R. D. R. Bhat

Pure spin current from one-photon absorption of linearly polarized light in noncentrosymmetric semiconductors

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.

Coherent control of an optically injected ballistic spin-polarized current in bulk GaAs

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.

Characterization of quantum interference control of injected currents in LT-GaAs for carrier-envelope phase measurements

We use two mutually coherent, harmonically related pulse trains to experimentally characterize quantum interference control (QIC) of injected currents in low-temperature-grown gallium arsenide. We observe real-time QIC interference fringes, optimize the QIC signal fidelity, uncover critical signal dependences regarding beam spatial position on the sample, measure signal dependences on the fundamental and second harmonic average optical powers, and demonstrate signal characteristics that depend on the focused beam spot sizes. Following directly from our motivation for this study, we propose an initial experiment to measure and ultimately control the carrier-envelope phase evolution of a single octave-spanning pulse train using the QIC phenomenon.

Optical injection and coherent control of a ballistic charge current in GaAs/AlGaAs quantum wells

We report all-optical injection and coherent control of a ballistic charge current in GaAs/AlGaAs quantum wells. This current arises through quantum interference of one- and two-photon absorption of ∼100 fs pulses with parallel linear polarizations, and its magnitude can be controlled by adjusting the relative phase of the incident pulses. By monitoring differential transmission using a spatially resolved optical pump–probe technique, we observe evidence of carrier motion associated with this ballistic current. Results are consistent with a theoretical treatment specific to quantum wells, and are qualitatively similar to previous measurements in bulk GaAs.

Excitonic effects on the two-color coherent control of interband transitions in bulk semiconductors

Quantum interference between one- and two-photon absorption pathways allows coherent control of interband transitions in unbiased bulk semiconductors; carrier population, carrier spin polarization, photocurrent injection, and spin-current injection can all be controlled. We extend the theory of these processes to include the electron-hole interaction. Our focus is on photon energies that excite carriers above the band edge, but close enough to it so that transition amplitudes based on low-order expansions in

Characterization of carrier-envelope phase-sensitive photocurrent injection in a semiconductor

\mathbf{k}

Enhanced coherent control of carrier and spin density in a zinc-blende semiconductor by cascaded second-harmonic generation

are applicable; both allowed-allowed and allowed-forbidden two-photon transition amplitudes are included. Analytic solutions are obtained using the effective-mass theory of Wannier excitons; degenerate bands are accounted for, but envelope-hole coupling is neglected. We find a Coulomb enhancement of each two-color coherent control process and relate it to the Coulomb enhancements of one- and two-photon absorption. In addition, we find a frequency-dependent phase shift in the dependence of photocurrent and spin current on the optical phases. The phase shift decreases monotonically from

All-Optical Control of Charge and Spin in GaAs: Densities and Currents

\ensuremath{\pi}∕2

Cascaded second-harmonic coherent control of carrier density and spin in a [110]-oriented semiconductor

at the band edge to zero over an energy range governed by the exciton binding energy. The phase shift is the difference between the partial-wave phase shifts of the electron-hole envelope function reached by one- and two-photon pathways.

Quantum kinetics of pure spin and charge currents

We characterize the manner in which the carrier-envelope phase of ultrashort pulses can control quantum interference of injected photocurrents in low-temperature-grown gallium arsenide. We verify the predicted linear and square-root dependences of the generated current on the average optical powers of the low (nu) and high (2nu) frequency wings of a pulse spectrum, respectively. When scanning the time delay between these two colors, the signal amplitude exhibits a temporal width of 72 fs. The generated signal behaves as an ideal current source for loads below ∼100 kOmega. This behavior allows us to increase the signal detection bandwidth from 25 kHz with a voltage amplifier to 830 kHz by use of a transimpedance amplifier; higher bandwidths are possible. We discuss how transimpedance amplification could also enable the quantum-interference photocurrent signal to be measured by use of materials with longer carrier lifetimes, such as intrinsic GaAs.