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

随着我们对技术的依赖增加，对具有更多功能的更快设备的需求也在增加。一个很好的例子是移动的手机--从只能打电话和发短信开始，我们现在有了智能手机，它有GPS，步监视器，可以搜索互联网，拍照和视频。尽管取得了如此迅速的进步，但“智能”设备仍然相对节能，功耗高，电池寿命短。随着当今的环境气候和技术使用的增加，对新型“21世纪产品”的需求很大，这些产品不仅可以看到设备速度的阶跃变化，而且还具有能源效率。特别是拓扑绝缘体（TI）已经成为下一代器件的潜在构建模块。材料的大部分是绝缘的，而表面承载着接近10，000，000 m/s的奇异狄拉克电子-比硅快100倍。由于其拓扑性质，表面电子不受非磁性杂质和晶体缺陷的散射。因此，它们的行为就像在电车轨道上行驶一样：比传统材料更快，阻力更小，产热更少，使它们更节能。电子也只能沿一个方向运动，这是由它们固有的角动量或“自旋”决定的。这一性质在信息处理、量子计算和自旋电子学应用中特别有用。为了在器件中利用这些有利的特性，深入了解关键参数，如电子迁移率（速度）和寿命，是必不可少的。虽然在探测这些材料的难以捉摸的性质方面已经取得了重大进展，但事实证明，将表面与本体分离是困难的。需要表面敏感技术来独立地检查表面电子，并深入了解这些材料中表面输运的基本物理机制。太赫兹（THz）频率范围-落在微波和红外辐射之间-为研究狄拉克材料提供了完美的探针。它能够穿透几种不透明材料，如塑料，纸张和纺织品，目前用于机场人体扫描仪。更令人兴奋的是，它还可以以非接触、非破坏性的方式测量材料的导电性。远场太赫兹探针已经被用来检查TI，并揭示了电子可以从体到表面弛豫，从而减少杂质散射。然而，这些太赫兹探测器都受到空间分辨率的限制。光的衍射极限将太赫兹辐射限制在150微米的光斑大小，因此他们只能测量由于体和表面的有效电导率。该项目旨在将太赫兹探测器的空间分辨率降低到纳米尺度。通过将太赫兹辐射耦合到原子力显微镜尖端，太赫兹探针可以被限制在仅受尖端曲率半径限制的光斑大小，提供<30 nm的空间分辨率。从针尖和样品散射回来的太赫兹辐射包含了关于材料电导率的所有局部信息。通过振荡尖端并改变敲击幅度，可以改变THz探针的穿透深度以提供表面灵敏度。大的敲击幅度探测材料的大部分，而小的敲击幅度仅探测表面。该技术将用于TI薄膜和纳米结构，以进行局部电子迁移率，寿命和电导率的差分深度分析。这将允许表面行为从主体中分离出来，并首次直接进行检查。这些信息将为利用这些狄拉克材料的有利特性开发新的"世纪产品"开辟一条途径。

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'.