Regensburg 2007 – wissenschaftliches Programm

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PV: Plenarvorträge

PV XIII

PV XIII: Preisträgervortrag

Donnerstag, 29. März 2007, 08:30–09:15, H1

Optically probing charge and spin interactions in semiconductor quantum dots and molecules — •Jonathan J. Finley — Walter Schottky Institut, Technical University of Munich, 85748 Garching, Germany — Träger des Walter-Schottky-Preises

I will review investigations of optically pumped spin-memory devices that enable the reversible transfer between photon polarisation and the spin orientation of isolated electrons (e) or holes (h) in self-assembled GaAs-GaInAs quantum dot (QD) nanostructures.[1] Following resonant optical excitation using circularly polarized light, individual spin orientated e-h pairs (excitons) are selectively generated in the QDs with a total spin that reflects the helicity of the optical field used to create them. After generation we inhibit spontaneous recombination by selectively removing one charge from the dots by tunnelling whilst the other remains stored. Preparing spins in the upper Zeeman level allows us to study spin relaxation dynamics by electrically neutralizing the dots after a defined storage time and recording the optical polarization of the resulting luminescence signal. These experiments reveal that both e and h spin relaxation is very slow in QD-nanostructures, with spin relaxation times in the millisecond regime having been measured for electrons and similar timescales for holes. Our findings firmly establish that spin-flip scattering in QD-nanostructures is strongly suppressed when compared to higher dimensional systems and is mediated predominantly by spin-orbit (SO) interaction when subject to a magnetic field in excess of 1 Tesla.

I will then continue to discuss optical investigations of individual, electrically tuneable double dot systems (QD-molecules) formed from a pair of vertically aligned GaInAs QDs separated by a thin GaAs tunnel barrier. As the electric field is tuned, we observe a clear anti-crossing between spatially direct (e,h in the same dot) and indirect (e,h in different dots) excitons with coupling energies in the range of a few millielectronvolts.[2] Our findings are in very good accord with realistic calculations, confirming that the inter-dot coupling is mediated by tunnel hybridization of the electron component of the exciton wavefunction over the two dots. Our structures also enable the introduction of additional electrons into the QD-molecule allowing us to probe negatively charged excitons. In this case, the results obtained are shown to be much richer due to the delicate interplay of intra- and inter-dot Coulomb interactions between localised charges in the QD-molecule.[3] The tunnel coupling energies are shown to be most sensitive to the electron population in the QD-molecule, confirming that the coupling mechanism is due to electron tunnelling. Moreover, the coupling is found to be spin-dependent providing much potential for the future electro-optical control and manipulation of few spin systems in coupled QD-nanostructures.

[1] M. Kroutvar, et al. Nature 432, (2004)

[2] H. J. Krenner et al. Phys. Rev. Lett. 94, 057402 (2005)

[3] H.J. Krenner et al. Phys. Rev. Lett. 97, 076403, (2006)

The research presented in this talk is supported by the DFG via SFB-631 and is the work of a number of students and colleagues over the past few years, including G. Abstreiter, M. Kroutvar, D. Heiss, H. J. Krenner and E. C. Clark.