A spatially indirect exciton (IX) can be formed when an electron and a hole are confined in two different spatially separated coupled quantum wells (CQWs). The spatial separation of an electron and a hole wavefunctions can be engineered to control the spin lifetimes of IXs. In GaAs/AlGaAs CQWs, IX spin relaxation time up to tens of nanoseconds have been recently reported [1,2]. However, the measurements of IX spin dynamics by nonlinear spectroscopy are challenging, due to low oscillator strength of the IX state. We demonstrate a proof-of-concept for time-resolved pump-probe spectroscopy of IXs in CQWs. Pump and probe light pulses are resonant with optically active direct exciton (DX) transition, and IXs are probed via their common ground state with DXs (Fig.1a). Photoinduced reflectivity measurements show that the IX lifetime time in biased CQWs is of order of 30 ns at Vg=0.8 V, consistent with the PL kinetics measurments [3]. Pump-probe experiments allow unraveling DX, IX and electron spin dynamics. It appears that not only DX, but also bare electron spin relaxation is much faster (≃ 200 ps) than one of IXs (up to 10 ns).

We report on time-resolved Kerr rotation (TRKR) experiments done on n-doped CdMnTe quantum wells (QWs), in the regime where strong coupling between the electron and the Mn spin-flip excitations shows up. It has been proposed previously to deduce the 2D electron gas spin susceptibility from the coupling energy between these spin excitations. Here we measure the coupling energy on a high mobility sample down to very low excitation density, and compare the results with spin-flip Raman scattering (SFRS) on the same sample. The electron spin polarizations measured by TRKR and SFRS are found in relatively good agreement. However the spin susceptibility measured by TRKR exceeds systematically the values predicted by many-body theory. This could be an indication that the two-oscillator model used to describe mixed electron-Mn spin excitations needs to be improved.

In various manganese-based II-VI dilutedmagnetic semiconductors and their quantum structures, the measured variations of electron spin dephasing time in an external magnetic field are conflicting with the most advanced spin relaxation theory based on quantum kinetic equations. We demonstrate, by time-resolved optical spin beat measurements performed on high-mobility n-doped CdMnTe quantum wells, that these contradictions are resolved if one takes into account the electron spin dephasing induced by laser heating of the manganese spins.We then provide a test of the spin relaxation theory inMn-based quantum wells by a careful comparison with existing data, including our measurements. It turns out that the theory systematically underestimates the relaxation rates by a factor of 5.

We report on the nondestructive measurement of nuclear magnetization in n-GaAs via cavity enhanced Faraday rotation. In contrast with the existing optical methods, this detection scheme does not require the presence of detrimental out-of-equilibrium electrons. Specific mechanisms of the Faraday rotation are identified for (i) nuclear spins situated within the localized electron orbits, these spins are characterized by fast dynamics, (ii) all other nuclear spins in the sample characterized by much slower dynamics. Our results suggest that even in degenerate semiconductors nuclear spin relaxation is limited by the presence of localized electron states and spin diffusion, rather than by Korringa mechanism.

Nuclear spin dynamics in semiconductors has been studied extensively by optical means for more than forty years. Yet generally the existing techniques require the injection of out of equilibrium carriers in the semiconductor in order to probe the nuclear polarization, which in turn disturbs the nuclei to be measured. This is the case for photoluminescence experiments, but also for time resolved Faraday/Kerr rotation, in which a shift in the electron Larmor frequency is induced by the nuclear field.1 In this work, we demonstrate a non-destructive detection of nuclear magnetization in n-GaAs via cavity enhanced Faraday rotation of an off-resonant light beam. We use a two-step protocol, as proposed by Artemova and Merkulov2, where preparation and measurement are separated in time. The nuclear spins are first cooled down by optical pumping, then the pump beam is switched off and the subsequent evolution of nuclear polarization is detected by the Faraday rotation of a below band gap probe beam. This experimental protocol applied to metallic GaAs just above the metal-insulator transition reveals a non- monotonous evolution of the nuclear induced Faraday rotation. We interpret this behavior as a consequence of the existence of two contributions in the Faraday rotation, which are related to optical transitions involving either localized or delocalized states. Therefore, this detection scheme turns out to be sensitive to nuclear spins situated within the localization radius of bound electron states, as well as to all other nuclear spins in the sample. The contributions to the Faraday rotation of different nuclear species with different localizations in the sample are identified in nuclear magnetic resonance experiments. Our results suggest that the nuclear relaxation in metallic n-GaAs is dominated by diffusion towards fast relaxing centers, rather than by the Korringa mechanism.

We report on the nondestructive measurement of nuclear magnetization in n-GaAs via cavity enhanced Faraday rotation. In contrast with the existing optical methods, this detection scheme does not require the presence of detrimental out-of-equilibrium electrons. Specific mechanisms of the Faraday rotation are identified for (i) nuclear spins situated within the localized electron orbits, they are characterized by fast dynamics, (ii) all other nuclear spins in the sample characterized by much slower dynamics. Our results suggest, that even in degenerate semiconductors nuclear spin relaxation is limited by the presence of localized electron states and spin diffusion, rather than by Korringa mechanism.

Mn spin dynamics is measured in very diluted (Cd,Mn)Te crystals by time-resolved Kerr rotation. Spin beats due to the hyperfine interaction between the 3d electrons of the Mn ions and their own nuclei are detected. It is shown that the effect of the crystal field can be strongly suppressed for "magic" orientations of the magnetic field. This particular orientation of the field permits the optical read-out of the Mn nuclear spin state. Manganese ions trapped on a semiconductor lattice have uniform properties and relatively long spin lifetimes, which make them promising for optical manipulation. In particular, Mn2+ ions embedded in a II-VI semiconductor are S-state ions, weakly coupled to the lattice. For this reason quite long Mn electronic spin relaxation times are expected and have been observed [1]. Here we show that the spin coherence time is mainly limited by dipole-dipole interactions at a Mn concentration x=0.001, and reaches up to 15 ns. At this low concentration, fine and hyperfine structures of Mn2+ are resolved in electron spin resonance experiments. In the time-domain it corresponds to low-frequency beats as shown in Fig. 1.