A. Schwan

Anisotropy of electron and hole g-factors in (In,Ga)As quantum dots

The g-factor tensors of electron and hole in self-assembled (In,Ga)As/GaAs quantum dots are studied by time-resolved ellipticity measurements in a three dimensional vector magnet system. Both g-factor tensors show considerable deviations from isotropy. These deviations are much more pronounced for the hole than for the electron and are described by different anisotropy factors, which can even have opposite signs.

Effect of pump-probe detuning on the Faraday rotation and ellipticity signals of mode-locked spins in (In,Ga)As/GaAs quantum dots

We have studied the Faraday rotation and ellipticity signals in ensembles of singly-charged (In,Ga)As/GaAs quantum dots by pump-probe spectroscopy. For degenerate pump and probe we observe that the Faraday rotation signal amplitude first grows with increasing the time separation between pump and probe before a decay is observed for large temporal separations. The temporal behavior of the ellipticity signal, on the other hand, is regular: its amplitude decays with the separation. By contrast, for detuned pump and probe the Faraday rotation and ellipticty signals both exhibit similar and conventional behavior. The experimental results are well described in the frame of a recently developed microscopic theory [Phys. Rev. B 80, 104436 (2009)]. The comparison between calculations and experimental data allows us to provide insight into the spectral dependence of the electron spin precession frequencies and extract the electron g-factor dependence on energy.

Dispersion of electron g-factor with optical transition energy in (In,Ga)As/GaAs self-assembled quantum dots

The electron spin precession about an external magnetic field was studied by Faraday rotation on an inhomogeneous ensemble of singly charged, self-assembled (In,Ga)As/GaAs quantum dots. From the data the dependence of electron g-factor on optical transition energy was derived. A comparison with literature reports shows that the electron g-factors are quite similar for quantum dots with very different geometrical parameters, and their change with transition energy is almost identical.

Optical Control of Coherent Interactions between Electron Spins in InGaAs Quantum Dots

S. Spatzek, A.Greilich, SophiaE.Economou, S. Varwig, A. Schwan, D.R.Yakovlev, D.Reuter, A.D.Wieck, T. L.Reinecke, and M.Bayer Experimentelle Physik 2, Technische Universität Dortmund, D-44221 Dortmund, Germany Naval Research Laboratory, Washington, D.C. 20375, USA A. F. Ioffe Physico-Technical Institute, RAS, St. Petersburg, 194021 Russia and Angewandte Festkörperphysik, Ruhr-Universität Bochum, D-44780 Bochum, Germany (Dated: October 3, 2011)

Generation and detection of mode-locked spin coherence in (In,Ga)As/GaAs quantum dots by laser pulses of long duration

Using optical pulses of variable duration up to 80 ps, we report on spin coherence initialization and its subsequent detection in n-type singly-charged quantum dots, subject to a transverse magnetic field, by pump-probe techniques. We demonstrate experimentally and theoretically that the spin coherence generation and readout efficiencies are determined by the ratio of laser pulse duration to spin precession period: An increasing magnetic field suppresses the spin coherence signals for a fixed duration of pump and/or probe pulses, and this suppression occurs for smaller fields the longer the pulse duration is. The reason for suppression is the varying spin orientation due to precession during pulse action.

Non-resonant optical excitation of mode-locked electron spin coherence in (In,Ga)As/GaAs quantum dot ensemble

Two-color pump probe experiments reveal the possibility to use non-resonant pulsed laser excitation to create mode-locking of the ground state electron spin precessions in an ensemble of singly charged (In,Ga)As/GaAs quantum dots. The mode-locking shows a resonance for excitation into the first excited shell while for excitation into higher shells or barriers it disappears; however, spin coherence can still be induced. We conclude that the optically excited carriers relax spin-conserved from the p-shell into their ground states on a picosecond time scale, much shorter than the spin revolution period about the magnetic field.

Optical orientation of hole magnetic polarons in (Cd,Mn)Te/(Cd,Mn,Mg)Te quantum wells

The optically induced spin polarization in (Cd,Mn)Te/(Cd,Mn,Mg)Te diluted-magnetic-semiconductor quantum wells is investigated by means of picosecond pump-probe Kerr rotation. At 1.8 K temperature, additionally to the oscillatory signals from photoexcited electrons and Manganese spins precessing about an external magnetic field, a surprisingly long-lived (up to 60 ns) nonoscillating spin polarization is detected. This polarization is related to optical orientation of equilibrium magnetic polarons involving resident holes. The suggested mechanism for the optical orientation of the equilibrium magnetic polarons indicates that the detected polaron dynamics originates from unexcited magnetic polarons. The polaron spin dynamics is controlled by the anisotropic spin structure of the heavy-hole resulting in a freezing of the polaron magnetic moment in one of the two stable states oriented along the structure growth axis. Spin relaxation between these states is prohibited by a potential barrier, which depends on temperature and magnetic field. The magnetic polaron relaxation is accelerated with increasing temperature and in magnetic field.