Time evolution of the Coulomb screening effects on terahertz generation at the surface of InAs

We study the effects of electron density and temperature, both experimentally and numerically, on the Coulomb screening process responsible for saturation of the terahertz generation process at the surface of InAs. We use a pair of ultrafast pulses with adjustable time delay to generate terahertz radiation and compare the results to a 1D drift-diffusion equation model modified to include radial diffusion and cooling of the electrons through scattering. We demonstrate the necessity of these modifications by implementing the original drift-diffusion equation model, as reported by Liu et al. [Phys. Rev. B73, 155330 (2006)1098-0121PRBMDO10.1103/PhysRevB.73.155330], and show that it underestimates saturation by an order of magnitude. We find excellent agreement between our experimental and numerical results, confirming the validity of our improved model and demonstrating its potential use in a number of applications, such as terahertz pulse shaping with an ultrafast pulse train.