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We review on time and spatially resolved two-color pump–probe magneto-optical Kerr effect (MOKE) microscopy studies of electron spins in bulk n-GaAs and GaAs (110) quantum wells (QWs) at low lattice temperatures. The influence of photocarrier heating by above-bandgap optical spin excitation on the spatially resolved magneto-optical spin detection is considered in detail. We demonstrate that a continuous-wave (cw) measurement of the local Kerr rotation at a fixed arbitrary probe wavelength in general does not correctly reveal the local spin polarization when hot electrons are present. For bulk GaAs we determine the true lateral electron spin polarization profile from cw measurements of the spatial dependence of the full excitonic Kerr rotation spectrum. For the (110) QWs, we directly obtain the electron spin diffusion coefficient from picosecond real space imaging of the time evolution of an optically excited electron spin packet, which we observe with a spectrally broad probe pulse.

We summarize experimental studies on the optical polarization properties of MBE-grown shape-asymmetric self-assembled CdSe/ZnSe quantum dots (QDs). The zero magnetic field fine structure splitting of excitonic transitions in structurally anisotropic QDs gives rise to an effective linear-to-circular and circular-to-linear optical polarization conversion upon resonant excitation of the QD ensemble. The effect is qualitatively modelled in terms of a pseudospin formalism. Furthermore, we show how an in-plane magnetic field leads to a complex rotation behavior of the axis of the optical polarization. Our analysis based on the exciton pseudospin Hamiltonian unambiguously displays that these effects are induced by isotropic and anisotropic contributions to the heavy hole Zeeman term. Likewise we show that the latter mechanism can be used to tune the optical respone of shape-anisotropic QDs such that the optical polarization properties are equal to QDs of perfect D2d symmetry. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation and properties of quantum dots (QDs) and related nanostructures by conventional molecular beam epitaxy (MBE) and two other variants of the technique, have been systematically investigated. While MBE growth of CdSe on ZnSe at 300 °C results in the formation of rough CdSe quantum-well-like structures, by one of the variants, which combines epitaxial growth of a CdSe layer at 230 °C and subsequent in-situ annealing at 310-340 °C, tiny but distinct three-dimensional (3D) QDs are formed. We demonstrate that QD formation by this method, developed originally by Rabe et al. (J. Cryst. Growth 184/185, 248 (1998) [30]), essentially takes place during the annealing step, by a process of “controlled roughening” of the as-grown quasi-two-dimensional (2D) CdSe layer. In comparison to conventional MBE, this process results in reduction of the areal density of the self-organized QDs by up to two orders of magnitude, thus making them distinctly discernible. In the second variant technique, similar to that applied originally for the self-organization of CdTe/ZnTe quantum dots by Tinjod et al. (Appl. Phys. Lett. 82, 4340 (2003) [34], QD-formation takes place as a result of deposition of an amorphous group-VI layer onto a strained 2D CdSe layer at ∼ 50 °C, and its subsequent re-desorption at temperatures of 280-320 °C. While deposition and desorption of amorphous Se leads to the formation of large isolated QDs, we show that use of amorphous Te results in the self-assembly and ordering of a rich variety of nanostructures. Luminescence, morphology, and composition of QDs and the underlying formation mechanism of the three processes are discussed in detail. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Resonant photoluminescence and photoluminescence excitation spectroscopy have been applied to CdSe/ZnSe quantumdots where separation between the quantized levels is close to the optical phonon energy. Astrong relaxation enhancement induced by the optical phonon is observed. The fine structure of the phonon replicas is resolved. It is attributed to the optical phonons with large wave vectors from the symmetrical points of the Brillouin zone. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present comprehensive studies of in-plane optical anisotropy in CdSe/ZnSe quantum dot ensembles. The anisotropy reveals itself as circular-to-linear polarization conversion and as an alignment occurring at resonant excitation without any applied magnetic fields. The variation of the anisotropy in different dots has been analyzed based on angle scans. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on spin memory relaxation time τ s of free electrons in CdTe-based quantum wells which is found to be in the nanosecond range. In these studies two different techniques have been exploited: (i) the Hanle effect under cw excitation and (ii) the time-resolved pump-&-probe Kerr rotation measured under excitation by 1.8-ps optical pulses. These independent techniques give very close results τ s =19 ns and τ s = 14 ns, respectively. To our knowledge this is by two orders of magnitude longer than the electron spin relaxation times reported for CdTe-based quantum wells so far.