Terahertz lights up the nanoscale: Exposing the ultrafast dynamics of Dirac systems using near-field spectroscopy

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As our reliance on technology has increased, so has the demand for faster devices with increased functionality. A perfect example is the mobile phone - starting with the capability to only make calls and send text messages, we now have smartphones that have GPS, step monitors, can search the internet, take photos and videos. Despite this rapid progress, 'smart' devices remain relatively energy-inefficient with high power consumption and low battery life. With today's environmental climate and the increased use of technology, there is a large need for novel '21st-century products' that not only see a step change in device speed but are also energy-efficient. Topological insulators (TIs), in particular, have emerged as potential building blocks for this next-generation of devices. The bulk of the material is insulating, whereas the surface hosts exotic Dirac electrons travelling close to 10,000,000 m/s - 100 times faster than silicon. Due to their topological nature, surface electrons are immune to scattering from non-magnetic impurities and crystal defects. They therefore behave as if travelling on a tramline: faster, with less resistance and less heat production than conventional materials, making them more energy-efficient. Electrons can also only travel in one direction, which is set by their inherent angular momentum or 'spin'. This property is particularly useful for information processing, quantum computing and spintronic applications. To exploit these advantageous properties in a device, an in-depth understanding of key parameters, such as electron mobility (speed) and lifetime, is essential. Although significant progress has been made to probe the elusive properties of these materials, it has proven difficult to isolate the surface from the bulk. Surface-sensitive techniques are required to examine the surface electrons independently and provide an in-depth understanding of the underlying physical mechanisms governing surface transport in these materials. The terahertz (THz) frequency range - falling in between microwave and infrared radiation - provides the perfect probe for investigating Dirac materials. It is capable of penetrating through several opaque materials, such as plastics, paper and textiles and is currently used in airport body scanners. Yet more excitingly, it can also measure how conductive a material is in a non-contact, non-destructive manner. Far-field THz probes have already been used to examine TI and have revealed that electrons can relax from the bulk to the surface, leading to a reduction in impurity scattering. However, these THz probes have all been limited in spatial resolution. The diffraction limit of light restricts THz radiation to a spot size of 150 microns, so they can only measure an effective conductivity due to both the bulk and the surface. This project aims to push the spatial resolution of THz probes down to nanometre-length scales. By coupling THz radiation to an atomic-force microscope tip, the THz probe can be confined to a spot size only limited by the radius curvature of the tip, providing <30nm spatial resolution. The THz radiation scattered back from the tip and sample contains all the local information about the material conductivity. By oscillating the tip and change the tapping amplitude, the penetration depth of the THz probe can be altered to provide surface-sensitivity. A large tapping amplitude probes the bulk of the material, where a small tapping amplitude probes only the surface. This technique will be utilised on TI thin films and nanostructures to perform differential depth-profiling of the local electron mobility, lifetime and conductivity. This will allow the surface behaviour to be isolated from the bulk and examined directly for the first time. This information will open up a pathway for harnessing the advantageous properties of these Dirac materials to develop novel '21st century products'.