Ultrafast terahertz dynamics of materials

点击翻译按钮获取中文摘要

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.