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arXiv:2002.04311 · Physical Review Research

In-plane anisotropy of the hole g factor in CdTe/(Cd,Mg)Te quantum wells studied by spin-dependent photon echoes

S. V. Poltavtsev, I. A. Yugova, A. N. Kosarev, D. R. Yakovlev, G. Karczewski, S. Chusnutdinow, T. Wójtowicz, I. A. Akimov, M. Bayer

Published May 12, 2020• 7 citations
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Abstract

The paper studies the in-plane anisotropy of the hole spin in a 20-nm-thick CdTe/(Cd,Te)Mg quantum well by two-pulse photon echoes in an external magnetic field $\mathbf{B}$. The authors show that the coherent optical response from donor bound excitons provides information on the spin dynamics of the hole in the exciton, allowing one to study the interactions contributing to the $g$ factor that characterizes the hole spin.

DOI
10.1103/physrevresearch.2.023160

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Coherent spin dynamics of carriers in GaAs and CdTe semiconductor quantum wells

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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

•Mar 24, 2010•0 citations
The paper studies the in-plane anisotropy of the hole spin in a 20-nm-thick CdTe/(Cd,Te)Mg quantum well by two-pulse photon echoes in an external magnetic field B\mathbf{B}B. The authors show that the coherent optical response from donor bound excitons provides information on the spin dynamics of the hole in the exciton, allowing one to study the interactions contributing to the ggg factor that characterizes the hole spin.
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.
Coherent optical spectroscopy such as four-wave mixing and photon echo generation deliver detailed information on the energy levels involved in optical transitions through the analysis of polarization of the coherent response. In semiconductors, it can be applied to distinguish between different exciton complexes, which is a highly non-trivial problem in optical spectroscopy. We develop a simple approach based on photon echo polarimetry, in which polar plots of the photon echo amplitude are measured as function of the angle φφφ between the linear polarizations of the two exciting pulses. The rosette-like polar plots reveal a distinct difference between the neutral and charged exciton (trion) optical transitions in semiconductor nanostructures. We demonstrate this experimentally by photon echo polarimetry of a 20-nm-thick CdTe/(Cd,Mg)Te quantum well at temperature of 1.5~K. Applying narrow-band optical excitation we selectively excite different exciton complexes including the exciton, the trion, and the donor-bound exciton D0^00X. We find that polarimetry of the photon echo on the trion and D0^00X is substantially different from the exciton: The echoes of the trion and D0^00X are linearly polarized at the angle 2φ2φ2φ with respect to the first pulse polarization and their amplitudes are weakly dependent on φφφ. While on the exciton the photon echo is co-polarized with the second exciting pulse and its amplitude scales as cos⁡φ\cosφcosφ.
We study Rabi oscillations detected in the coherent optical response from various exciton complexes in a 20-nm-thick CdTe/(Cd,Mg)Te quantum well using time-resolved photon echoes. In order to evaluate the role of exciton localization and inhomogeneous broadening we use selective excitation with spectrally narrow ps pulses. We demonstrate that the transient profile of the photon echo from the localized trion (${\mathrm{X}}^{\ensuremath{-}}$) and the donor-bound exciton (D0X{\mathrm{D}}^{0}\mathrm{X}D0X) transitions strongly depends on the strength of the first pulse. It acquires a non-Gaussian shape and experiences significant advancement for pulse areas larger than $\ensuremath{\pi}$ due to non-negligible inhomogeneity-induced dephasing of the oscillators during the optical excitation. Next, we observe that an increase of the area of either the first (excitation) or the second (rephasing) pulse leads to a significant damping of the photon echo signal, which is strongest for the neutral excitons and less pronounced for the donor-bound exciton complex (D0X{\mathrm{D}}^{0}\mathrm{X}D0X). The measurements are analyzed using a theoretical model based on the optical Bloch equations which accounts for the inhomogeneity of optical transitions in order to reproduce the complex shape of the photon echo transients. In addition, the spreading of Rabi frequencies within the ensemble due to the spatial variation of the intensity of the focused Gaussian beams and excitation-induced dephasing are incorporated in our model, which is able to explain the fading and damping of Rabi oscillations. By analyzing the results of the simulation for ${\mathrm{X}}^{\ensuremath{-}}$ and D0X{\mathrm{D}}^{0}\mathrm{X}D0X complexes we are able to establish a correlation between the degree of localization and the transition dipole moments determined as $\ensuremath{\mu}({\mathrm{X}}^{\ensuremath{-}}$)=73 D and $\ensuremath{\mu}({\mathrm{D}}^{0}\mathrm{X}$)=58 D.
Spin-dependent photon echoes in combination with pump-probe Kerr rotation are used to study the microscopic electron spin transport in a CdTe/(Cd,Mg)Te quantum well in the hopping regime. We demonstrate that, independent of the particular spin relaxation mechanism, the hopping of resident electrons leads to a shortening of the photon echo decay time, while the transverse spin relaxation time evaluated from pump-probe transients increases due to motional narrowing of spin dynamics in the fluctuating effective magnetic field of the lattice nuclei.
We report on magnetic field-induced oscillations of the photon echo signal from negatively charged excitons in a CdTe/(Cd,Mg)Te semiconductor quantum well. The oscillatory signal is due to Larmor precession of the electron spin about a transverse magnetic field and depends sensitively on the polarization configuration of the exciting and refocusing pulses. The echo amplitude can be fully tuned from the maximum down to zero depending on the time delay between the two pulses and the magnetic-field strength. The results are explained in terms of the optical Bloch equations accounting for the spin level structure of electrons and trions.
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