Terahertz, Topology, Technology: Realising the potential of nanoscale Dirac materials using near-field terahertz spectroscopy

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Technology is constantly evolving. Even within our lifetime, devices have become noticeably faster and smaller with increased functionality; yet these 'smart' devices still suffer from high power consumption and poor energy storage. Integrative photonic, electronic and quantum technologies are key to creating the next-generation of devices that are more energy-efficient with unprecedented performance. These '21st century products' will have a huge impact on a range of sectors, including healthcare, wireless communication, defence, security and clean energy. Advanced functional materials, including graphene, 2D materials and III-V nanowires, will form the basis of these new technologies. Dirac materials, in particular, have attracted significant attention as candidates for novel devices, owing to their extraordinary optoelectronic properties. Dirac semi-metals (DSM) form a 3D analogue of graphene. Whereas topological insulators (TI) are insulating in the bulk, yet possess perfectly conducting surface states. For both materials, the surface hosts Dirac electrons that travel close to the speed of light and are immune to backscattering from non-magnetic impurities and defects. Their direction of travel is fixed by their inherent angular momentum or 'spin', so they behave as if on a railway line - travelling with less resistance and heat production. This coupling between an electron's charge and spin renders TIs and DSMs useful for quantum computing and spintronic applications. In particular, these materials have emerged as promising candidates for novel terahertz (THz) devices. THz technologies are poised to impact several sectors, including security, food processing, healthcare and wireless communication. To realise their full potential, an in-depth understanding of key device parameters (e.g. conductivity) in active THz materials is vital. THz time-domain spectroscopy (THz-TDS) has arisen as a powerful ultrasensitive, non-contact probe of electrical conductivity. It has already been used to examine TIs and DSMs and has shown they possess a high effective electron mobility as a result of reduced impurity scattering. However, so far these measurements have been limited in spatial resolution by the diffraction limit of light (150um for 1THz). The measured conductivity averages over any inhomogeneity and is dominated by the bulk response. Local information is therefore lost and it has proven difficult to isolate the surface conductivity from that of the bulk. This research project aims to push THz-TDS down to the nanoscale, extending the spatial resolution to nanometre length scales. It will employ scattering-type near-field optical microscopy (SNOM) with ultrafast optical-pump terahertz-probe (OPTP) spectroscopy (OPTP-SNOM) to provide a non-destructive, surface-sensitive, nanoscale probe of electrical conductivity. This unique tool will be applied to individual TI and DSM nanostructures to isolate and map their surface photoconductivity response for the first time with <30nm spatial and <1ps temporal resolution. Nano-tomography will form a 3D map of local carrier concentration, carrier lifetime and electron mobility, providing deeper insight into surface carrier transport. Utilising this newfound knowledge, the exclusive P-NAME facility will be used to spatially dope optimised TI and DSM nanostructures for use in THz emitters and detectors. This tool enables a single ion to be positioned with <40nm spatial accuracy, providing control of electronic properties on nanometre length scales. OPTP-SNOM will be used to image dopants and examine nanoscale conductivity, providing a direct feedback loop between material and device optimisation. This capability will allow the advantageous properties of Dirac materials to be fully exploited, leading to a step-change in performance. These THz devices are expected to surpass performance of current state-of-the-art THz devices, opening a pathway for THz technologies to impact on today's society.