Optical investigations of quantum dot spin dynamics as a function of external electric and magnetic fields

Abstract

We have performed all-optical measurements of spin relaxation in single self-assembled $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dots (QDs) as a function of static external electric and magnetic fields. To study QD spin dynamics, we measure the degree of resonant absorption which results from a competition between optical spin pumping induced by the resonant laser field and spin relaxation induced by reservoirs. Fundamental interactions that determine spin dynamics in QDs are hyperfine coupling to QD nuclear spin ensembles, spin-phonon coupling, and exchange-type interactions with a nearby Fermi sea of electrons. We show that the strength of spin relaxation generated by the three fundamental interactions can be changed by up to 5 orders of magnitude upon varying the applied electric and magnetic fields. We find that the strength of optical spin pumping that we use to study the spin relaxation is determined predominantly by hyperfine-induced mixing of single-electron spin states at low magnetic fields and heavy-light hole mixing at high magnetic fields. Our measurements allow us to determine the rms value of the hyperfine (Overhauser) field to be $\ensuremath{\sim}15\phantom{\rule{0.3em}{0ex}}\mathrm{mT}$ with an electron $g$ factor of ${g}_{e}=0.6$ and a hole mixing strength of ${\ensuremath{\mid}{ϵ}_{H}\ensuremath{\mid}}^{2}=5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$.