We show that quantum interference of one and two photon absorption from a two color field allows one to optically inject ballistic spin currents in unbiased semiconductors. The spin currents can be generated with or without an accompanying electrical current and can be controlled using the relative phase of the two colors. We characterize the injected spin currents using symmetry arguments and an eight-band Kane model.

We demonstrate all-optical quantum interference injection and control of a ballistic pure spin current (without an accompanying charge current) in GaAs/AlGaAs quantum wells, consisting of spin-up electrons traveling in one direction and spin-down electrons traveling in the opposite direction. This current is generated through quantum interference of one- and two-photon absorption of approximately 100 fs phase-locked pulses that have orthogonal linear polarizations. We use a spatially resolved pump-probe technique to measure carrier movement of approximately 10 nm. Results agree with recent theoretical predictions.

Quantum interference of one- and two-photon excitation of unbiased semiconductors yields ballistic currents of carriers. The magnitudes and directions of the currents and the spin orientations of the carriers are controlled by the polarization and relative phase of the exciting femtosecond laser fields. We provide direct experimental evidence for the spin polarization of the optically injected spin currents by detecting a phase-dependent spatial shift of the circularly polarized photoluminescence in cubic ZnSe.

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We demonstrate quantum interference control of injected photocurrents in a semiconductor using the phase stabilized pulse train from a mode-locked Ti:sapphire laser. Measurement of the comb offset frequency via this technique results in a signal-to-noise ratio of 40 dB (10 Hz resolution bandwidth), enabling solid-state detection of carrier-envelope phase shifts of a Ti:sapphire oscillator.

We show that one-photon absorption of linearly polarized light should produce pure spin currents in noncentrosymmetric semiconductors, including even bulk GaAs. We present 14x14 k.p model calculations of the effect in GaAs, including strain, and pseudopotential calculations of the effect in wurtzite CdSe.

We demonstrate the injection and control of pure spin currents in ͓110͔-oriented GaAs quantum wells at room temperature by one-photon absorption of a single linearly polarized optical pulse. These currents result from the interference of absorption processes associated with the right and left circularly polarized components of the pulse, with the current direction determined by their relative phase. The current generation process differs from the circular photogalvanic effect, which relies only on the intensity of circularly polarized beams. By using spatially resolved pump-probe techniques, we obtain signatures for the currents by measuring the resulting spin separations of 1-4 nm. The separation decreases with increasing excitation fluence, consistent with a reduction in the momentum relaxation time with increasing carrier density.

We demonstrate coherent all-optical injection and control of a ballistic spin-polarized current in bulk, low-temperature-grown GaAs at room temperature. The spin current is injected by interfering the two-photon absorption of the fundamental (1.55 μm) and the single photon absorption of the second harmonic (0.775 μm) of ∼180 fs pulses that propagate collinearly and have the same circular polarization. Adjusting the relative phase of the two pulses controls the direction of this current. The component of the electrical current transverse to the pulse propagation direction is investigated by monitoring charge collection across a pair of gold electrodes deposited on the GaAs surface. Results are in agreement with recent theoretical predictions.