Dynamics of charge current gratings generated in GaAs by ultrafast quantum interference control

Abstract

Ballistic charge current gratings are induced in GaAs at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ by quantum interference of single- and two-photon absorption using noncollinearly incident 775 and $1550\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, $150\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ pulses. First-order diffraction of time-delayed $830\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, $150\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ probe pulses is used to observe carrier evolution for injected densities near ${10}^{17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. The current grating forms electron and hole charge-density gratings during pumping, and because the pumping is uniform while the carrier density and hence electronic specific heat is not, a carrier temperature grating also forms. The peak diffraction efficiency from both grating types is only $\ensuremath{\sim}{10}^{\ensuremath{-}9}$. The temperature grating, with modulation amplitude $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, decays through cooling in $\ensuremath{\sim}500\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$. Space-charge fields neutralize the electron and hole density gratings by the end of pumping, but nonetheless leave a neutral, electron-hole pair density grating with amplitude of $\ensuremath{\sim}{10}^{\ensuremath{-}3}$ of the injected carrier density. At the highest injected carrier densities, the pair grating amplitude builds on a few picosecond time scale before decaying by recombination and ambipolar diffusion with an $\ensuremath{\sim}15\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ time constant. A model based on continuity equations for carrier density, momentum, and energy during ballistic and drift motion is used to help interpret the experimental data. Besides qualitatively confirming the above dynamics, the model suggests that the pair grating amplitude and evolution is determined by two factors: (1) the warping or nonparabolicity of the hole bands and (2) the transfer of some electrons from the $\ensuremath{\Gamma}$-valley electron-density grating to the $L$, $X$ conduction band valleys during excitation, and their subsequent return to the $\ensuremath{\Gamma}$ valley on a few picosecond time scale.