Time-resolved Faraday Rotation Experiments Research Articles

Spin Dynamics of Charged and Neutral Excitons in Colloidal Nanocrystals

We review our studies on spin-dependent phenomena in various colloidal nanocrystals (quantum dots, dot-in-rod, and nanoplatelets). Several magneto-optical techniques are used in high magnetic fields: magnetic-field-induced polarization of neutral and charged excitons, spin-flip Raman scattering, and pump-probe Faraday rotation. We focus on spin properties and spin dynamics of neutral and charged excitons in order to identify them and measure g-factors and spin relaxation times. Pump-probe time-resolved Faraday rotation experiments with additional prepump allow us to measure coherent spin dynamics of carriers in CdS NCs at room temperature, and to study interplay between the negative and positive photocharging processes. We discuss the role of the dangling bond spins for recombination of the dark excitons and for formation of the dangling bond magnetic polaron.

Detection of large magnetoanisotropy of electron spin dephasing in a high-mobility two-dimensional electron system in a [001]GaAs∕AlxGa1−xAsquantum well

In time-resolved Faraday rotation experiments, we have detected an in-plane anisotropy of the electron spin-dephasing time (SDT) in an n-modulation-doped GaAs∕Al0.3Ga0.7As single quantum well. The SDT was measured with magnetic fields of B⩽1 T, applied in the [110] and [11 0] in-plane crystal directions of the GaAs quantum well. For fields along [11 0], we have found up to a factor of about 2 larger SDT than in the perpendicular direction. The observed SDTs also show strong anisotropic magnetic-field dependence. Fully microscopic calculations, by numerically solving the kinetic spin Bloch equations considering the D’yakonov-Perel’ and the Bir-Aronov-Pikus mechanisms, reproduce the experimental findings quantitatively. This quantitative analysis of the data allowed us to determine the relative strengths of Rashba and Dresselhaus terms in our sample. Moreover, we could predict the SDT for spins aligned in the [110] in-plane direction to be on the order of several nanoseconds, which is up to 2 orders of magnitude larger than that in the perpendicular in-plane direction.

Motional-Narrowing-Type Dephasing of Electron and Hole Spins of Itinerant Excitons in Magnetically Doped II-VI Bulk Semiconductors

Time-resolved optical spin-quantum-beat measurements performed on magnetically doped II-VI bulk semiconductors reveal an increase of the electron spin dephasing time with rising temperature typical for motional narrowing. With the dephasing being notably faster than in undoped II-VI semiconductors, the magnetic dopants must play a key role, modifying the known dephasing mechanisms and introducing new ones. Focusing on the latter, we theoretically explore the spin dephasing channel arising from magnetization fluctuations sampled by the itinerant excitons. This mechanism suffices to explain quantitatively the results of our time-resolved Faraday-rotation experiments on optically excited Cd(1-x)Mn(x)Te which we present here as a function of magnetic field, temperature and manganese dopant density. In addition to electron spin-quantum beats, some of our experiments reveal hole spin beats as well.

Spin transfer and coherence in coupled quantum wells

Spin dynamics of optically excited electrons confined in asymmetric coupled quantum wells are investigated through time resolved Faraday rotation experiments. The inter-well coupling is shown to depend on applied electric field and barrier thickness. We observe three coupling regimes: independent spin precession in isolated quantum wells, incoherent spin transfer between single-well states, and coherent spin transfer in a highly coupled system. Relative values of the inter-well tunneling time, the electron spin lifetime, and the Larmor precession period appear to govern this behavior.

Magneto-optical response of CdSe nanostructures

We present theoretical calculations of the Land\'e g-factors of semiconductor nanostructures using a time-dependent empirical tight-binding method. The eigenenergies and eigenfunctions of the band edge states are calculated as a function of an external magnetic field with the electromagnetic field incorporated into the tight-binding Hamiltonian in a gauge-invariant form. The spin-orbit interaction and magnetic field are treated non-perturbatively. The g-factors are extracted from the energy splitting of the eigenstates induced by the applied magnetic field. Both electron and hole g-factors are investigated for CdSe nanostructures. The size and aspect ratio dependence of g-factors is studied. We observe that the electron g-factors are anisotropic and find that the calculated values agree quantitatively with experimental data. We conclude that the two distinct g-factor values extracted from time resolved Faraday rotation experiments should be assigned to the anisotropic in plane ($g_\parallel$) and out of plane ($g_z$) electron g-factors, rather than to isotropic electron and exciton g-factors. We find that the anisotropy in the electron g-factor depends on the aspect ratio of the nanocrystal. The g-factor anisotropy derived from the wurtzite structure and from the non-unity aspect ratio may cancel each other in some regime. We observe that hole g-factors oscillate as a function of size, due to size dependent mixing between the heavy hole-light hole components of the valence band edge states. Extension to the calculation of exciton g-factors is discussed.

Ultrafast spin phenomena in highly excited n‐doped GaAs

We perform time-resolved Faraday rotation experiments on undoped and n-doped GaAs samples at high optical excitation densities. The Faraday rotation signal linearly increases with optical excitation density. The excitation dependence of the electron g-factor shows that many-body effects strongly affect spin phenomena in semiconductors.

Spin polarization of semiconductor carriers by reflection off a ferromagnet.

We present a theory of generation or alteration of the electron spin coherence and population in an n-doped semiconductor by reflection at the interface with a ferromagnet. The dependence of the spin reflection on the Schottky barrier height and the doping concentration in the semiconductor was computed for a generic model. The theory provides an explanation for the spontaneous electron spin coherence and nuclear polarization in the semiconductor interfaced with a ferromagnet and associated phenomena recently observed by time-resolved Faraday rotation experiments. The study also points to an alternative approach to spintronics different from spin injection.

All-optical magnetic resonance in semiconductors

A scheme is proposed wherein nuclear magnetic resonance (NMR) can be induced and monitored using only optical fields. In analogy to radio-frequency fields used in traditional NMR, circularly polarized light creates electron spins in semiconductors whose hyperfine coupling could tip nuclear moments. Time-resolved Faraday rotation experiments were performed in which the frequency of electron Larmor precession was used as a magnetometer of local magnetic fields experienced by electrons in n-type gallium arsenide. Electron spin excitation by a periodic optical pulse train appears not only to prepare a hyperpolarized nuclear moment but also to destroy it resonantly at magnetic fields proportional to the pulse frequency. This resonant behavior is in many ways supportive of a simple model of optically induced NMR, but a curious discrepancy between one of the observed frequencies and classic NMR values suggests that this phenomenon is more complex.

Optical spin resonance and transverse spin relaxation in magnetic semiconductor quantum wells

Ultrafast optical pulses are used to initiate and measure free-induction decays of coherent conduction electron spins and of embedded magnetic ${\mathrm{Mn}}^{2+}$ ions in a series of magnetic-semiconductor quantum wells. These time-resolved Faraday rotation experiments in transverse applied magnetic fields complement previous studies of spin dynamics in longitudinal fields by unambiguously distinguishing between the spin relaxation of electrons and holes, and by identifying a mechanism by which angular momentum is transferred from spin-polarized carriers to the sublattice of local moments. In transverse fields (Voigt geometry), the precession of the photoexcited spins about the field axis can be measured as an oscillatory induced Faraday rotation signal. We observe the THz free-induction decay of spin-polarized electrons in modest (4 T) magnetic fields and separately identify the more rapid spin relaxation of the holes as functions of field and temperature. The $g$ factors of the electrons and holes are accurately measured as a function of well width. The role of quantum confinement on the stability of the hole spin is discussed, with particular attention given to the observed ability of the transient hole-exchange field to coherently rotate a macroscopic ensemble of local ${\mathrm{Mn}}^{2+}$ moments. This ``tipping pulse'' initiates a free-induction decay in the sublattice of ${\mathrm{Mn}}^{2+}$ spins and enables electron paramagnetic resonance (EPR) studies of the fractional monolayer magnetic planes. These time-domain EPR measurements reveal a significant magnetic field dependence of the Mn transverse spin relaxation time.