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

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.

Knowledge: This fellowship proposal will provide a unique facility for non-destructive, surface-sensitive advanced material characterisation at ultrafast timescales and nanometre length scales. This capability will be unique to the UK/EU and internationally competitive, one of only 3 systems worldwide. It will open up a large parameter range to examine the optoelectronic properties of advanced functional materials, including Dirac materials, nanomaterials and biomaterials. It will therefore ensure that the UK remains at the forefront of research and innovation in this area and is the 'go-to' place for nanoscale THz frequency characterisation. The fellowship will also provide a new insight into the fundamental physical mechanisms governing electronic transport in topological Dirac materials. It will enable the exotic surface conductivity to be examined independently from the bulk for the first time, revealing their ultrafast surface carrier dynamics. It will also enable the first observation of topologically-protected collective surface modes (Dirac SPPs and HP3 modes), opening a route to their control in plasmonic devices. Exploration of nanoscale spatial doping will allow their electronic properties to be tailored on nm-length scales, enabling design of bespoke systems for optoelectronic, photonic and spintronic applications. This knowledge will widely distributed throughout the UK 2D materials and THz communities, through a 2nd network meeting I will host at UoM, international and national conferences, Royce partners, publications and my group website. This will ensure immediate impact in these fields, opening up new research avenues and allowing the UK to compete internationally, becoming a leader in this area.

Economy: This increased knowledge and control (through spatial doping) of electronic properties at the nanoscale will accelerate materials optimisation, facilitating the development of '21st century devices' based on Dirac materials. These materials are predicted to deliver a step-change in device performance, owing to their high electron mobility (electrons travelling close to speed of light), doping tunability and topological protection (suppressed scattering), which can lead to integrated electronic/photonic or quantum technologies with increased device speed and reduced energy consumption. They therefore are set to impact several key economic sectors, in particular in energy (eg. increased absorption for solar cells), ICT (eg. candidates for ultrafast wireless communication) and technology (eg. faster transistors and nanoelectronics). To maximise impact, key industrial stakeholders (e.g. Teraview) have been engaged to scope out a roadmap for potential optoelectronic, photonic and spintronic applications of these materials and their translation to market.

Society: These next-generation devices will be transformative, disrupting existing technologies. They will ultimately have a large impact on UK society. The potential increased speed of these devices could lead to ultrafast wireless communication their reduced energy consumption will address the GCRF challenge around sustainable cities. Exploitation of their quantum effects will ultimately revolutionise how we use technology (e.g. quantum computing). To maximise this societal change, public engagement is crucial. During this fellowship programme, I will promote the research through outreach activities and public lectures (e.g. Pint of Science). I also aim to become an inspirational role model within the STEM community and will promote accessibility in STEM, particularly focusing on the use of British Sign Language in STEM through my SignScience Campaign. I will utilise key BSL terms in all public and academic lectures to promote these signs to a wide audience and develop BSL signs for missing terms. I hope that this will have a wider impact on society, making STEM more accessible for future researchers and engaging new audiences.