Combined Far-Infrared & Optical Probes of Semiconductor Quantum Structures

Abstract

Abstract : We report on a program in which we have developed a terahertz time-resolved spectroscopy system with electro-optic generation and detection, incorporating a femtosecond optical pump pulse suitable for studying the dynamics of impulsively photo-generated carriers using the THz probe. In the course of this work, we have developed an entirely new way of generating tunable coherent narrow-band (multi-cycle) THz waveforms using femtosecond pulses, suitable for semiconductor spectroscopy. The technique utilizes optical rectification in periodically poled lithium niobate. We have also utilized all-optical femtosecond pump-probe spectroscopy to perform the most in-depth study to date of carrier capture and relaxation dynamics in self-organized III-V quantum dots. In a major breakthrough, we have made the first direct, spectroscopic observation of the phonon bottleneck, and also the first observation of the fast electron-hole scattering that occurs when both species of carrier are captured in the same quantum dot; this electron-hole scattering circumvents the phonon bottleneck and enables fast relaxation in bipolar device structures. Additionally, we have directly observed for the first time the tunnel coupling of vertically aligned quantum dots, and performed the first study of the temperature dependence of carrier capture in these systems.