Ultrafast optical control of electron spins in quantum wells and quantum dots

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.