  The dynamic polarization of nuclear spins interacting with resident electrons
under resonant excitation of trions is studied in a nominally undoped
GaAs/(Al,Ga)As quantum well. Unlike in common time-resolved pump-probe
techniques, we used a single beam approach where the excitation light is
modulated between the circular and linear polarization states. The
time-integrated intensity of the excitation laser reflected from the sample
surface, proportional to the optical generation rate and changes due to the
pumping of the resident electrons, is detected. Polarized electrons on the
other hand transfer their spin to the lattice nuclei via the hyperfine
interaction. Exciting the sample with a train of pulses in an external magnetic
field leads to resonant spin amplification observed when the Larmor precession
frequency is synchronized with the laser pulse repetition rate. Build-up of the
nuclear spin polarization causes a shifting of the RSA peaks since the
resulting nuclear field alters the strength of the external magnetic field
experienced by the electrons. It was established that the nuclear spin
polarization time $T_1$ is temperature dependent and owing to the electron
localization at lower temperatures becomes shorter. "Locking" of the nuclear
field manifested as the limited by the strength of the external field growth of
the nuclear field, that is related to the anisotropy of the electron
$g$-factor, was observed. The $g$-factor ratio between the in plane
$g_{\parallel}$ and out-of-plane $g_{\bot}$ components was estimated to be
$g_{\bot}/g_{\parallel}=1.3$.
