Broadband on-chip terahertz spectroscopy

An enhancement of on-chip terahertz time domain spectroscopy (THz-TDS) systems for the analysis of polycrystalline materials has been made. An in depth review of planar Goubau lines is presented for which there are no analytically defined terms for transmission line parameters such as characteristic impedance and effective permittivity. Using a simulation package, Ansoft HFSS, the transmission line is optimised for spectroscopy applications and the bandwidth enhanced using a variety of methods. The theory for calculating the effective permittivity of the transmission line is derived based on a two dimensional interpretation of electromagnetic field patterns and cross-section transmission line geometry. The resulting formulae have a significant impact on both bandwidth and resonant filter designs.
The excitation of the planar Goubau line’s quasi transverse magnetic mode, which is typically excited using coplanar to planar Goubau line transition, has been modified in favour of a novel all-planar Goubau line on-chip spectroscopy system utilising photoconductive generation and detection methodologies. In doing so, the frequency resolution of the system is heightened enabling a narrow line width system to be resolved.
The planar Goubau lines are fabricated on a quartz substrate with epitaxial transferred low-temperature-grown GaAs based photoconductive switches for both THz generation and detection. The bandwidth of the planar Goubau line is enhanced using a substrate thinning methodology to 2 THz for a 1-mm-long planar Goubau line. Using the enhanced bandwidth, THz-TDS spectroscopy using a planar Goubau line is demonstrated for the first time, where spectra of polycrystalline lactose monohydrate is obtained with a 3.75 GHz frequency-resolution over variable temperature range (4 – 298 K). The THz-TDS spectra are compared with spectra found using alternative THz spectroscopy systems to highlight the improved benefits of using this device.
The theoretical development of a narrow bandstop filter design is presented, with the analytical terms defined. This novel filter enables the dielectric sensing of overlaid materials at a multiple of predefined frequencies to be pushed away from the transmission lines, which would otherwise reduce the bandwidth of the system.
Results presented in this thesis present a strong candidacy for planar Goubau lines to be utilised in a broad range of applications which hold information in the THz regime.