Time-resolved Faraday Rotation Measurements Research Articles

Phase effects due to previous pulses in time-resolved Faraday rotation measurements

Time-resolved Faraday rotation measurements have proved transformative in the investigation of spin dynamics in semiconductors. In materials with spin lifetimes which are on the order of, or greater than, the laser repetition time, the collective effect of spin polarization due to the whole pump pulse train becomes important. Here, we discuss a relative phase shift which results from these spins. We derive and experimentally validate a closed-form expression which describes this phase shift and characterize it throughout parameter space. A spin lifetime measurement based on this phase shift is described, and we discuss situations in which the model used must be augmented to be applicable.

Highly spin-polarized carrier dynamics and ultralarge photoinduced magnetization in CH3NH3PbI3 perovskite thin films.

Low-temperature solution-processed organic-inorganic halide perovskite CH3NH3PbI3 has demonstrated great potential for photovoltaics and light-emitting devices. Recent discoveries of long ambipolar carrier diffusion lengths and the prediction of the Rashba effect in CH3NH3PbI3, that possesses large spin-orbit coupling, also point to a novel semiconductor system with highly promising properties for spin-based applications. Through circular pump-probe measurements, we demonstrate that highly polarized electrons of total angular momentum (J) with an initial degree of polarization Pini ∼90% (i.e., -30% degree of electron spin polarization) can be photogenerated in perovskites. Time-resolved Faraday rotation measurements reveal photoinduced Faraday rotation as large as 10°/μm at 200 K (at wavelength λ = 750 nm) from an ultrathin 70 nm film. These spin polarized carrier populations generated within the polycrystalline perovskite films, relax via intraband carrier spin-flip through the Elliot-Yafet mechanism. Through a simple two-level model, we elucidate the electron spin relaxation lifetime to be ∼7 ps and that of the hole is ∼1 ps. Our work highlights the potential of CH3NH3PbI3 as a new candidate for ultrafast spin switches in spintronics applications.

Room-Temperature Electron Spin Generation by Femtosecond Laser Pulses in Colloidal CdS Quantum Dots.

We present an experimental investigation of optical spin orientation in colloidal CdS quantum dots (QDs) by a femtosecond laser pulse at room temperature. The spin carrier and its spin-generation process are clarified. Firstly, the observed spin signals of CdS QDs in time-resolved Faraday rotation measurements are shown to belong to electron carriers, by comparing the spin dephasing dynamics and Landé g factor between CdS QDs and bulk materials. Secondly, spin dynamics unaffected by the faster carrier recombination suggests that the spin-polarized electrons are not photoexcited but resident in the dots. Moreover, hole spins should dephase very fast compared with electron spins, otherwise the trion (two electrons with opposite spin orientations and one hole) recombination process will affect the resident electron spin signals. The electron spin is generated in a short time of which the excitation light is absorbed and the resident electron is excited to trion states, i.e., of pulse durations. Due to fast hole spin dephasing, trion recombination gives null spin signals, and the subsequent electron spin dynamics is controlled by its intrinsic mechanisms.

Ultrafast spin dynamics in optically excited bulk GaAs at low temperatures

This paper presents a study of electron spin dynamics in bulk GaAs at low temperatures for elevated optical excitation conditions. Our time-resolved Faraday rotation measurements yield sub-nanosecond electron spin dephasing-times over a wide range of n-doping concentrations in quantitative agreement with a microscopic treatment of electron spin dynamics. The calculation shows the occurrence and breakdown of motional narrowing for spin dephasing under elevated excitation conditions. We also find a peak of the spin dephasing time around a doping density for which, under lower excitation conditions, a metal-insulator transition occurs. However, the experimental results for high excitation can be explained without a metal-insulator transition. We therefore attribute the peak in spin-dephasing times to the influence of screening and scattering on the spin-dynamics of the excited electrons.

Electron-Spin Beat Susceptibility of Excitons in Semiconductor Quantum Wells

Recent time-resolved differential transmission and Faraday rotation measurements of long-lived electron-spin coherence in quantum wells displayed intriguing parametric dependencies. For their understanding we formulate a microscopic theory of the optical response of a gas of optically incoherent excitons whose constituent electrons retain spin coherence, under a weak magnetic field applied in the quantum well's plane. We define a spin beat susceptibility and evaluate it in linear order of the exciton density. Our results explain the many-body physics underlying the basic features observed in the experimental measurements.

Time-Resolved Study of Surface Spin Effect on Spin−Lattice Relaxation in Fe3O4 Nanocrystals

The role of surface spins in the relaxation of magnetization in optically excited Fe(3)O(4) nanocrystals has been investigated via time-resolved Faraday rotation measurements. The time scale of the magnetization recovery following the optically induced demagnetization increased from 250 to 350 ps as the size of the nanocrystals increased from 5 to 15 nm. From analysis of the data with a simple model relating the spin-lattice relaxation rate to the average spin-orbit coupling strength of the interior and surface spins, we estimated the relative efficiency of surface in spin-lattice relaxation with respect to the interior spins. Our analysis indicates that, in Fe(3)O(4) nanocrystals passivated with oleic acid, the surface is 3 times more efficient in spin-lattice relaxation than the interior.

Ultrafast Spin Dynamics in Colloidal ZnO Quantum Dots

Time-resolved Faraday rotation measurements in the ultraviolet have been performed to reveal the ultrafast spin dynamics of electrons in colloidal ZnO quantum dots. Oscillating Faraday rotation signals are detected at frequencies corresponding to an effective g factor of g = 1.96. Biexponential oscillation decay is observed that is due to (i) rapid depopulation of the fundamental exciton (tau = 250 ps) and (ii) slow electron spin dephasing ( T 2 = 1.2 ns) within a metastable state formed by hole-trapping at the quantum dot surface.

Size-Dependent Ultrafast Magnetization Dynamics in Iron Oxide (Fe3O4) Nanocrystals

Optically induced ultrafast demagnetization and its recovery in superparamagnetic colloidal iron oxide (Fe3O4) nanocrystals have been investigated via time-resolved Faraday rotation measurements. Optical excitation with near-infrared laser pulse resulted in ultrafast demagnetization in approximately 100 fs via the destruction of ferrimagnetic ordering. The degree of demagnetization increased with the excitation density, and the complete demagnetization reached at approximately 10% excitation density. The magnetization recovered on two time scales, several picoseconds and hundreds of picoseconds, which can be associated with the initial reestablishment of the ferrimagnetic ordering and the electronic relaxation back to the ground state, respectively. The amplitude of the slower recovery component increased with the size of the nanocrystals, suggesting the size-dependent ferrimagnetic ordering throughout the volume of the nanocrystal.

Ultrafast optical studies of diffusion barriers between ferromagnetic Ga(Mn)As layers and non-magnetic quantum wells

In recent years, ferromagnetic Ga(Mn)As has emerged as a highly interesting material for semiconductor spintronics. One possible application is to use Ga(Mn)As as an injector layer to inject spin-polarized carriers into a non-magnetic semiconductor heterostructure. As Ga(Mn)As layers are typically grown at much lower substrate temperatures than high-mobility GaAs heterostructures, a combination of both requires that the ferromagnetic layer is grown last. We have prepared samples by molecular beam epitaxy which consist of two quantum wells (QWs) of different widths grown at high substrate temperature. The upper QW is separated by a thin barrier (few nm) from a ferromagnetic Ga(Mn)As layer grown at low substrate temperature, while the lower QW is widely separated (more than 100 nm) from the Ga(Mn)As. We observe that the photoluminescence of the upper QW is red-shifted and partially quenched as compared to a control sample without a Ga(Mn)As layer, and time-resolved Faraday rotation measurements reveal that the spin lifetime in the upper QW is up to 50 times longer than the one in the lower QW. We attribute these observations to Mn back-diffusion into the upper QW during sample growth. Both, the PL and the Faraday rotation technique, are highly sensitive to small quantities (below 0.05%) of Mn and allow us to study the effectiveness of different types (e.g., a short-period superlattice) and thicknesses of barrier layers in suppressing Mn diffusion.

Large in-plane spin-dephasing anisotropy in a [0 0 1]-grown two-dimensional electron system

A large anisotropy in the spin dephasing times for the different in-plane directions of [0 0 1] quantum wells has been predicted in calculations by Averkiev and Golub [Phys. Rev. B 60 (1999) 15582] for systems with both Rashba and Dresselhaus spin-orbit fields. Here, we present time-resolved Faraday rotation measurements performed in the Voigt geometry on a high-mobility 2D electron system grown on a [0 0 1] substrate. An out-of-plane spin polarization is created in the sample by a circularly polarized pump pulse. The magnetic field applied in the sample plane forces the spins to precess perpendicular to it. For a magnetic field applied along [1 1 0], the spins precess into the [ 1 1 ¯ 0 ] direction, which is in the sample plane, for a magnetic field along [ 1 1 ¯ 0 ] , the spins precess into the [1 1 0] direction. The experimentally determined spin dephasing times for these two different cases differ by about 60%, demonstrating that the predicted anisotropy is present in our sample.

Time-resolved Faraday rotation measurements of spin relaxation in InGaAs∕GaAs quantum dots: Role of excess energy

The authors report measurements of room temperature spin dynamics in InGaAs quantum dots using time-resolved differential transmission and Faraday rotation techniques. They observe an enhancement of the electron spin lifetime by an order of magnitude for direct optical pumping of the quantum dot ground state compared to optical pumping of the GaAs barriers. These findings indicate that the optical excitation conditions can have a critical influence on the spin kinetics, a result which may account for the wide variation of spin lifetimes reported to date. The enhancement in spin lifetime observed here is attributed to the reduction of phonon-mediated spin-flip scattering.

Initialization and read-out of spins in coupled core–shell quantum dots

In the field of quantum information science, semiconductor quantum dots (QDs) are of particular interest for their ability to confine a single electron for use as a qubit1,2. However, to realize the potential offered by quantum information processing, it is necessary to couple two or more qubits. In contrast to coupling individual QDs, we demonstrate the integration of two coupled electronic states within a single QD heterostructure. These chemically synthesized nanocrystals, known as quantum-dot quantum wells (QDQWs)3,4,5,6,7, comprise concentric layers of different semiconducting materials. We investigate carrier and spin dynamics in these structures using transient absorption and time-resolved Faraday rotation measurements. By tuning the excitation and probe energies, we find that we can selectively initialize and read out spins in different coupled states within the QDQW. These results open a pathway for engineering coupled qubits within a single nanostructure.

Energy-resolved electron spin dynamics at surfaces ofp-doped GaAs

Electron-spin relaxation at different surfaces of $p$-doped GaAs is investigated by means of spin, time, and energy-resolved two-photon photoemission. These results are contrasted with bulk results obtained by time-resolved Faraday rotation measurements as well as calculations of the Bir-Aronov-Pikus spin-flip mechanism. Due to the reduced hole density in the band bending region at the (100) surface the spin-relaxation time increases over two orders of magnitude towards lower energies. At the flat-band (011) surface a constant spin relaxation time in agreement with our measurements and calculations for bulk GaAs is obtained.

Hysteretic dynamic nuclear polarization inGaAs/AlxGa1−xAs(110) quantum wells

We investigated the dynamics of a coupled electron-nuclear spin system in a GaAs/AlGaAs (110) quantum well by time-resolved Faraday rotation measurements. Photoexcited electron spins with long coherence time ${(T}_{2}^{*}=6\mathrm{ns})$ are strongly subject to a nuclear field as well as polarizing the nuclei. A hysteretic behavior was observed in the magnetic-field dependence of the nuclear polarization under optical orientation. Our quantitative analysis based on self-consistent calculations reproduced the experimental observation of the bistable configurations of dynamic nuclear polarization.

Magneto-optical spin spectroscopy in hybrid ferromagnetic semiconductor heterostructures

We report on magneto-optical measurements of diluted magnetic semiconductor quantum well structures in which an iron film is epitaxially grown and subsequently patterned on top of the heterostructure. The magnetic field due to the iron penetrates the quantum well near the edges of the patterned regions. Photoluminescence, Hanle effect, and time-resolved Faraday rotation measurements demonstrate that the films can be processed without significantly altering the electronic properties of the quantum well. The smallest length scale probed in these far-field optical measurements is of order 10 μm, so that no effects of the local magnetic fields at the film edges are observed for structures with 50-nm-thick ferromagnetic layers.

Dynamics of photoinduced Faraday rotation in semimagnetic semiconductors

Time-resolved Faraday rotation measurements on a nanosecond time scale are performed for Cd1−xMnxTe samples grown by two different techniques. The dependence of the observed photoinduced dynamics on the purity, Mn++ concentration (x=0.03, 0.10, 0.15, 0.3), excitation density, and temperature (100–290 K) reveals several novel features. The relaxation for pure samples is monoexponential at 100 K and is slowing down with increasing temperature, in strong contrast to impure samples, where the opposite behavior is observed. All experimental observations can be well explained in a simple model based on rate equations for the free-carrier populations, provided that recombination centers and impurity traps are included in the model.

Time-resolved Faraday rotation spectroscopy of spin dynamics in digital magnetic heterostructures

A time-resolved resonant Faraday rotation spectroscopy is employed to study the dynamical interplay between local magnetic moments and photoexcited carrier spins in quantum-confined semiconductor geometries. This highly sensitive technique functions as an energy selective, noninvasive, all-optical probe of spin dynamics ranging from femtosecond to microsecond timescales and is particularly suited to low-dimensional systems having small numbers of magnetic spins. Carrier spin-scattering rates, lifetimes, and the orientation and relaxation of perturbed magnetic ions are directly observed in the time domain. The utility of this technique is demonstrated through the study of a newly developed class of magnetic heterostructure, in which fractional monolayer planes of magnetic Mn/sup 2+/ ions are incorporated digitally into nonmagnetic II-VI ZnSe-ZnCdSe quantum wells. These digital magnetic heterostructures (DMH) possess large g-factors and exhibit enormous low-field resonant Faraday rotations in excess of 1.7/spl times/10/sup 7/ deg/T/spl middot/cm at low temperatures. Time-resolved Faraday rotation measurements identify a wealth of unexpected electronic and magnetic spin dynamics that are different from those generated in traditional semiconductors or alloyed diluted magnetic semiconductor structures.