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

技术是不断发展的。即使在我们的有生之年，随着功能的增加，设备也变得越来越快、越来越小；然而，这些“智能”设备仍然存在高功耗和低能量存储的问题。集成光子、电子和量子技术是创造下一代更节能、性能前所未有的设备的关键。这些“21世纪的产品”将对一系列领域产生巨大影响，包括医疗保健、无线通信、国防、安全和清洁能源。先进的功能材料，包括石墨烯、二维材料和III-V纳米线，将构成这些新技术的基础。特别是狄拉克材料，由于其非凡的光电特性，作为新型器件的候选者引起了极大的关注。狄拉克半金属（DSM）形成石墨烯的三维模拟物。而拓扑绝缘体（TI）在整体上是绝缘的，但具有完美的导电表面状态。对于这两种材料，表面都承载着接近光速的狄拉克电子，并且不受非磁性杂质和缺陷的后向散射的影响。它们的运动方向是由它们固有的角动量或“自旋”决定的，所以它们的行为就像在铁路线上一样——运动时阻力更小，热量产生更少。电子电荷和自旋之间的这种耦合使得ti和dsm在量子计算和自旋电子学应用中很有用。特别是，这些材料已经成为新型太赫兹（THz）器件的有希望的候选者。太赫兹技术将影响多个领域，包括安全、食品加工、医疗保健和无线通信。为了充分发挥其潜力，深入了解活性太赫兹材料中的关键器件参数（例如电导率）至关重要。太赫兹时域光谱学（THz- tds）作为一种强大的超灵敏、非接触式电导率探针而兴起。它已经被用来检查ti和DSMs，并表明它们具有高的有效电子迁移率，因为减少了杂质散射。然而，到目前为止，这些测量在空间分辨率上受到光的衍射极限（1THz为150um）的限制。测量的电导率平均超过任何非均匀性，并由体响应主导。因此，局部信息丢失，并且事实证明很难将表面电导率与体的电导率分离开来。本研究项目旨在将太赫兹- tds推进到纳米尺度，将空间分辨率扩展到纳米长度尺度。它将采用散射型近场光学显微镜（SNOM）和超快光泵太赫兹探针（OPTP）光谱（OPTP-SNOM）来提供一种非破坏性的、表面敏感的纳米级电导率探针。这种独特的工具将首次应用于单个TI和DSM纳米结构，以<30nm的空间分辨率和<1ps的时间分辨率分离和绘制其表面光电性响应。纳米层析成像将形成局部载流子浓度、载流子寿命和电子迁移率的三维地图，为表面载流子迁移提供更深入的了解。利用这些新发现的知识，独家的P-NAME设施将用于空间掺杂优化的TI和DSM纳米结构，用于太赫兹发射器和探测器。该工具能够以<40nm的空间精度定位单个离子，提供纳米长度尺度上的电子特性控制。OPTP-SNOM将用于成像掺杂剂和检查纳米级电导率，在材料和器件优化之间提供直接反馈回路。这种能力将使狄拉克材料的优势特性得到充分利用，从而导致性能的逐步变化。这些太赫兹器件的性能有望超过目前最先进的太赫兹器件，为太赫兹技术对当今社会的影响开辟了一条途径。

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.