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

太赫兹照亮了纳米尺度：利用近场光谱揭示狄拉克系统的超快动力学

• 批准号: EP/S037438/1
• 负责人: Jessica Boland
• 金额: $ 36.73万
• 依托单位: University of Manchester
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2019
• 资助国家: 英国
• 起止时间: 2019 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=EP%2FS037438%2F1
• 关键词: Terahertz; lights; up; nanoscale; Exposing

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'.

随着我们对技术的依赖不断增加，对速度更快、功能更强的设备的需求也在增加。一个完美的例子就是手机——从只能拨打电话和发送短信的功能开始，我们现在有了带有 GPS、计步器、可以搜索互联网、拍摄照片和视频的智能手机。尽管取得了如此快速的进步，“智能”设备仍然相对能源效率低下，功耗高且电池寿命短。随着当今的环境气候和技术使用的增加，对新颖的“21 世纪产品”的需求量很大，这些产品不仅可以使设备速度发生巨大变化，而且还具有能源效率。尤其是拓扑绝缘体 (TI) 已成为下一代器件的潜在构建模块。该材料的大部分是绝缘的，而表面则承载着奇特的狄拉克电子，其行进速度接近 10,000,000 m/s，比硅快 100 倍。由于其拓扑性质，表面电子不会受到非磁性杂质和晶体缺陷的散射。因此，它们的表现就像在有轨电车轨道上行驶一样：比传统材料速度更快、阻力更小、产生的热量更少，从而更加节能。电子也只能沿一个方向行进，该方向由其固有的角动量或“自旋”决定。这一特性对于信息处理、量子计算和自旋电子应用特别有用。为了在器件中利用这些有利特性，深入了解电子迁移率（速度）和寿命等关键参数至关重要。尽管在探索这些材料难以捉摸的特性方面已经取得了重大进展，但事实证明很难将表面与本体隔离。需要表面敏感技术来独立检查表面电子，并深入了解控制这些材料表面传输的潜在物理机制。太赫兹 (THz) 频率范围位于微波和红外辐射之间，为研究狄拉克材料提供了完美的探头。它能够穿透多种不透明材料，例如塑料、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。远场太赫兹探头已被用于检查 TI，并表明电子可以从本体弛豫到表面，从而减少杂质散射。然而，这些太赫兹探头的空间分辨率都受到限制。光的衍射极限将太赫兹辐射限制在 150 微米的光斑尺寸，因此它们只能测量由于体积和表面而产生的有效电导率。该项目旨在将太赫兹探测器的空间分辨率降低至纳米长度尺度。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尺寸，提供<30nm的空间分辨率。从尖端和样品散射回的太赫兹辐射包含有关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，可以改变太赫兹探头的穿透深度以提供表面灵敏度。大的敲击幅度可探测材料的大部分，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，以对局部电子迁移率、寿命和电导率进行差分深度分析。这将使表面行为与本体分离并首次直接检查。这些信息将为利用这些狄拉克材料的优势特性来开发新颖的“21 世纪产品”开辟一条途径。

项目成果

Topological Dirac semi-metals as novel, optically-switchable, helicity-dependent terahertz sources

拓扑狄拉克半金属作为新型、光学可切换、螺旋度相关的太赫兹源

• DOI: 10.1109/irmmw-thz50927.2022.9895566
• 发表时间: 2022
• 作者: Boland J

Unveiling the ultrafast optoelectronic properties of 3D Dirac semi-metal Cd 3 As 2

揭示 3D 狄拉克半金属 Cd 3 As 2 的超快光电特性

• DOI: 10.1109/irmmw-thz46771.2020.9370806
• 发表时间: 2020
• 作者: Boland J

Investigating the Effect of Crystal Morphology on Optoelectronic Properties of Zinc Phosphide Thin Films via Optical-pump Terahertz Probe Spectroscopy

通过光泵太赫兹探针光谱研究晶体形态对磷化锌薄膜光电性能的影响

• DOI: 10.1109/irmmw-thz57677.2023.10299122
• 发表时间: 2023
• 作者: Huang Y

Topological materials as promising candidates for tuneable helicity-dependent terahertz emitters

• DOI: 10.1117/12.2681745
• 发表时间: 2023-10
• 期刊: Review of Palaeobotany and Palynology
• 影响因子: 1.9
• 作者: J. Boland;D. Damry;Chelsea Q. Xia;Y. Saboon;A. Mannan;Piet Schoenherr;D. Prabhakaran;Laura M. Herz;T. Hesjedal;Michael B. Johnston

The 2023 terahertz science and technology roadmap

• DOI: 10.1088/1361-6463/acbe4c
• 发表时间: 2023-06-01
• 期刊: JOURNAL OF PHYSICS D-APPLIED PHYSICS
• 影响因子: 3.4
• 作者: Leitenstorfer, Alfred;Moskalenko, Andrey S.;Cunningham, John
• 通讯作者: Cunningham, John