High-Field Terahertz Carrier Dynamics in Ge

The conductivity and functionality of a semiconductor are directly related to the population and dynamics of carriers within the conduction band. For example, electron mobility is determined by the curvature of a particular valley within the band structure. Therefore, changing carrier populations in specific valleys (with inherent mobility) will heavily influence the material conductivity. Terahertz (THz) spectroscopy has proven invaluable in measuring and understanding how conduction-band dynamics influence conductivity on ultrafast time scales. Furthermore, THz pulses have been used to excite carrier motion in germanium (Ge) and gallium arsenide (GaAs) with weak fields to avoid carrier generation or strong fields to induce the generation of carriers via impact ionization and Zener tunneling. Our goal is to model the carrier dynamics in Ge with low-field to high-field THz to show how carrier populations in Γ, L, and X valleys evolve over time. We show that we can move carrier populations between valleys using ultrafast THz pulses.