Reversal of Coherently Controlled Ultrafast Photocurrents by Band Mixing in Undoped GaAs Quantum Wells S. Priyadarshi, A. M. Racu, K. Pierz, U. Siegner, and M. Bieler * Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, D-38116 Braunschweig, Germany H.T. Duc, J. Fo ¨rstner, and T. Meier Department Physik and CeOPP, Universita ¨t Paderborn, Warburger Straße 100, D-33098 Paderborn, Germany (Received 5 March 2010; published 28 May 2010) 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. DOI: 10.1103/PhysRevLett.104.217401 PACS numbers: 78.67.De, 73.63.Hs, 78.47.J, 82.53.Mj For a few decades quantum-mechanical band-structure engineering has already been widely employed to design semiconductor heterostructures. The choice of material composition and combination, dimensionality, and geome- try offers an immense flexibility in optimizing, e.g., elec- tronic and optical properties. In unstrained semiconductor nanostructures the quantum confinement induces a cou- pling of the heavy-hole (hh) and light-hole (lh) valence bands [1–3]. This band mixing results not only in an anticrossing in momentum space, but also leads to a mix- ing of their spin properties and therefore modifies the optical selection rules. It has, thus, far reaching conse- quences for the light-matter interaction and optoelectronic and spintronic phenomena as demonstrated in a variety of studies [4–9]. Although the influence of band mixing on optical and magnetic properties of semiconductors is well known, a strong influence of band mixing on trans- port phenomena that goes beyond a simple change of effec- tive masses [1] or tunneling times [10] has not been re- ported so far. In this Letter, we demonstrate that band mixing induces a reversal of ultrafast photocurrents being optically in- jected into undoped GaAs quantum wells upon excitation of interband transitions with femtosecond laser pulses. The current reversal becomes apparent in an oscillatory behav- ior of the injection current amplitude and direction on the excitation frequency. A microscopic theory based on the 14  14 band k  p band structure [2] and the semiconduc- tor Bloch equations [3] reproduces the experimental data in which the current transients have been measured by detect- ing the emitted THz radiation very well [11]. The numeri- cal results demonstrate that the oscillations of the frequency-dependent photocurrents arise from an interplay of the electron, heavy- and light-hole currents. The detec- tion of these oscillations is unique for photocurrent experi- ments which measure the magnitude and direction of charge transport and are therefore sensitive to optical transitions with relatively large momenta which only weakly influence conventional optical experiments. Our combined experimental and theoretical studies demon- strate that despite the higher mass of the holes as compared to the electrons, the holes dominate the overall current response. Thus, ultrafast coherent photocurrent spectros- copy constitutes a powerful probe of details of the valence- band structure in semiconductor nanostructures. For the generation of ultrafast photocurrents we employ a coherent control scheme. In GaAs quantum wells (QWs) the quantum interference between two orthogonally polar- ized electric field components of a laser beam induces a polar carrier distribution in momentum space, which, in turn, leads to a current flow. The current, being injected with a net velocity [12], is maximized for circularly polar- ized light. Therefore this injection current is also referred to as circular photogalvanic current [13]. Macroscopically, the injection current can be linked to second-order non- linear optical effects [12]. Microscopically, the injection current results from a spin splitting of subbands due to the spin-orbit interaction [13]. Our sample consists of 40 periods of undoped, nomi- nally symmetric, (110)-oriented GaAs=Al 0:3 Ga 0:7 As QWs with a well thickness of 15 nm grown on a 500 m thick substrate. We have chosen the (110) orientation due to experimental reasons. This orientation allows us to gener- ate injection currents in the plane of the QW along the ½1  10 direction for normally incident light being circularly polarized [14]. In contrast, (001)-oriented QWs require oblique excitation for the generation of injection currents. We like to emphasize that our calculations show that the band-mixing-induced current reversal discussed further below is not limited to the (110) orientation but also occurs PRL 104, 217401 (2010) PHYSICAL REVIEW LETTERS week ending 28 MAY 2010 0031-9007= 10=104(21)=217401(4) 217401-1 Ó 2010 The American Physical Society