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 in which 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 changing 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. A buildup of the nuclear spin polarization causes a shifting of the resonant spin amplification 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 (Overhauser) field in the oblique external magnetic field, 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 ($g_{\perp }$) components was estimated to be $g_{\perp }/g_{\parallel }=1.3$.

• Received 26 September 2018

DOI:https://doi.org/10.1103/PhysRevB.98.205304