Ultrafast terahertz dynamics of materials

自然界中的许多基本过程发生在以皮秒或万亿分之一秒为单位的超快时间尺度上。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运动约十分之一皮秒，然后被原子的微小振动撞向不同的方向。电子在如此短的时间尺度上的散射会影响材料中的电流流动。纯半导体实际上可以成为良好的绝缘体，但光的吸收可以产生负(电子)和正(空穴)载流子，然后这些载流子可以自由移动，使材料导电。这种现象被称为光电导性，构成了大多数光敏技术的基础。光激发的电子和空穴可以在材料内部移动长达几纳秒，然后被困在缺陷位置或在重新结合时发光。在半导体纳米材料中，光激发载流子的寿命可以是几十皮秒，这取决于材料的纳米级形貌。因此，了解材料中的超快过程有助于深入了解材料的性质。超快激光光源产生持续时间小于皮秒的非常短的光脉冲，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产生皮秒持续时间的电磁瞬变，称为太赫兹(THz)脉冲，这种脉冲非常适合探测材料的超快动力学。提出的研究计划的目标之一是使用非常强的太赫兹脉冲来探索体相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。强太赫兹脉冲的大峰值电场可以在散射之前将载流子加速到非常高的能量，这为研究材料中载流子的产生、输运和复合提供了独特的见解。然而，在纳米尺度上直接探测材料中的超快过程，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，已被证明是具有挑战性的。最近，我们发展了一种名为太赫兹扫描隧道显微镜(THz-STM)的新技术，它可以在纳米空间分辨率和亚皮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太赫兹扫描隧道显微镜以原子分辨率成像材料和纳米结构中的超快动力学，并将探索太赫兹脉冲感应的瞬时隧道电流的本质。事实上，纳米级的超快成像将对能源转换新材料的开发和纳米级器件技术产生巨大影响。

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.