  Circularly polarized photoluminescence (PL) spectroscopy measured at 19 K on
GaAs/AlGaAs symmetric and asymmetric double quantum wells (DQW) is reported.
The PL is obtained by exciting the sample with a circularly polarized (left or
right) laser in order to create an initial unbalanced distribution of electron
spins in the conduction band and, in this way, obtain the electron spin
lifetime $\tau_s$. The effects of the excitation laser wavelength were
estimated by exciting with laser wavelengths of 701.0 nm, 787.0 nm, 801.5 nm
and 806.5 nm. The increase of $\tau_s$ with the excitation wavelength is
attributed to the lower initial quasi-momentum $\bf{k}$ of the excited
carriers, which also reduces spin-orbit relaxation processes. $\tau_s$ was
found to be higher in asymmetric DQWs: this is attributed to the wider QWs in
these samples, which reduces spin relaxation due to the Dresselhaus mechanism.
In addition, we also detected a smaller contribution from the Rashba mechanism
by comparing samples with built-in electric fields of different orientations
defined by doped barrier layers.
