We report dynamics of the transient polar Kerr rotation (KR) and of the transient reflectivity induced by femtosecond laser pulses in ferromagnetic (Ga,Mn)As with no external magnetic field applied. It is shown that the measured KR signal consist of several different contributions, among which only the oscillatory signal is directly connected with the ferromagnetic order in (Ga,Mn)As. The origin of the light-induced magnetization precession is discussed and the magnetization precession damping (Gilbert damping) is found to be strongly influenced by annealing of the sample.Comment: 6 pages, 4 figures. accepted in Applied Physics Letter

We used time-resolved Kerr rotation technique to study the electron spin coherence in a comprehensive set of bulk CdTe samples with various concentrations of electrons that were supplied by n-type doping. The electron spin coherence time of 40 ps was observed at temperature of 7 K in p-type CdTe and in n-type CdTe with a low concentration of electrons. The increase in the concentration of electrons leads to a substantial prolongation of the spin coherence time, which can be as long as 2.5 ns at 7 K in optimally doped samples, and to a modification of the g factor of electrons. The influence of the concentration of electrons is the most pronounced at low temperatures but it has a sizable effect also at room temperature. The optimal concentration of electrons to achieve the longest spin coherence time is 17 times higher in CdTe than in GaAs and the maximal low-temperature value of the spin coherence time in CdTe is 70 times shorter than the corresponding value in GaAs. Our data can help in cross checking the predictions of various theoretical models that were suggested in literature as an explanation of the observed nonmonotonous doping dependence of the electron spin coherence time in GaAs.

Exciton spin dynamics in quasi-spherical CdS quantum dots is studied in detail experimentally and theoretically. Exciton states are calculated using the 6-band k.p Hamiltonian. It is shown that for various sets of Luttinger parameters, when the wurtzite lattice crystal field splitting and Coulomb interaction between the electron-hole pair are taken into account exactly, both the electron and hole wavefunction in the lowest exciton state are of S-type. This rules out the spatial-symmetry-induced origin of the dark exciton in CdS quantum dots. The exciton bleaching dynamics is studied using time- and polarization-resolved transient absorption technique of ultrafast laser spectroscopy. Several samples with a different mean size of CdS quantum dots in different glass matrices were investigated. This enabled the separation of effects that are typical for one particular sample from those that are general for this type of material. The experimentally determined dependence of the electron spin relaxation rate on the radius of quantum dots agrees well with that computed theoretically.Comment: 24 pages, 10 figure

Spin relaxation in undoped quasi-spherical CdS quantum dots at zero magnetic fields is investigated using time-and polarization-resolved transient absorption measurements. Unlike in previous studies of these systems, the measured signals were corrected for spin-insensitive contributions to the exciton bleaching dynamics, allowing us to determine the pure spin-related exciton dynamics. To explain the observed room-temperature spin-relaxation time of several nanoseconds, we propose a novel mechanism based on intralevel exciton transitions with the emission of one LO phonon, the absorption of another LO phonon, and an electron spin flip, which is driven by the electronhole exchange interaction. The transition rates, calculated in the present work for different sizes of quantum dots and temperatures, are in fair agreement with our experimental results. PACS numbers: 72.25.Rb, 78.47.+p, 72.25.Fe, 78.67.Hc, 78.55.Et Spin relaxation and decoherence in quantum dots (QDs) have attracted increasing attention over the last few years. This is i.a. due to the long spindecoherence times observed in QDs, which have led to suggestions about their possible applications in quantum computation 1,2 . The underlying mechanisms of spin relaxation in semiconductor QDs are still being debated (see Ref.3 for a recent review). The term "quantum dot" is actually used for a variety of different objects. While most device concepts assume highly symmetric QDs, real QDs are usually strongly anisotropic. For example, the most often investigated self-assembled QDs have a base elongated along the [110] axis 4 , and QDs formed by interface fluctuations in narrow quantum wells are elongated along the [110] axis 5 . As a result, the anisotropic exchange interaction splits the | ± 1 radiative exciton doublet, with opposite total angular momentum projections, into two linearly polarized eigenstates |X = (|1 + | − 1 ) / √ 2 and |Y = (|1 − | − 1 ) /(i √ 2). On the other hand, chemically synthesized QDs 6,7,8,9 are nearly spherical, retaining higher symmetry, and therefore the exciton states | ± 1 are degenerate eigenstates of the Hamiltonian. Research on chemically synthesized quasi-spherical QDs is motivated also by their possible use as building blocks for constructing artificial solids of semiconductor QDs in the bottom-up self-assembly approach 10 . However, there exist only few reports on spin relaxation in quasi-spherical QDs. Measurements of differential transmission at zero magnetic field in neutral QDs revealed rather small circular polarization of the signal, from which only qualitative conclusions could be drawn 6,11 . The results retrieved from the decay of the Faraday rotation indicate that the spin dynamics is considerably slower in quasi-spherical QDs 6,7,8 than in anisotropic QDs 12,13,14 . However, as the signal of Faraday rotation contains also contributions from the spininsensitive exciton dynamics, these experiments cannot be used for the precise determination of the electron spinrelaxation time T 1 in quasi-spherical QDs. The maj...

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