The coherent optical injection and temporal decay of spin and charge currents in semiconductor heterostructures is described microscopically, including excitonic effects, carrier LO-phonon and carrier-carrier scattering, as well as nonperturbative light-field-induced intraband and interband excitations. A nonmonotonous dependence of the currents on the intensities of the laser beams is predicted. Enhanced damping of the spin current relative to the charge current is obtained as a consequence of spin-dependent Coulomb scattering. PACS numbers: 72.25.Fe, 72.25.Rb One of the fundamental principles of quantum mechanics is that superpositions of wave functions lead to interference phenomena which depend on the relative phase differences. In the coherent regime, e.g., shortly after an external perturbation has generated a nonequilibrium situation, such quantum mechanical interference effects can be employed for the coherent control of dynamical processes in atomic, molecular, biological, and semiconductor systems [1,2,3,4,5,6]. Many of the measurements and proposals in this area make use of the coherent evolution of electronic excitations induced by specially designed optical laser pulses, e.g., sequences of phase-locked or suitably chirped beams.In semiconductors, the ultrafast coherent generation of photocurrents using two light beams with frequencies ω and 2ω has attracted considerable attention [7,8,9]. As shown in Ref. 10, the same type of interference scheme can also be employed to create pure spin currents which are not accompanied by any charge current. Such spin currents generated on ultrafast time scales have been observed in semiconductors [11,12] and could be useful for future applications in the area of spintronics [13,14]. As shown recently, it is even possible to control photocurrents via the carrier-envelope phase [15,16] which makes this scheme also interesting for optical metrology. Furthermore, for disordered semiconductors it has been predicted that sequences of temporally delayed excitation pulses can be used to induce current echoes [17].The coherent generation of photocurrents in semiconductors by two light fields with frequencies ω and 2ω satisfying 2hω > E gap >hω, where E gap is the band gap energy, has first been described in terms of nonlinear optical susceptibilities which have been obtained on the basis of band structure calculations [8,18]. In this framework, the optically-induced intra-and interband transitions are treated within Fermi's golden rule. The interference between intra-and interband excitations leads to electron and hole distributions which are not symmetric in k-space corresponding to a nonequilibrium situation with a finite current. The dynamics of the generation process has been analyzed using Bloch equations. This approach has been applied to disordered semiconductors within a two-band model [17] and to ordered quantum wells within a multiband formalism [19]. Although the relaxation of the photocurrent by carrier LO-phonon scattering bulk GaAs has been analyzed [20], ...

The dynamics of charge and spin injection currents excited by circularly polarized, one-color laser beams in semiconductor quantum wells is analyzed. Our microscopic approach is based on a 14ϫ 14 k · p band-structure theory in combination with multisubband semiconductor Bloch equations which allows a detailed analysis of the photogenerated carrier distributions and coherences in k space. Charge and spin injection currents are numerically calculated for ͓110͔and ͓001͔-grown GaAs quantum wells including dc population contributions and ac contributions that arise from intersubband coherences. The dependencies of the injection currents on the excitation conditions, in particular, the photon energy are computed and discussed. A. Quantum-well band structureWe start by determining the electronic band structure and wave functions of the considered QW systems using 14 ϫ 14 band k · p theory. 35 The electron wave functions are de-

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We demonstrate by spin quantum beat spectroscopy that in undoped symmetric (110)-oriented GaAs/AlGaAs single quantum wells even a symmetric spatial envelope wavefunction gives rise to an asymmetric in-plane electron Landé-g-factor. The anisotropy is neither a direct consequence of the asymmetric in-plane Dresselhaus splitting nor of the asymmetric Zeeman splitting of the hole bands but is a pure higher order effect that exists as well for diamond type lattices. The measurements for various well widths are very well described within 14 × 14 band k · p theory and illustrate that the electron spin is an excellent meter variable to map out the internal -otherwise hidden-symmetries in two dimensional systems. Fourth order perturbation theory yields an analytical expression for the strength of the g-factor anisotropy, providing a qualitative understanding of the observed effects.PACS numbers: 78.55. Cr,78.47.jd,78.20.Ci,71.18.+y Symmetry is a fundamental principle which runs through all fields of sciences like a common thread. The balance of proportions is attracting great interest ever since reaching from Euclid's geometry theorems and the Archimedes lever principle in ancient times to Mandelbrot sets in present day mathematics and parity violation in modern particle physics. At the beginning of the last century the topic was significantly pushed by Emmy Noether's discovery of the deep connection between symmetry and conservation laws [1] and the classification of nearly all entities in today's physics in terms of its symmetry properties is a very powerful and widely applied method in a vast number of fields. Among the plethora of interesting physical observables the pure quantum mechanical entity spin in connection with the relativistic effect of spin-orbit interaction (SOI) [2] bears an exceeding connection to symmetry. In a free atom, SOI can break the degeneracy of states with the same orbital wave function owing opposite spins. In solids, however, such a splitting interferes with crystal symmetry. The most prominent example is the conduction band Dresselhaus splitting in zinc-blende (ZB) type lattice semiconductors [3], which is not present in their diamond lattice type equivalents [4]. The alteration of the symmetry allows a clear assignment of the investigated spin properties to the symmetry at hand and the change of symmetry properties on micro-and macroscopic scales is easy to produce in solid state physics by the introduction of low dimensional structures, potential gradients, or the choice of peculiar crystallographic quantization axes. This fact has boosted a great interest in recent semiconductor spintronic research [5][6][7] since crystal symmetry yields a control on the spin dynamics [8][9][10][11][12] and contrariwise the entity spin yields jointly with the time-reversal breaking property of a magnetic a unique meter variable to probe internal symmetries which might be inaccessible by other means.In this letter, we exploit the intriguing property that quantum wells (QW) grown with their quantizatio...

It is demonstrated that valence-band mixing in GaAs quantum wells tremendously modifies electronic transport. A coherent control scheme in which ultrafast currents are optically injected into undoped GaAs quantum wells upon excitation with femtosecond laser pulses is employed. An oscillatory dependence of the injection current amplitude and direction on the excitation photon energy is observed. A microscopic theoretical analysis shows that this current reversal is caused by the coupling of the light- and heavy-hole bands and that the hole currents dominate the overall current response. These surprising consequences of band mixing illuminate fundamental physics as they are unique for experiments which are able to monitor electronic transport resulting from carriers with relatively large momenta.

A microscopic approach that is based on the multisubband semiconductor Bloch equations formulated in the basis of a 14-band k · p model is employed to compute the temporal dynamics of photocurrents in GaAs quantum wells following the excitation with femtosecond laser pulses. This approach provides a transparent description of the interband, intersubband, and intraband excitations, fully includes all resonant as well as off-resonant excitations, and treats the light-matter interaction non-perturbatively. For linearly polarized excitations the photocurrents contain contributions from shift and rectification currents. We numerically compute and analyze these currents generated by the excitation with femtosecond laser pulses for [110]-and [111]-oriented GaAs quantum wells. It is shown that the often employed perturbative χ (2) -approach breaks down for peak fields larger than about 10 kV/cm and that non-perturbative effects lead to a reduction of the peak values of the shift and rectification currents and to temporal oscillations which originate from Rabi flopping. In particular, we find a complex oscillatory photon energy dependence of the magnitudes of the shift and rectification currents. Our simulations demonstrate that this dependence is the result of mixing between the heavy-and light-hole valence bands. This is a surprising finding since the bandmixing has an even larger influence on the strength of the photocurrents than the absorption coefficient. For [110]-oriented GaAs quantum wells the calculated photon energy dependence is compared to experimental results and a good agreement is obtained which validates our theoretical approach.

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Based on the multiband semiconductor Bloch equations a microscopic approach to high-harmonic generation in crystalline solids which is able to properly describe degenerate bands and band crossings is presented and analyzed. It is well-known that numerical band structure calculations typically provide electronic wave functions with an undetermined k-dependent phase which results in matrix elements which contain arbitrary k-dependent phases. In addition, such approaches usually mix degenerate bands and bands with an energy difference smaller than the numerical precision in an arbitrary way for each point in k-space. These ambiguities are problematic if one considers the dynamics induced by electric fields since the matrix elements of the position operator involve a derivative of the wave functions with respect to k. When the light-matter interaction is described in the length gauge, the problem of arbitrary phases and degenerate subspace mixing of Bloch states is solved by adopting a smooth gauge transformation along the field direction. The results obtained within this method are validated by comparing with calculations in the velocity gauge. Although we obtain in both gauges the same overall result, the length gauge is advantageous since it converges with a smaller number of bands and thus requires significantly less numerical effort than the velocity gauge. Also an unique distinction between inter-and intraband contributions and thus an instructive physical interpretation is possible in the length gauge whereas in the velocity gauge this is unclear. The computed polarization-direction-dependent high-harmonic spectra agree well with experimental data reported for GaAs. Furthermore, it is demonstrated that, under proper conditions, the Berry curvature is largely responsible for the even-order harmonics which are polarized perpendicular to the driving field.