  We report on magnetization precession induced in (Ga,Mn)As by an optical spin
transfer torque (OSTT). This phenomenon, which corresponds to a transfer of
angular momentum from optically-injected spin-polarized electrons to
magnetization, was predicted theoretically in 2004 and observed experimentally
by our group in 2012. In this paper we provide experimental details about the
observation of OSTT by a time-resolved pump-and-probe magneto-optical
technique. In particular, we show that the precession of magnetization due to
OSTT can be experimentally separated from that induced by the well known
magnetic-anisotropy-related mechanism in a hybrid structure
piezo-stressor/(Ga,Mn)As with an in situ electrical control of the magnetic
anisotropy. We also illustrate that OSTT is clearly apparent in the measured
dynamical magneto-optical signal in a large variety of (Ga,Mn)As samples with a
Mn concentration from 3% to 9%.
