Spectroscopic measurements in the terahertz-frequency-range can offer insight into the picosecond dynamics, molecular conformation, and biological function of chemical systems. Despite the recent emergence of terahertz-frequency time-domain spectroscopy as a tool for the measurement of dry, solid samples, the investigation of liquid analytes is complicated by the strong attenuation of terahertz-frequency signals in aqueous environments. The integration of microfluidic systems with on-chip waveguides offers a potential solution as picosecond pulses confined to a waveguide can interact with nano- or microlitre liquid sample volumes over a distance of several millimetres, with significantly reduced attenuation compared to free-space techniques. Specifically, the single-wire planar Goubau line waveguide has attracted attention in recent years owing to the relatively large extent of the supported evanescent field, enabling sensitive interaction between a propagating electric field and nearby samples. In this work, the first on-chip microfluidic spectrometer, capable of measuring the complex permittivity of liquids in the terahertz-frequency range is introduced.
The fabrication of planar Goubau line devices with integrated photoconductive switches for the generation and detection of terahertz-frequency electric fields is discussed in detail. Given the importance of maximising the signal-to-noise ratio in spectroscopic measurements, an investigation of the signals excited from these switches is conducted, and factors such as the pump-power, generating beam polarisation, and switch geometry are found to have a significant impact on signal generation efficiency and noise. In addition to problematic signal noise, waveguide geometries can introduce artefacts that complicate further analysis. To simplify later modelling of these structures, the sources of unwanted reflections and propagation modes are identified, and prevented by design.
The integration of microfluidic systems with on-chip waveguides presents several interesting challenges. Intimate contact between the waveguide and analyte allows for sensitive measurement of the sample properties, yet the electronic circuitry required to generate and detect a probing terahertz field must be isolated from the risk of a short-circuit presented by the potentially conductive liquid. A device structure is proposed that simultaneously overcomes these design limitations, and comprises a geometry that can be accurately modelled. Given the lack of analytical models with which the planar Goubau line can be described, numerical modelling techniques are used to create an accurate simulation of the device structure. A method is then introduced that allows interpretation of experimental data, such that the complex permittivity of unknown liquid samples can be calculated.
This new technique is used to measure the complex permittivity of a selection of well-studied polar alcohols, and the results are found to compare well to those available in literature. A free-space terahertz spectroscopy system is then used to measure liquid samples that have not been published in order to verify the results of the on-chip spectrometer when used to measure a wider range of liquid samples
