Electron spin orientation under in-plane optical excitation in GaAs quantum wells

We study the optical orientation of electron spins in GaAs/AlGaAs quantum wells for excitation in the growth direction and for in-plane excitation. Time- and polarization-resolved photoluminescence excitation measurements show, for resonant excitation of the heavy-hole conduction band transition, a negligible degree of electron spin polarization for in-plane excitation and nearly 100% for excitation in the growth direction. For resonant excitation of the light-hole conduction band transition, the excited electron spin polarization has the same (opposite) direction for in-plane excitation (in the growth direction) as for excitation into the continuum. The experimental results are well explained by an accurate multiband theory of excitonic absorption taking fully into account electron-hole Coulomb correlations and heavy-hole light-hole coupling.

Figure 1

(a) Experimental setup of the time-resolved PL measurements, (b) geometry for laser excitation as well as PL detection, and (c) sketch of the precession of the spin orientation (blue arrow) around the applied in-plane magnetic field (black arrow).

Figure 2

Typical experimental data and fits for an excitation energy of $1.567$ eV and an excitation intensity of 6 mW. The time of laser excitation defines $t=0$. The (red) squares and (blue) circles show the measured SQBs with an applied magnetic field of $+2$ T and $-2$ T, respectively, while the dashed lines represent the corresponding fits. Within the shaded area, laser stray light may influence the measured degree of polarization. Therefore, the data are fitted only outside the shaded area for $t-t_0\ge 80$ ps.

Figure 3

Comparison of (a) experimental results and (b) calculated values for the initial degree of electron spin polarization as a function of the excitation energy. The left (right) column shows the data for excitation in the growth direction (in the QW plane). The lower panels show the absorption coefficient obtained from PLE measurements (c) and theory (d) in the growth direction (left) and for in-plane excitation (right).

Figure 4

$P_0$ as a function of excitation energy for resonant LH1:E1 excitation at $1.542$ eV. The excitation power of 1 mW for the maximum $P_0$ is comparable to the excitation power used in Fig. 3.