$L$-valley electron spin dynamics in GaAs

Optical orientation experiments have been performed in GaAs epilayers with photoexcitation energies in the 3 eV region, yielding the photogeneration of spin-polarized electrons in the satellite $L$ valley. We demonstrate that a significant fraction of the electron spin memory can be conserved when the electron is scattered from the $L$ to the $\Gamma$ valley following an energy relaxation of several hundreds of meV. Combining these high energy photoexcitation experiments with time-resolved photoluminescence spectroscopy of $\Gamma$-valley spin-polarized photogenerated electrons allows us to deduce a typical $L$-valley electron spin relaxation time of 200 fs, in agreement with theoretical calculations.

Figure 1

(a) Schematics of the GaAs band structure; the arrows present the optical excitation and detection energies used in (b) and (c). (b) Time-integrated photoluminescence spectrum and the corresponding circular polarization following a ${\sigma }^{+}$-polarized laser excitation at an energy $E_{\text{exc}}=2.987$ eV. (c) PL circular polarization as a function of the excitation energy. The vertical arrows indicate the energy position of $L$-valley transitions. Inset: Dependence of the calculated photogenerated electron spin polarization after Ref. 21.

Figure 2

(a) Hanle curve: Variation of the PL circular polarization as a function of the transverse magnetic field $B$. The full line is a Lorentzian curve with $T_S=140$ ps (see text). Inset: Schematic representation of the two-level model including the spin relaxation times in both $L$ and $\Gamma$ valleys (see text). (b) Sketch of the Brillouin zone of GaAs displaying the eight $L$ valleys. The blue arrows represent the photogenerated spins in $L$ valleys. (c) Time evolution of the PL circular components $I^{+}$ and $I^{-}$ and the corresponding circular polarization $P_c$ for a near band-gap excitation.