Dynamics of charge currents ballistically injected in GaAs by quantum interference

Abstract

The dynamics of charge currents ballistically injected in GaAs bulk and quantum wells are spatially and temporally resolved. The electrons and holes are injected with oppositely directed velocities without the use of accelerating fields by quantum interference between two photon absorption of a 200fs, 1430nm fundamental pulse and one photon absorption of the corresponding second harmonic pulse. The subsequent charge motion is followed with ∼200fs temporal and ∼1nm spatial resolution by using tightly focused optical differential transmission techniques that are dependent on the relative phase of the incident pump pulses. Initially, the electrons and holes ballistically separate by up to ∼20nm, and a space charge field forms, which decelerates the carriers. Within this ∼1ps regime, the momentum relaxes by electron-hole and phonon scatterings, and the space charge field restores the electrons and holes to a common position; on time scales long compared to 1ps, ambipolar diffusion and recombination complete the return of the system to equilibrium. A rigid shift (damped simple harmonic oscillator) model for the electron motion reproduces the key features in the data, and the procedure for extracting the spatiotemporal dynamics of the electrons is shown to be immune to energy relaxation effects and forgiving of nonlinear saturation.

Second-Harmonic Generation Induced by Electric Currents in GaAs

“…Such a lag has been confirmed in our previous high-resolution pump-probe experiments, where the charge separation was found to reach a peak after more than 100 fs. [26,27] However, here we observe the peak SHG around zero probe delay. Hence, the all-optical time-resolved technique has the advantage to unambiguously distinguish the field-induced and the current-induced SHG effects.…”

Power dependence of pure spin current injection by quantum interference

“…27,28 However, unlike pure spin current, in charge current the transport gives rise to a space-charge field that significantly changes the dynamics of the transport. 45 Since the space-charge field cannot be isolated from the average velocity, it is difficult to relate the electron accumulation to the initial average velocity. 45 Nevertheless, the same power dependence of charge current injection has indeed been observed in GaAs, silicon, germanium, and carbon nanotubes by measuring terahertz emission from the samples.…”

“…45 Since the space-charge field cannot be isolated from the average velocity, it is difficult to relate the electron accumulation to the initial average velocity. 45 Nevertheless, the same power dependence of charge current injection has indeed been observed in GaAs, silicon, germanium, and carbon nanotubes by measuring terahertz emission from the samples. [46][47][48] We suggest that the consistency between our experiment and the terahertz-based charge current experiments [46][47][48] is an indication that the terahertz signal detected in those experiments [46][47][48] is proportional to the charge current density initially injected, and is not influenced by the sequential charge transport dynamics.…”

Ultrafast optical microscopy of single monolayer molybdenum disulfide flakes

“…We can gain more insight by fitting our fluence-dependent Δ T/T data ( Fig. 3(b)) to obtain the saturation fluence F sat 40,41 . This gives F sat = 51.5 μ J/cm 2 , corresponding to a saturated carrier density of n sat~1 .6 × 10 13 cm −2…”