Spin dynamics in many-electron quantum dots of tunable size

The electron spin coherence time in GaAs-based quantum dots is limited by hyperfine interaction with statistical fluctuations of the nuclear spin polarization. For larger dots in the transition to a 2D system, spin-orbit interaction becomes the dominant dephasing mechanism. In the region in between, a minimum dephasing is expected. Here we investigate the spin coherence time in quantum dots of tunable size obtained from dry etching of a 2D electron gas (2DEG) confined in a GaAs quantum well. Using the pump-probe magneto-optical Kerr rotation technique, electron spin dynamics in ensembles of uniform dots is measured. The spin coherence time increases by two orders of magnitude, from 200 ps in the 2DEG, to more than 20 ns for dots with a diameter of 400 nm. A periodic optical orientation of electron spin polarization at a rate of 80 MHz induces a mode-locking of the spin precession that is enhanced by orientation of nuclear spin polarization, similar to what has been observed for singly charged self-assembled quantum dots but here realized in large many-electron quantum dots with tunable spin-orbit interaction.