Topological insulators conduct electricity along their surfaces with almost no heat loss, and this project will turn them into tiny terahertz sources for next-generation communications. Semiconductor performance is hitting fundamental limits as transistors shrink to atomic scales. Topological materials offer a way around this bottleneck—their spin-polarised surface currents generate terahertz radiation that can be switched between broadband and narrowband operation by simply changing the laser beam that excites them. But these materials remain poorly understood at the nanoscale, and no one has yet built a practical device from them. The researcher will use a custom-built near-field microscope—the only national facility of its kind—to map electrical properties across topological insulator surfaces with sub-30-nanometre resolution. By correlating local defects and crystal structure with emission performance, the team will identify how to optimise growth conditions such as doping and temperature. They will then fabricate prototype terahertz sources and test them in a communications testbed and imaging setup. If successful, this work could produce on-chip terahertz emitters for 6G telecommunications, security imaging, and quality control in manufacturing—systems that currently rely on bulky, inefficient sources.
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Semiconductor materials underpin all five ‘technologies of tomorrow’: quantum, AI, engineering biology, semiconductors and future telecommunications. For example, the development of mm-wave/terahertz sources for 6G telecommunications relies on III/V compound semiconductors. The quantum computing architecture at the highest maturity level is based on donors in silicon. Today’s most pressing societal challenges, such as net zero, also require semiconductor development. For instance, wind and solar technologies require high-power operation, whereas digital ‘smart’ devices need to improve efficiency to reduce energy consumption. However, performance gains for semiconductor devices are slowing, as transistors reach fundamental molecular/atomic limits. To achieve smaller, faster, smarter, more energy-efficient devices, advanced functional materials, such as graphene, 2D materials, III-V nanowires, are therefore essential. Topological materials form potential building blocks for all 5 technologies. Examples include topological insulators (TI), which are insulating / semiconducting in the bulk with topologically-protected conductive surface states, and Dirac semi-metals (DSM) that are 3D analogues of graphene. They can therefore host spin-polarised surface currents, which act like a tramline – they travel in one direction set by their spin with less heat and resistance. This makes them promising candidates for low-loss electronics, spintronics and quantum technologies. However, they are still at a low maturity level, requiring further understanding and control of their optoelectronic properties. This proposal aims to address this issue by developing novel terahertz spectroscopy and microscopy techniques to examine the optoelectronic properties of topological insulators for technological applications. When operated in the far-field, terahertz spectroscopy can provide information on the average electrical conductivity, carrier density and carrier mobility in the material on mm-length scales. By utilising this technique, we have extracted the bulk electronic properties of topological insulator thin films and Dirac semi-metal nanowires. We have demonstrated that they can generate THz radiation, making them promising candidates for development of THz sources. By altering the properties of the photoexcitation beam, we can also control the emitted THz radiation, optically-switching between broadband and narrowband operation and tuning the peak emission frequency. This exciting behaviour is directly related to their topological nature, with polarisation control provided by spin-polarised currents at the surface. We will therefore focus on developing controllable THz sources from topological insulator thin films with the following objectives: 1) To examine the local THz photocurrent emission on TI and DSM nanostructures. 2) To control / enhance their THz emission via improved material/device design. 3) To develop a prototype THz source based on TI / DSM NWs for on-chip integration. To optimise these materials, we will measure their electronic and emission properties, as we change key growth parameters, such as doping and growth temperature. We will also investigate how properties on the nanoscale influence performance, such as defects from growth, changes in crystal structure and localised doping. To achieve this, we have established a national near-field microscopy facility that operates across the visible to terahertz wavelength range. When operating in the THz range, it enables us to map the surface-sensitive electrical conductivity, reflectivity and absorption with <30nm lateral spatial resolution. This will enable us to cross-correlate the emission performance of our topological materials with their local electronic properties and engineer these materials at these nanoscale. Once optimised, we will fabricate prototype THz sources from these materials and assess their performance in real-world applications, such as a communications testbed and imaging setup.
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