On-chip terahertz characterisation of liquids

Licentiate thesis, 2020

Spectroscopy at terahertz frequencies can be used in a wide range of applications including radio-astronomy, pharmaceutical manufacturing control, and the study of processes in molecular biology. Biomolecular samples should preferably be studied in their native environment, water, however, water poses extreme attenuation for THz-frequency waves, deteriorating or even impeding analysis using these waves. The most common THz spectroscopy method, time-domain spectroscopy, can measure water samples using free-space measurements, lacks sensitivity when trying to measure on a chip environment. To exploit the advantages that chip measurements offer, such as integration and cost, this thesis works on developing on-chip THz spectroscopy of aqueous samples using a frequency-domain approach, with vector network analysers. Vector network analysers exhibit a higher dynamic range than time-domain spectroscopy systems, making them a promising alternative for sensitive THz measurements. For maximising the sensitivity of the measurements, the losses must be minimised. One important source of losses at THz frequencies are conductor and radiation loss. In this thesis, two planar waveguides were designed, coplanar waveguide and planar Goubau line, minimising their losses at THz frequencies by avoiding the coupling to other parasitic modes, obtaining attenuation constants as low as 0.85 Np/mm for coplanar waveguide and 0.33 Np/mm for planar Goubau line. Additionally, planar Goubau line calibration structures were developed for setting the measurement plane along this planar waveguide. Finally, coplanar waveguides were integrated with microfluidic channels to perform spectroscopy measurements of water samples, showing good performances as THz sensors of high-loss liquids.

This thesis is a first step towards a sensitive and miniaturised system for measuring the electrical properties of high-loss liquids, which could shed light on the fundamental biomolecular processes in the picosecond time-scale.