Abstract
Using two-color time-resolved Faraday rotation and ellipticity, we demonstrate ultrafast optical control of electron spins
in GaAs quantum wells and InAs quantum dots. In quantum wells, a magnetic-field induced electron spin polarization is
manipulated by off-resonant pulses. By measuring the amplitude and phase of the spin polarization as a function of pulse
detuning, we observe the two competing optical processes: real excitation, which generates a spin polarization through
excitation of electron-hole pairs; and virtual excitation, which can manipulate a spin polarization through a stimulated
Raman process without exciting electron-hole pairs. In InAs quantum dots, the spin coherence time is much longer, so that
the effect of many repetitions of the pump pulses is important. Through real excitation, the pulse train efficiently polarizes
electron spins that precess at multiples of the laser repetition frequency, leading to a "mode-locking" phenomenon. Through
virtual excitation, the spins can be partially rotated toward the magnetic field direction, leading to a sensitive dependence of
the spin orientation on the precession frequency and detuning. The electron spin dynamics strongly influence the nuclear
spin dynamics as well, leading to directional control of the nuclear polarization distribution.
