Ultrafast terahertz dynamics of materials

材料的超快太赫兹动力学

• 批准号: RGPIN-2016-05842
• 负责人: Hegmann, Frank
• 金额: $ 3.64万
• 依托单位: University of Alberta
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2016
• 资助国家: 加拿大
• 起止时间: 2016-01-01 至 2017-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=615936
• 关键词: Ultrafast; terahertz; dynamics; materials

Many fundamental processes in nature occur over ultrafast time scales measured in picoseconds, or trillionths of a second. For example, in semiconductor materials like those used in computer chips and digital cameras, electrons can move freely in one direction for about a tenth of a picosecond before getting bumped in a different direction by small vibrations of the atoms. This scattering of electrons over such short time scales affects the flow of electrical current in materials. A pure semiconductor can actually be a good insulator, but absorption of light can generate both negative (electron) and positive (“hole”) charge carriers that are then free to move, making the material conducting. This phenomenon, which is called photoconductivity, forms the basis of most light sensing technologies. The photoexcited electrons and holes can move around inside the material for up to a few nanoseconds before becoming trapped at defect sites or emitting light as they recombine. In semiconductor nanomaterials, the lifetimes of photoexcited charge carriers can be tens of picoseconds, depending on the nanoscale morphology of the material. Understanding ultrafast processes in materials, therefore, provides valuable insight into the nature of materials. Ultrafast laser sources, which generate very short pulses of light less than a picosecond in duration, are the only experimental tool that can directly probe ultrafast dynamics in materials. In our lab, we use ultrafast laser sources to generate picosecond-duration electromagnetic transients called terahertz (THz) pulses that are ideally suited for probing ultrafast dynamics of materials. One of the goals of the proposed research program is to use very intense THz pulses to explore the nonlinear transport dynamics of photoexcited electrons and holes in bulk semiconductors and semiconductor nanomaterials. The large peak electric fields of intense THz pulses can accelerate charge carriers to very high energies before scattering, providing unique insight into charge carrier generation, transport and recombination in materials. However, directly probing ultrafast processes in materials on the nanoscale, which would provide completely new insight into how morphology and local environments affect carrier dynamics, has proven to be challenging. Recently, we developed a new technique called THz scanning tunneling microscopy (THz-STM) that allows direct imaging of ultrafast dynamics on surfaces with nanometer spatial resolution and sub-picosecond time resolution. The proposed research program will use THz-STM for imaging ultrafast dynamics in materials and nanostructures with atomic resolution, and will also explore the nature of THz-pulse-induced transient tunnel currents. Indeed, ultrafast imaging on the nanoscale would have an enormous impact on the development of new materials for energy conversion and nanoscale device technologies.

自然界中的许多基本过程都发生在以picseconds或几分之一的超快时间尺度上发生。例如，在像计算机芯片和数码相机中使用的半导体材料中，电子设备可以在一个方向上自由移动约十分之一的皮秒，然后通过原子的小振动朝着不同的方向撞击。电子设备在如此短的时间尺度上的这种散射会影响材料中电流的流动。纯半导体实际上可以是良好的绝缘体，但是光的遗憾可以产生负（电子）和正（“孔”）电荷载体，然后可以自由移动，从而使材料导电。这种现象称为光电导率，构成了大多数光传感技术的基础。光激发的电子和孔可以在材料内部移动，最多几纳米秒，然后被困在缺陷地点或重新组合时发出光线。半导体纳米材料，光激发电荷载体的寿命可以是数十秒钟，具体取决于材料的纳米级形态。 因此，了解材料中的超快过程提供了对材料本质的宝贵见解。超快激光源会产生非常短的光脉冲，而持续时间小于皮秒，是唯一可以直接探测材料中超快动力学的实验工具。在我们的实验室中，我们使用超快激光源来生成称为Terahertz（THZ）脉冲的Picsecond-Dration电子瞬态，非常适合探测材料的超快动力学。拟议的研究计划的目标之一是使用非常激烈的THZ脉冲来探索散装半导体和半导体纳米材料中光激发电子和孔的非线性传输动力学。强烈的THZ脉冲的大峰值电场可以在散射之前将电荷载体加速到非常高的能量，从而为材料中的电荷载体产生，运输和重组提供独特的见解。但是，已被证明是对纳米级材料的直接探测材料的超快过程，这些过程将对形态和当地环境如何影响载体动态提供全新的见解。最近，我们开发了一种称为THZ扫描隧道显微镜（THZ-STM）的新技术，该技术允许在具有纳米空间分辨率和次秒时间分辨率的表面上直接成像超快动力学。拟议的研究计划将使用THZ-STM在具有原子分辨率的材料和纳米结构中成像超快动力学，还将探索THZ-Pulse诱导的瞬态隧道电流的性质。实际上，纳米级上的超快成像将对能量转化和纳米级设备技术的新材料的开发产生增强的影响。