  We have performed all-optical measurements of spin relaxation in single
self-assembled InAs/GaAs quantum dots (QD) 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 five 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 ~15 mTesla
with an electron g-factor of g_e=0.6 and a hole mixing strength of |epsilon|^2
= 0.0005.
