscopy measurements were performed over a variable temperature range of (~ 6-293 K). Spectral features obtained from the on-chip system were also compared to those obtained from a free-space THz-TDS system to highlight the benefits of using an on-chip system over free space THz-TDS. A theoretical model developed using Ansoft HFSS tool was then used to optimise device design parameters in the second generation of CPW devices. In doing so, the bandwidth of the system was enhanced from ~ 0.42 to 1.6 THz and a much higher frequency resolution of (~ 2 GHz) was obtained compared to that of the first-generation CPW devices (~ 55 GHz) with the modified device design of second generation devices. Branching waveguide systems (THz Y-splitter and coupler) were also simulated, fabricated and measured in order to investigate THz pulse splitting in branching waveguides. These systems allow the measurement of both a sampled and reference pulse. During these measurements, the splitting of the THz pulses propagating in an on-chip THz system was also demonstrated for the first time.