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AIP conference proceedings

Spin Coherence of Holes in GaAs/AlGaAs Quantum Wells

M. Syperek, Dmitri R. Yakovlev, A. Greilich, M. Bayer, J. Misiewicz, D. Reuter, Andreas Dirk Wieck

Published Jan 1, 2007• 5 citations
Publisher

Abstract

Pump‐probe time‐resolved Kerr rotation technique has been used for experimental investigation of hole spin coherence in 15‐nm wide GaAs/AlGaAs quantum well structure in regime of diluted two‐dimensional hole gas. Due to the spectral width of 1.5 ps laser pulses which correspond to ∼1 meV, selective excitation of trion and exciton states has been performed. In the case of resonant excitation of exciton only the electron spin beats have been observed in the presence of magnetic field. In the case of resonant excitation of the positively charged exciton two component decay of time‐resolved Kerr rotation (TRKR) signal appeared. From the period of the quantum beat oscillation related to long decay component (up to 650 ps) of TRKR signal we obtained in‐plain hole g‐factor of ∼0.048±0.005.

DOI
10.1063/1.2730380

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Pump‐probe time‐resolved Kerr rotation technique has been used for experimental investigation of hole spin coherence in 15‐nm wide GaAs/AlGaAs quantum well structure in regime of diluted two‐dimensional hole gas. Due to the spectral width of 1.5 ps laser pulses which correspond to ∼1 meV, selective excitation of trion and exciton states has been performed. In the case of resonant excitation of exciton only the electron spin beats have been observed in the presence of magnetic field. In the case of resonant excitation of the positively charged exciton two component decay of time‐resolved Kerr rotation (TRKR) signal appeared. From the period of the quantum beat oscillation related to long decay component (up to 650 ps) of TRKR signal we obtained in‐plain hole g‐factor of ∼0.048±0.005.
Carrier spin coherence in a p-doped GaAs/(Al,Ga)As quantum well with a diluted hole gas is studied by picosecond pump-probe Kerr rotation. For resonant optical excitation of the positively charged exciton the spin precession shows two types of oscillations: Electron spin beats decaying with the charged exciton radiative lifetime of 50 ps, and long-lived hole spin beats with dephasing times up to 650 ps, which decrease with increasing temperature, underlining the importance of hole localization. The mechanism of hole spin coherence generation is discussed.
Generation mechanisms of long‐lived spin coherence of two‐dimensional electron gas (2DEG) have been studied theoretically and experimentally, the latter by means of a picosecond pump‐probe Kerr rotation technique. CdTe/(Cd,Mg)Te quantum wells with a diluted two‐dimensional electron gas have been addressed. Strong Coulomb interaction between electrons and holes, which results in large binding energies of neutral and negatively charged excitons (trions), allows selective addressing of exciton and trion states with resonant optical excitation. Different scenarios of coherence generation have been analyzed, namely, direct trion photocreation, formation of trions from photogenerated excitons and electron‐exciton exchange scattering.
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.
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
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.
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.