Electron spin relaxation in a single InAs quantum dot measured by tunable nuclear spins

The electron-spin dynamics of positively charged excitons in a single InAs quantum dot (QD) were measured by time-resolved photoluminescence spectroscopy. Using alternating sigma(+)/sigma(-) pulse sequences to excite the QD, the lattice nuclear spins remained randomly oriented. This method enables us to check the theory of electron-spin relaxation in the randomly distributed frozen fluctuation of the nuclear field. The experimental results are in qualitative agreement with theoretical prediction, showing that the electron-spin polarization decreases to a minimum value at first, and then increases again up to a steady value of about 1/3 of its initial value.