Terahertz emission from ultrafast optical orientation of spins in semiconductors:: Experiment and theory

We discuss the optical injection of magnetization into a nonmagnetic semiconductor by the absorption of circularly polarized light. A microscopic approach, which is based on Fermi's golden rule and k.p band models, is used to quantify the magnetization- injection rate in GaAs. We find that under conditions typical in optical orientation experiments, the magnetization-injection rate of holes is approximately 20 times larger than it is for electrons, reflecting the large hole magnetic moment. We then turn to the ultrafast excitation regime and explore the possibility that the injected magnetization can radiate a detectable terahertz field. By using a phenomenological approach for the magnetization relaxation dynamics, we predict that the terahertz field from magnetic injection is at the limit of current terahertz detection technology. We provide initial experimental measurements in search of this terahertz radiation.