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Physical Review Letters

Ultrafast Coherent Electron Spin Flip in a Modulation-Doped CdTe Quantum Well

Carey E. Phelps, Timothy M. Sweeney, R.T. Cox, Hailin Wang

Published Jun 12, 2009• 30 citations
Publisher

Abstract

We report the experimental realization of coherent electron spin flip in a modulation-doped CdTe quantum well. Coherent spin rotation is realized with an off-resonant laser pulse, which induces a polarization-dependent optical Stark shift in the trion resonance. Complete electron spin flip is made possible by a laser pulse designed to avoid excessive excitations of nearby exciton resonances and minimizes detrimental many-body effects. These results demonstrate an effective approach for ultrafast optical spin control in a complex spin system.

DOI
10.1103/physrevlett.102.237402

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We report the experimental realization of coherent electron spin flip in a modulation-doped CdTe quantum well. Coherent spin rotation is realized with an off-resonant laser pulse, which induces a polarization-dependent optical Stark shift in the trion resonance. Complete electron spin flip is made possible by a laser pulse designed to avoid excessive excitations of nearby exciton resonances and minimizes detrimental many-body effects. These results demonstrate an effective approach for ultrafast optical spin control in a complex spin system.
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We have experimentally studied the spin dynamics of excitons, electrons, and trions in charge-tunable InP/InGaP quantum dots (QDs) excited by picosecond resonant laser pulses by observing the time-resolved Kerr rotation. In singly charged QDs, inversion of the spin polarization direction of doped electrons is found to be caused simply by variation in the pulse intensity, which is accompanied by an abrupt change of the spin coherence time. This phenomenon is reproduced by density-matrix calculations allowing for the reaction on the QD electron-trion four-level system during its coherent radiation emission. This result means that the optical coherence is another critical factor affecting electron spin coherence.
Impulsive stimulated Raman excitation with coherent optical fields is used for controlling the electron spin coherence in a charged GaAs\mathrm{GaAs}GaAs quantum dot ensemble through an intermediate charged exciton (trion) state. The interference between two stimulated Raman two-photon quantum mechanical pathways leading to the spin coherence allows us to control the electron spin coherence on the time scale of the Larmor precession frequency. We also demonstrate, both theoretically and experimentally, that ultrafast manipulation of the spin coherence is possible on the time scale of the optical laser frequency, and analyze the limitations due to the trion and spin decoherence times.
The excitation of electron spin polarization and coherence by picosecond light pulses and their dynamics in a wide remotely doped quantum well are studied theoretically and experimentally. Assuming that all electrons in the quantum well are localized, the theory considers the resonant interaction of light pulses with the four-level system formed by the electron spins of the ground state and the hole spins of the trion excited state. The theory describes the effects of spontaneous emission, a transverse magnetic field and hole spin relaxation on the dynamics detected by the Kerr rotation of a probe pulse. Time resolved Kerr rotation experiments were carried out on a remotely doped 14nm14\phantom{\rule{0.3em}{0ex}}\mathrm{nm}14nm GaAs quantum well in the frequency range of optical transitions to the heavy hole (HH) trion and to the light-hole (LH) trion degenerate with the HH exciton. The experiments on the resonant excitation of the HH trion show a very slow heavy hole spin relaxation and, consequently, a weak electron spin polarization after the trion relaxation. In contrast, the resonant excitation of the LH trion/HH exciton results in a fast hole spin relaxation that increases electron spin polarization.
Based on a multiparticle-state stimulated Raman adiabatic passage approach, a comprehensive theoretical study of the ultrafast optical manipulation of electron spins in quantum wells is presented. In addition to corroborating experimental findings [Gupta et al., Science 292, 2458 (2001)], we improve the expression for the optical-pulse-induced effective magnetic field, in comparison with the one obtained via the conventional single-particle ac Stark shift. Further study of the effect of hole-spin relaxation reveals that, while the coherent optical manipulation of electron spin in undoped quantum wells would deteriorate in the presence of relatively fast hole-spin relaxation, the coherent control in doped systems can be quite robust against decoherence. The implications of the present results on quantum dots will also be discussed.
Abstract Ultrafast laser pulses were used to control the spin state of a pair of entangled, donor‐bound electrons in a CdMnTe quantum well. The entanglement, mediated by an optically‐excited exciton, was characterized by pumpprobe spectroscopy. The data exhibits two dominant oscillations, one at the frequency of the spin‐flip of a donor electron and the second one at twice this frequency. Their amplitudes depend linearly on the pump power. The quantum state of the two‐electron spin system can be controlled by appropriate spatial and temporal shaping of the pump pulses. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)