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，在用于计算机芯片和数码相机的半导体材料中，电子可以在一个方向上自由移动大约十分之一皮秒，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影响了材料中的电流流动。一个纯半导体实际上可以是一个很好的绝缘体，但是光的吸收可以产生负（电子）和正（空穴）载流子，然后自由移动，使材料导电。这种现象被称为光导电性，是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，然后被困在缺陷位置或在它们重新组合时发光。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态。

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.