The Development of Microfluidic and Plasmonic Devices for Terahertz Frequencies

Abstract

The wealth of opportunities associated with the terahertz (THz) region of the electromagnetic spectrum have only recently, thanks to advances in technology, begun to be fully recognised and exploited. The advent of terahertz time-domain spectroscopy (THz-TDS) has led to a wide spectrum of research, spanning chemical, biological and physical systems. However, the relative immaturity of THz techniques results in a variety of inherent problems which limit the potential applications. With an equality existing between the wavelength of THz radiation, and the length scales associated with modern microfabrication techniques, such technology can be exploited to facilitate in finding solutions to these problems.

This thesis seeks to address one of these problems, namely the strong absorptions associated with liquid water in the THz region. A simple design idea, that if the optical path length through a fluidic sample were reduced, strong signals could be detected after direct transmission, resulted in a micromachined fluidic cell being devised. The design, fabrication and testing of a microfluidic device inherently transparent to THz radiation, and designed for use in a standard THz-TDS arrangement, is presented. A range of samples, including primary alcohol-water mixtures, commercial whiskies and organic materials are analysed, which, when used in conjunction with data extraction algorithms, allows for accurate dielectric information to be yielded.

Further exploitation of micromachining techniques are presented, where a variety of structures, seeking to initiate and utilise a class of surface electromagnetic wave known as surface plasmon polaritons (SPPs), are realised. By flanking a single sub-wavelength aperture with sub-wavelength periodic corrugations, extraordinary optical transmission (EOT) can be observed. This technique allows smaller apertures to be used for THz near-field imaging applications, with a view to increase spatial resolution. The first demonstration of THz near-field imaging using sub-wavelength plasmonic apertures in conjunction with a THz quantum cascade laser source, is presented.

Detailed investigations into EOT for the case of two-dimensional, sub-wavelength aperture arrays are documented. A qualitative time-of-flight model describing the transmission properties of these structures is presented, resulting from systematic investigations into a variety of geometrical effects. This model has allowed sharp resonances to be engineered in the frequency domain. A hybrid device featuring a combination of sub-wavelength periodic apertures and corrugations is also investigated. Such a structure is not known to have been described previously in the literature, either in the optical or THz domains. The device demonstrates unparallelled transmission efficiencies, termed `super' EOT.

Finally, a device combining the microfluidic technology with the highly resonant SPP structures is presented. This device seeks to exploit the innate dependence of SPPs to a metal-dielectric interface, for use as a sensor. By introducing a range of fluids into the device, the change in the metal-dielectric interface induced a change in the frequency response of the resonant structure. The magnitude of the observed frequency shift can be related back to the dielectric properties of the fluid. This result displays how microfabrication techniques can be successfully exploited to create devices for THz applications, seeking to provide solutions to the inherent problems associated with this part of the electromagnetic spectrum.