Optical Orientation Experiments Research Articles

Driving a pure spin current from nuclear-polarization gradients

A pure spin current is predicted to occur when an external magnetic field and a linearly inhomogeneous spin-only field are appropriately aligned. Under these conditions (such as originate from nuclear contact hyperfine fields that do not affect orbital motion) a linear, spin-dependent dispersion for free electrons emerges from the Landau Hamiltonian. The result is that spins of opposite orientation flow in opposite directions giving rise to a pure spin current. A classical model of the spin and charge dynamics reveals intuitive aspects of the full quantum mechanical solution. We propose optical orientation or electrical polarization experiments to demonstrate this outcome.

Bismuth content dependence of the electron spin relaxation time in GaAsBi epilayers and quantum well structures

Time-resolved optical orientation experiments have been performed in dilute bismide structures. Bulk layers with bismuth fractions in the range 1%–3.8% and quantum wells with bismuth fractions in the range 2.4%–7% were investigated. A clear decrease of the electron spin relaxation time is evidenced in both cases when the bismuth content increases. These results can be well interpreted by the increased efficiency of the spin relaxation mechanisms due to the bismuth induced larger spin-orbit interaction in these alloys.

Optical orientation of spins in GaAs:Mn/AlGaAs quantum wells via impurity-to-band excitation

The paper reports optical orientation experiments performed in the narrow GaAs/AlGaAs quantum wells doped with Mn. We experimentally demonstrate a control over the spin polarization by means of the optical orientation via the impurity-to-band excitation and observe a sign inversion of the luminescence polarization depending on the pump power. The g factor of a hole localized on the Mn acceptor in the quantum well was also found to be considerably modified from its bulk value due to the quantum confinement effect. This finding shows the importance of the local environment on magnetic properties of the dopants in semiconductor nanostructures.

Control of the electron spin relaxation by the built-in piezoelectric field in InGaAs quantum wells

The electron spin dynamics is studied by time-resolved optical orientation experiments in strained InGaAs/GaAs quantum wells (QWs) grown on (111) or (001) substrates. For a given well width, the electron spin relaxation time in (111) InGaAs QWs decreases by an order of magnitude when the indium fraction in the well varies only from 4% to 12%. In contrast, the electron spin relaxation time depends weakly on the indium fraction in similar InGaAs quantum wells grown on (001) substrates. The strong variation of the electron spin relaxation time in (111) strained quantum well can be well interpreted by the Dyakonov-Perel spin relaxation mechanism where the conduction band spin-orbit splitting is dominated by the structural inversion asymmetry (Rashba term) induced by the piezoelectric field. In (001) QWs, due to the absence of piezoelectric field, the electron spin relaxation time is solely controlled by the Dresselhaus term. These results demonstrate the possibility to engineer the electron spin relaxation time in (111)-oriented quantum wells by the piezoelectric field induced by the built-in strain.

Gate control of the electron spin-diffusion length in semiconductor quantum wells

The spin diffusion length is a key parameter to describe the transport properties of spin polarized electrons in solids. Electrical spin injection in semiconductor structures, a major issue in spintronics, critically depends on this spin diffusion length. Gate control of the spin diffusion length could be of great importance for the operation of devices based on the electric field manipulation and transport of electron spin. Here we demonstrate that the spin diffusion length in a GaAs quantum well can be electrically controlled. Through the measurement of the spin diffusion coefficient by spin grating spectroscopy and of the spin relaxation time by time-resolved optical orientation experiments, we show that the diffusion length can be increased by more than 200% with an applied gate voltage of 5 V. These experiments allow at the same time the direct simultaneous measurements of both the Rashba and Dresselhaus spin-orbit splittings.

L-valley electron spin dynamics in GaAs

Optical orientation experiments have been performed in GaAs epilayers with photoexcitation energies in the 3 eV region, yielding the photogeneration of spin-polarized electrons in the satellite $L$ valley. We demonstrate that a significant fraction of the electron spin memory can be conserved when the electron is scattered from the $L$ to the $\ensuremath{\Gamma}$ valley following an energy relaxation of several hundreds of meV. Combining these high energy photoexcitation experiments with time-resolved photoluminescence spectroscopy of $\ensuremath{\Gamma}$-valley spin-polarized photogenerated electrons allows us to deduce a typical $L$-valley electron spin relaxation time of 200 fs, in agreement with theoretical calculations.

Electron spin control in dilute nitride semiconductors

We report on a study of spin-dependent recombination processes (SDR)for conduction band electrons on deep paramagnetic centers in a series ofGaAs1−yNy epilayers by time-resolved optical orientation experiments. We demonstrate that thisdilute nitride compound can be used as an effective electron spin filter undera polarized optical excitation of appropriate intensity. This optimum intensitycan moreover be controlled by adjusting the nitrogen composition in the layer.

Optical orientation of electron and nuclear spins in strain free GaAs quantum dots grown by droplet epitaxy

AbstractWe report optical orientation experiments in individual, strain free GaAs quantum dots in AlGaAs grown by modified droplet epitaxy. Circularly polarized optical excitation yields strong circular polarization of the resulting photoluminescence at 4 K. Optical injection of spin polarized electrons into the dot gives rise to dynamical nuclear polarization that considerably changes the exciton Zeeman splitting (Overhauser shift). We show that the created nuclear polarization is bistable and present a direct measurement of the build‐up time of the nuclear polarization in a single GaAs dot in the order of a second. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Theory of electron spin relaxation in ZnO

Doped ZnO is a promising material for spintronics applications. For such applications, it is important to understand the spin dynamics and particularly the spin relaxation times of this II-VI semiconductor. The spin relaxation time ${\ensuremath{\tau}}_{s}$ has been measured by optical orientation experiments, and it shows a surprising nonmonotonic behavior with temperature. We explain this behavior by invoking spin exchange between localized and extended states. Interestingly, the effects of spin-orbit coupling are by no means negligible, in spite of the relatively small valence-band splitting. This is due to the wurtzite crystal structure of ZnO. Detailed analysis allows us to characterize the impurity binding energies and densities, showing that optical orientation experiments can be used as a characterization tool for semiconductor samples.

Hole spin quantum beats in bulk ZnO

We report on experimental investigations of the hole spin coherence in ZnO by time-resolved optical orientation experiments in a magnetic field perpendicular to the growth direction. We observe periodic oscillations of the circular polarization of the donor-bound-exciton luminescence in both ZnO epilayer and nonintentionally doped bulk ZnO. The measurement of these quantum beats, which originate from the Larmor precession of hole spins, yields an accurate determination of the transverse Land\'e $g$ factors of the hole in both the ground and excited states of the donor-bound-exciton complex.

Optically monitored nuclear spin dynamics in individual GaAs quantum dots grown by droplet epitaxy

We report on optical orientation experiments in individual strain-free GaAs quantum dots in AlGaAs grown by droplet epitaxy. Circularly polarized optical excitation yields strong circular polarization of the resulting photoluminescence at 4 K. Optical injection of spin-polarized electrons into a dot gives rise to dynamical nuclear polarization that considerably changes the exciton Zeeman splitting (Overhauser shift). We show that the created nuclear polarization is bistable and we present a direct measurement of the buildup time of the nuclear polarization in a single GaAs dot in the order of 1 s.

Exciton and hole spin dynamics in ZnO investigated by time-resolved photoluminescence experiments

The carrier spin dynamics in ZnO is investigated by time-resolved optical orientation experiments. We evidence a clear circular polarization of the donor-bound exciton luminescence in both ZnO epilayer and nonintentionally doped bulk ZnO. This allows us to measure the localized hole spin relaxation time. We find h s 350 ps at T = 1.7 K in the ZnO epilayer. The strong energy and temperature dependences of the photoluminescence polarization dynamics are well explained by the fast free exciton spin relaxation time and the ionization of bound excitons.

Terahertz emission from ultrafast optical orientation of spins in semiconductors: Experiment and theory

We discuss the optical injection of magnetization into a nonmagnetic semiconductor by the absorption of circularly polarized light. A microscopic approach, which is based on Fermi's golden rule and $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ band models, is used to quantify the magnetization-injection rate in GaAs. We find that under conditions typical in optical orientation experiments, the magnetization-injection rate of holes is approximately 20 times larger than it is for electrons, reflecting the large hole magnetic moment. We then turn to the ultrafast excitation regime and explore the possibility that the injected magnetization can radiate a detectable terahertz field. By using a phenomenological approach for the magnetization relaxation dynamics, we predict that the terahertz field from magnetic injection is at the limit of current terahertz detection technology. We provide initial experimental measurements in search of this terahertz radiation.

OPTICAL SPECTROSCOPY OF 2D NANOISLANDS IN QUANTUM WELLS: LATERAL ISLAND PROFILES AND NATURE OF EMITTING STATES

Results of experimental studies of photoluminescence (PL) and PL excitation (PLE) spectra of MBE grown single quantum wells (QWs) formed by insertion of few CdSe monolayers in the ZnSe matrix are reviewed. PL spectra of such quantum objects originate from the luminescence of CdSe -rich nanoislands. Two types of island emitting states, namely ground and metastable ones, contribute to the low- and high-energy parts of the PL band, respectively. An interplay between these contributions is responsible for the anomalous temperature dependence of the maximum position of the PL band. The optical orientation and optical alignment experiments at resonant excitations allow to elucidate the nature of the two types of the emitting states. PLE spectra of ground and metastable states have strongly differing characters at excitation below some characteristic energy E ME which is identified as the exciton percolation threshold. A theoretical model of the absorption spectra of emitting island states is presented, and practical applications of the model for the characterization of the island lateral concentration profiles are reported.

Influence of buffer surface preparation on the quality of Al x Ga1-x As/GaAs quantum wells studied by optical orientation experiments

Quantum well (QW) structures of Al x Ga1-x As/GaAs with x = 0.3 were characterized by photoluminescence spectroscopy (PL) with circularly polarized excitation at a temperature of 1.6 K. The samples contained three QWs with thickness of 7, 5, and 3 nm grown by molecular beam epitaxy (MBE) on a 500 nm thick buffer layer. Four samples with identical geometry but different surface treatments (in-situ etching the GaAs buffer with Cl2 at different temperatures, and air-exposed buffer, respectively) were compared. The degree of circular polarization of the PL and its decrease in a magnetic field applied perpendicularly to the direction of propagation of light (Hanle effect) allows the determination of the interband lifetime τ and the spin lifetime τ s of the electrons. These lifetimes were different in the different QWs and strongly depend on the growth procedure.

Exciton spin dynamics in ZnO epilayers

AbstractWe used time‐resolved optical orientation experiments to study the low temperature spin dynamics of a ZnO epilayer. The sample shows a circular polarisation of the donor‐bound exciton of 11% with a decay time of 275 ps. A very narrow spectral dependence of the initial polarisation and a rapid decrease of the polarisation decay time with temperature are also observed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical spin orientation in (110) GaAs quantum wells at room temperature

AbstractWe used time‐resolved optical orientation experiments to study the spin dynamics in (110) oriented GaAs/Al0.4GaAs quantum wells at room temperature. We observe a clear dependence of the initial spin polarisation P(0) and of the polarisation decay time τs as a function of the excitation wavelength and power. As high values as P(0) ∼ 80% and τs ∼ 1.6 ns have been observed when exciting quasi‐resonantly with the heavy‐hole transition. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron spin dynamics in GaAsN and InGaAsN structures

AbstractWe report on optical orientation experiments in undoped GaAsN epilayers and InGaAsN quantum wells (QW), showing that a strong electron spin polarisation can persist at room temperature. We demonstrate that the spin dynamics in these dilute nitride structures is governed by a spin‐dependent recombination process of free conduction electrons on deep paramagnetic centres. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultrafast Processes in Semiconductor Structures

We review the dynamics of some of the more relevant optical processes in semiconductor quantum wells. We concentrate on the linear regime and study the time evolution of the light emission, using time-resolved photoluminescence spectroscopy. In intrinsic materials, excitonic effects determine their optical properties. Here we describe the formation and recombination of excitons, and the dependence of these processes on lattice temperature, exciton density, and energy of the excitation light pulses. We also describe the dynamics of the exciton's spin by optical orientation experiments. We discuss the principal mechanisms responsible for the spin flip of the excitons and clarify the role of the exciton localization. Finally, we will show that exciton—exciton interaction produces a breaking of the spin degeneracy in two-dimensional semiconductors. In doped quantum wells, we show that the two spin components of an optically created two-dimensional electron gas are well described by the Fermi—Dirac distributions with a common temperature but different chemical potentials. The rate of the spin depolarization of the electron gas is found to be independent of the mean electron kinetic energy but accelerated by thermal spreading of the carriers.

Spin Orientation of Excitons in Asymmetric Semiconductor Nanostructures

We performed optical orientation experiments for excitons in a biased asymmetric GaAs-Ga(Al)As double quantum well structure. We excited quasi-resonathly (with circularly polarized light) the ground wide well exciton transition and recorded the evolution in time of the polarization of the resulting luminescence line. We show that the measured initial polarization and depolarization time sensibly vary with the applied field around the flat-band regime. We examine theoretically the role of the intra-well (induced by the field) and inter-well (tunnel) couplings on the relaxation of the wide well exciton spin.