Ultrafast terahertz dynamics of materials

自然界中的许多基本过程都发生在以皮秒或万亿分之一秒测量的超快时间尺度上。例如，在计算机芯片和数码相机中使用的半导体材料中，电子可以在一个方向上自由移动约十分之一皮秒，然后通过原子的微小振动在不同方向上碰撞。电子在如此短的时间尺度上的这种散射影响了材料中电流的流动。纯半导体实际上可以是一个很好的绝缘体，但光的吸收可以产生负（电子）和正（“空穴”）电荷载流子，然后自由移动，使材料导电。这种被称为光电导的现象构成了大多数光传感技术的基础。光激发的电子和空穴可以在材料内部移动长达几纳秒，然后被困在缺陷部位或在重组时发光。在半导体纳米材料中，光激发电荷载流子的寿命可以是几十皮秒，这取决于材料的纳米级形态。因此，了解材料中的超快过程可以提供对材料性质的宝贵见解。超快激光源产生持续时间小于1皮秒的超短光脉冲，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产生皮秒持续时间的电磁瞬态，称为太赫兹（THz）脉冲，非常适合探测材料的超快动力学。拟议研究计划的目标之一是使用非常强的THz脉冲来探索大块半导体和半导体纳米材料中光激发电子和空穴的非线性输运动力学。强THz脉冲的大峰值电场可以在散射之前将电荷载流子加速到非常高的能量，从而为材料中电荷载流子的产生、传输和复合提供独特的见解。然而，直接探测纳米级材料中的超快过程，这将为形态和局部环境如何影响载流子动力学提供全新的见解，已被证明具有挑战性。最近，我们开发了一种称为太赫兹扫描隧道显微镜（THz-STM）的新技术，该技术可以直接成像表面上的超快动力学，具有纳米空间分辨率和亚皮秒时间分辨率。拟议的研究计划将使用THz-STM以原子分辨率对材料和纳米结构中的超快动力学进行成像，并且还将探索THz脉冲诱导的瞬态隧道电流的性质。事实上，纳米尺度上的超快成像将对能源转换新材料和纳米器件技术的发展产生巨大影响。

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.