We derive simple formulas to explain terahertz (THz) emission from semiconductor surfaces excited by a femtosecond (fs) laser. Femtosecond optical pulses with energies larger than the bandgap create photocarriers which travel and generate THz radiation, according to the time derivative of the photocurrent. By assuming that only electrons traveling in an ultrafast time scale, less than a few hundred fs, contribute to THz radiation, one can obtain simple expressions for the emission originating from the photocarrier drift accelerated with a built-in field or from the photocarrier diffusion. The emission amplitude of the former is in proportion with electron mobility, the Schottky-Barrier height, and the laser intensity and one of the latter with the laser intensity and diffusion coefficient squared. We also discuss the formula for emission from metal-insulator-semiconductor structures. The derived expressions are useful in understanding the THz emission properties observed by a laser THz emission microscope (LTEM), bringing LTEM into real applications in the field of semiconductor research and development.
Generation and manipulation of chiral terahertz waves emitted from the three-dimensional topological insulator Bi2Te3
在宽带太赫兹波的任意操控下，利用灵活的偏振整形技术，在成像、信息加密、太赫兹非线性现象的全光相干控制等方面具有广阔的应用前景。拓扑绝缘体具有独特的自旋-动量锁定表面态，在太赫兹辐射、探测和调制等方面具有广阔的应用前景，为未来基于片上拓扑绝缘体的太赫兹系统奠定了基础。然而，基于拓扑绝缘体的具有任意操纵的幅度和电场矢量方向的时间演化的规定方式的偏振成形太赫兹发射尚未被探索。本文系统研究了飞秒激光脉冲驱动下拓扑绝缘体Bi 2 Te 3纳米薄膜的太赫兹辐射，成功实现了手征性、椭圆率和主轴可控的高效手征太赫兹波的产生。手征太赫兹波的方便工程解释为光电流效应引起的光电流，而线性极化太赫兹波起源于线性光电流效应引起的移位电流。我们相信我们的工作不仅有助于进一步理解光-物质相互作用中超快自旋流的飞秒相干控制，而且还提供了一种有效的方法来产生自旋极化的太赫兹波，并加速扭曲的太赫兹波在源头的扩散。
