We report on the unusual and counterintuitive behavior of spin lifetime of excitons in coupled semiconductor quantum wells (CQWs) in the presence of in-plane magnetic field. Instead of conventional acceleration of spin relaxation due to the Larmor precession of electron and hole spins, we observe a strong increase of the spin relaxation time at low magnetic fields followed by saturation and decrease at higher fields. We argue that this nonmonotonic spin relaxation dynamics is a fingerprint of the magnetic quantum confined Stark effect. In the presence of electric field along the CQW growth axis, an applied magnetic field efficiently suppresses the exciton spin coherence, due to inhomogeneous broadening of the g-factor distribution.

Spin noise spectroscopy is an optical technique which can probe spin resonances non-perturbatively. First applied to atomic vapours, it revealed detailed information about nuclear magnetism and the hyperfine interaction. In solids, this approach has been limited to carriers in semiconductor heterostructures. Here we show that atomic-like spin fluctuations of Mn ions diluted in CdT e (bulk and quantum wells) can be detected through the Kerr rotation associated to excitonic transitions. Zeeman transitions within and between hyperfine multiplets are clearly observed in zero and small magnetic fields and reveal the local symmetry because of crystal field and strain. The linewidths of these resonances are close to the dipolar limit. The sensitivity is high enough to open the way towards the detection of a few spins in systems where the decoherence due to nuclear spins can be suppressed by isotopic enrichment, and towards spin resonance microscopy with important applications in biology and materials science.

Spin noise spectroscopy (SNS) is an attractive optical technique, which in recent years has been widely used to probe the carrier spin relaxation in various semiconductors and heterostructures [1]. Typically SNS probes spontaneous spin fluctuations by off-resonant Faraday rotation. Spin noise spectra have to be equivalent to electron spin resonance spectra, by virtue of the fluctuation-dissipation theorem, and are quite interesting for quantum non-demolition measurements of either atomic or electronic spins [2]. Here we show that spin fluctuations of magnetic impurities in semiconductors can also be efficiently detected by SNS. This is demonstrated for manganese spins diluted in CdTe. Mn ions possess an electronic spin S=5/2, and a nuclear spin I=5/2. Thus one expects that the spin noise spectra will exhibit spectral signatures associated with the hyperfine interaction, and a dependence on the magnetic field direction dictated by the point-symmetry of the Mn ion in the lattice. This is illustrated in Figure 1, where contour plots of the spin noise spectra are shown for different magnetic field orientations in the (1-10) plane of the sample, q being the angle between the applied field and the [001] crystal axis. The spectra measured on bulk Cd0:999Mn0:001Te (Fig. 1b) exhibit many of the spectral features predicted within the spin hamiltonian approximation (Fig. 1a). Fig. 1c shows that quite good agreement between theory and experiment is obtained for a Mn spin coherence time T2 = 20 ns, which is the only fitting parameter. We will present our results on two CdMnTe quantum wells grown in coherent epitaxy on either CdTe or Cd0:96Zn0:04Te substrate. The first QW is almost lattice matched to the substrate and nearly unstrained, while the other is unmatched and strained. About 4106 Mn spins are detected in the spot size of 5 μm. We will show that, because of the hyperfine coupling between an electronic spin and a nuclear spin with the same value (5/2), below 15 mT Mn ions exhibit a Zeeman effect with an effective g-factor equal 1, rather than 2. The Mn spin noise spectra appear to be quite sensitive to the presence of local strains, and gives insights into the strain distribution in the QWs. Finally, we will discuss the possible spin relaxation mechanisms, which ultimately limit the Mn spin relaxation time. This work is supported by the French ANR research project SNS (Grant N° 2011-BS04-018 01).

Spin noise spectroscopy (SNS) is an attractive optical technique, which in recent years has been widely used to probe the carrier spin relaxation in various semiconductors and heterostructures [1]. Typically SNS probes spontaneous spin fluctuations by off-resonant Faraday rotation. Spin noise spectra have to be equivalent to electron spin resonance spectra, by virtue of the fluctuation-dissipation theorem, and are quite interesting for quantum non-demolition measurements of either atomic or electronic spins [2]. Here we show that spin fluctuations of magnetic impurities in semiconductors can also be efficiently detected by SNS. This is demonstrated for manganese spins diluted in CdTe. Mn ions possess an electronic spin S=5/2, and a nuclear spin I=5/2. Thus one expects that the spin noise spectra will exhibit spectral signatures associated with the hyperfine interaction, and a dependence on the magnetic field direction dictated by the point-symmetry of the Mn ion in the lattice. This is illustrated in Figure 1, where contour plots of the spin noise spectra are shown for different magnetic field orientations in the (1-10) plane of the sample, q being the angle between the applied field and the [001] crystal axis. The spectra measured on bulk Cd0:999Mn0:001Te (Fig. 1b) exhibit many of the spectral features predicted within the spin hamiltonian approximation (Fig. 1a). Fig. 1c shows that quite good agreement between theory and experiment is obtained for a Mn spin coherence time T2 = 20 ns, which is the only fitting parameter. We will present our results on two CdMnTe quantum wells grown in coherent epitaxy on either CdTe or Cd0:96Zn0:04Te substrate. The first QW is almost lattice matched to the substrate and nearly unstrained, while the other is unmatched and strained. About 4106 Mn spins are detected in the spot size of 5 μm. We will show that, because of the hyperfine coupling between an electronic spin and a nuclear spin with the same value (5/2), below 15 mT Mn ions exhibit a Zeeman effect with an effective g-factor equal 1, rather than 2. The Mn spin noise spectra appear to be quite sensitive to the presence of local strains, and gives insights into the strain distribution in the QWs. Finally, we will discuss the possible spin relaxation mechanisms, which ultimately limit the Mn spin relaxation time. This work is supported by the French ANR research project SNS (Grant N° 2011-BS04-018 01).

We exploit the potential of the spin noise spectroscopy (SNS) for studies of nuclear spin dynamicsinn-GaAs. The SNS experiments were performed on bulkn-type GaAs layers embedded into ahigh-finesse microcavity at negative detuning. In our experiments, nuclear spin polarisation ini-tially prepared by optical pumping is monitored in real time via a shift of the peak position in theelectron spin noise spectrum. We demonstrate that this shift is a direct measure of the Overhauserfield acting on the electron spin. The dynamics of nuclear spin is shown to be strongly dependenton the electron concentration.

Indirect excitons in coupled quantum wells are long-living quasiparticles, explored in the studies of collective quantum states. We demonstrate that, despite the extremely low oscillator strength, their spin and population dynamics can by addressed by time-resolved pump-probe spectroscopy. Our experiments make it possible to unravel and compare spin dynamics of direct excitons, indirect excitons, and residual free electrons in coupled quantum wells. Measured spin relaxation time of indirect excitons exceeds not only one of direct excitons but also one of free electrons by two orders of magnitude.

Exciton recombination and spin dynamics in electrically biased semiconductor coupled quantum wells (CQWs) is dramatically slowed down by the formation of spatially indirect excitons (IX), consisting of electron and hole confined in different quantum wells. We report on time-resolved two-colors pump-probe Kerr rotation and reflectivity measurements in CQWs. Our results demonstrate that (i) Despite their vanishing oscillator strength, IXs possess remarkable nonlinear properties and induce measurable nolinear signal. (ii) IXs spin relaxation time of order of 10 ns is much longer than electron spin relaxation in the studied structure. (iii) Coherent precession of DX spin in the effective in-plane magnetic field due to linear mode splitting is washed out by the application of electrical bias: this is a direct evidence of the electron-hole exchange energy reduction.