Optical alignment of excitons

John J. Sein

K. Kowalik, O. Krebs, Aristide Lemaître +2 more

We investigate exciton spin memory in individual $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ self-assembled quantum dots via optical alignment and conversion of exciton polarization in a magnetic field. Quasiresonant phonon-assisted excitation is successfully employed to define the initial spin polarization of neutral excitons. The conservation of the linear polarization generated along the bright exciton eigenaxes of up to 90% and the conversion from circular to linear polarization of up to 47% both demonstrate a very long spin relaxation time with respect to the radiative lifetime. Results are quantitatively compared to a model of pseudospin $1∕2$ including heavy-to-light hole mixing.

Martin Kroner, K. M. Weiss, B. Biedermann +7 more

We report high-resolution resonant two-color laser spectroscopy on single self-assembled InGaAs quantum dots. The negatively charged exciton in a quantum dot can decay radiatively into both electron-spin ground states, the states with an electron in the quantum dot with its spin parallel or antiparallel to an applied magnetic field. The two decay paths can be used for optical spin alignment via optical pumping. We apply two laser fields at two different colors resonant with the two Zeeman split optical transitions to study the properties of the spin of the resident electron in the quantum dot in both the Faraday and Voigt geometries. The resonant two-color signal is monitored as a function of the laser power as well as the applied magnetic and electric field allowing us to determine spin decay and dephasing rates. We find the rate at which the optical spin alignment can be performed depending on the direction and the magnitude of the applied magnetic field. Finally we demonstrate the feasibility of performing coherent all-optical spin manipulation of an electron spin in a quantum dot.

P. I. Khadzhi, G. K. Shibarshina, A. Kh. Rotaru

Hamid Ohadi, Yago del Valle‐Inclan Redondo, Alexander Dreismann +6 more

Tunable spin correlations are found to arise between two neighboring trapped exciton-polariton condensates which spin polarize spontaneously. We observe a crossover from an antiferromagnetic to a ferromagnetic pair state by reducing the coupling barrier in real time using control of the imprinted pattern of pump light. Fast optical switching of both condensates is then achieved by resonantly but weakly triggering only a single condensate. These effects can be explained as the competition between spin bifurcations and spin-preserving Josephson coupling between the two condensates, and open the way to polariton Bose-Hubbard ladders.

Hermann Haken, Haken, Hermann