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Physical Review B

Optical initialization and dynamics of spin in a remotely doped quantum well

Thomas A. Kennedy, Andrew Shabaev, Michael Scheibner, Al. L. Éfros, A. S. Bracker, D. Gammon

Published Jan 10, 2006• 43 citations
Publisher

Abstract

The excitation of electron spin polarization and coherence by picosecond light pulses and their dynamics in a wide remotely doped quantum well are studied theoretically and experimentally. Assuming that all electrons in the quantum well are localized, the theory considers the resonant interaction of light pulses with the four-level system formed by the electron spins of the ground state and the hole spins of the trion excited state. The theory describes the effects of spontaneous emission, a transverse magnetic field and hole spin relaxation on the dynamics detected by the Kerr rotation of a probe pulse. Time resolved Kerr rotation experiments were carried out on a remotely doped $14\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ GaAs quantum well in the frequency range of optical transitions to the heavy hole (HH) trion and to the light-hole (LH) trion degenerate with the HH exciton. The experiments on the resonant excitation of the HH trion show a very slow heavy hole spin relaxation and, consequently, a weak electron spin polarization after the trion relaxation. In contrast, the resonant excitation of the LH trion/HH exciton results in a fast hole spin relaxation that increases electron spin polarization.

DOI
10.1103/physrevb.73.045307

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The excitation of electron spin polarization and coherence by picosecond light pulses and their dynamics in a wide remotely doped quantum well are studied theoretically and experimentally. Assuming that all electrons in the quantum well are localized, the theory considers the resonant interaction of light pulses with the four-level system formed by the electron spins of the ground state and the hole spins of the trion excited state. The theory describes the effects of spontaneous emission, a transverse magnetic field and hole spin relaxation on the dynamics detected by the Kerr rotation of a probe pulse. Time resolved Kerr rotation experiments were carried out on a remotely doped 14nm14\phantom{\rule{0.3em}{0ex}}\mathrm{nm}14nm GaAs quantum well in the frequency range of optical transitions to the heavy hole (HH) trion and to the light-hole (LH) trion degenerate with the HH exciton. The experiments on the resonant excitation of the HH trion show a very slow heavy hole spin relaxation and, consequently, a weak electron spin polarization after the trion relaxation. In contrast, the resonant excitation of the LH trion/HH exciton results in a fast hole spin relaxation that increases electron spin polarization.
Optical control of the spin coherence of quantum well electrons by short laser pulses with circular or linear polarization is studied experimentally and theoretically. For that purpose the coherent electron spin dynamics in a nnn-doped CdTe/(Cd,Mg)Te quantum well structure was measured by time-resolved pump-probe Kerr rotation, using resonant excitation of the negatively charged exciton (trion) state. The amplitude and phase shifts of the electron spin beat signal in an external magnetic field, that are induced by laser control pulses, depend on the pump-control delay and polarization of the control relative to the pump pulse. Additive and nonadditive contributions to pump-induced signal due to the control are isolated experimentally. These contributions can be well described in the framework of a two-level model for the optical excitation of the resident electron to the trion.
We have experimentally studied the spin dynamics of excitons, electrons, and trions in charge-tunable InP/InGaP quantum dots (QDs) excited by picosecond resonant laser pulses by observing the time-resolved Kerr rotation. In singly charged QDs, inversion of the spin polarization direction of doped electrons is found to be caused simply by variation in the pulse intensity, which is accompanied by an abrupt change of the spin coherence time. This phenomenon is reproduced by density-matrix calculations allowing for the reaction on the QD electron-trion four-level system during its coherent radiation emission. This result means that the optical coherence is another critical factor affecting electron spin coherence.
Pulsed optical excitation of the negatively charged trion has been used to generate electron spin coherence in an n-doped (In,Ga)As/GaAs quantum well. The coherence is monitored by resonant spin amplification detected at times exceeding the trion lifetime by 2 orders of magnitude. Still, even then signatures of the hole spin dynamics in the trion complex are imprinted in the signal leading to an unusual batlike shape of the magnetic field dispersion of spin amplification. From this shape information about the spin relaxation of both electrons and holes can be derived.
Systematic investigation of the spin dephasing times in GaAs/(Al,Ga)As quantum wells and its depending on parameters like temperature, magnetic fields and doping will be shown in Chapter 3.In Chapter 4 are presented results of investigation of the nuclear spin systems in quantum well heterostructures.The chapter 5 presents the results of studying the carrier spin dynamics in a n-doped (In,Ga)As/GaAs quantum wells by time-resolved Faraday rotation and ellipticity techniques in the temperature range down to 430 mil-liKelvin.In Chapter 6 is devoted to experimental and theoretically studying of optical control of the spin coherence of quantum well electrons by short laser pulses with circular or linear polarization.For that purpose the coherent electron spin dynamics in a n-doped CdTe/(Cd,Mg)Te quantum well structure was measured by time-resolved pump-probe Kerr rotation, using resonant excitation of the negatively charged exciton (trion) state.on the eigenstates |s z ; ↑ and |s z ; ↓ yields the states themselves independent of time.Along the field axis (longitudinal direction) these quantum-mechanical spin states are thus stationary.Any loss of coherence is induced by coupling of those states with the environment.Applying the time-evolution operator on the spin states perpendicular
The resident electron spin polarization (RESP) in quantum wells (QWs) has gained considerable attention for the potential applications of spin degree of freedom in the spintronic devices and quantum information processing. This is because of the optical controllability, the robust and long spin coherence free from the recombination annihilation. Therefore, massive research has been conducted on resident electron spin relaxation, transport, control, coherence, initialization, the interaction with other excited states, donors, lattice nuclei and so on, especially in GaAs and InAs QWs. Owing to these studies, the RESP can be better manipulated within the decoherence time after its generation. Unfortunately, it is almost a blank to the important spin formation dynamics in the initial temporal region, due largely to the ultrafast internal transitions and complicated many-body interactions. This issue, specifically, unclear initial phase, has been emphasized in our study with overcoming the problems successfully.The formation dynamics of RESP and ensemble spin coherence in a naturally n-doped CdTe single QW have been studied with picosecond laser pulses by a time-resolved Kerr rotation (TRKR) method. The careful analysis of TRKR signals as compared to the theoretical fittings around time origin reveals the appealing phenomenon for the first time: a negative initial phase shift (IPS) in the precessional motion of RESP. After detailed experimental studies on the IPS of RESP, along with rate equations analysis, the theoretical model of the interplay between the excitons and RESP has been simulated. Consequently, the possibility of the negative IPS and of ensemble spin dynamics discussed gave a deep understanding of RESP generation, involving the formation and evolution of excitons, particularly in the elusive early time region.The experimental Kerr signals with high precision both in time and phase show that the magnitude of IPS as well as the amplitude of RESP precession can be affected by external parameters such as excitation power, the strength of magnetic field. Such overall trends in the experiments agree well with the model calculations. Moreover, the negative IPS is almost constant for the pulse photon energy around the transition energy in resonance with trion. Through studying nonzero IPSs, we have confirmed the RESP formation dynamical process, associated especially with a fast asymmetric hole-spin flip in trions spin dynamics.There is still a dominant remained obstacle characterized by hyperfine interaction (HFI), the coupling of electron spins to the lattice nuclear spins, although the particular benefit of RESP is the long lifetime of spin coherence without energy relaxation or forceful coupling with semiconductor environments. The HFI interaction was often ignored in the previous electron spin research of CdTe semiconductors. But the nuclear spin polarization (NSP) in an oblique magnetic field configuration has been evaluated after exploring the longitudinal and in-plane electron g factors precisely. This investigation indicates that RESP gives rise to a small effective nuclear field of a few mT.This study makes the integral RESP formation process, initial behaviors and ensemble spin coherence well-established quite fully. Moreover, the IPS is founded to provide an important tool for researching into the spin dynamics of excitons. In a word, RESP performances with the feature of low nuclear field are preferred for spintronic devices that require the long ensemble spin coherence and the manipulation at rather low magnetic fields.
The carrier spin dynamics in a nnn-doped (In,Ga)As/GaAs quantum well has been studied by time-resolved Faraday rotation and ellipticity techniques in the temperature range down to 430 milliKelvin. These techniques give data with very different spectral dependencies, from which nonetheless consistent information on the spin dynamics can be obtained, in agreement with theoretical predictions. The mechanisms of long-lived spin coherence generation are discussed for the cases of trion and exciton resonant excitation. We demonstrate that carrier localization leads to a saturation of spin relaxation times at 45 ns for electrons below 4.5 K and at 2 ns for holes below 2.3 K. The underlying spin relaxation mechanisms are discussed.