Quantum beats in the exciton emission of type II GaAs/AlAs quantum wells

Poel, W. A. J. A. van der; Severens, A.L.G.J.; Foxon, C.T.

doi:10.1016/0030-4018(90)90304-c

“…However the strong localization of the carrier wavefunction at the QW interface will (i) modify drastically the exciton fine structure and (ii) yield very long electron spin relaxation times (≈ a few tens of ns) compared to type I QWs [64]. It has been shown that the symmetry of the system is reduced form D 2d to C 2v and [51,58,63]. The symmetry reduction in these type II GaAs superlattices was first explained by the presence of a random local deformation (due to the presence of bonds of different nature (Al-As or Ga-As) on each side of the interface taken as an As plane) which mixes the heavy and light hole states [58].…”

Section: Exciton Spin Dynamics In Type II Qwsmentioning

confidence: 99%

“…• angle with respect to the exciton eigenstates orientations) [63]. As a consequence, the time-resolved luminescence signal, detected with polarization either parallel or perpendicular to the excitation, decays and oscillates with a period T inversely proportional to the splitting ∆ 1 between the two sublevels (T = h/∆ 1 , see section 1.1.1).…”

Section: Exciton Spin Dynamics In Type II Qwsmentioning

confidence: 99%

“…Since the splitting of the X' and Y' excitonic sublevels (which is much smaller than k B T ), is much smaller than the laser spectral width, the two sublevels can be coherently excited at time t = 0 by a short pulse polarized along one of the [1, 0, 0] axes (i.e. 45• angle with respect to the exciton eigenstates orientations) [63]. As a consequence, the time-resolved luminescence signal, detected with polarization either parallel or perpendicular to the excitation, decays and oscillates with a period T inversely proportional to the splitting ∆ 1 between the two sublevels (T = h/∆ 1 , see section 1.1.1).…”

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Exciton Spin Dynamics in Semiconductor Quantum Wells

Amand

¹

,

Marie

²

2008

Springer Series in Solid-State Sciences

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Two-Dimensional exciton fine structureThe spin properties of excitons in nanostructures are determined by their fine structure. Before analysing the exciton spin dynamics, we give first a brief description of the exciton states in quantum wells. We will mainly focus in this review on GaAs or InGaAs quantum well which are model systems. For more details, the reader is referred to the reviews in ref. [1,2]. As in bulk material, exciton states in II-VI and III-V quantum wells (QW) correspond to bound states between valence band holes and conduction band electrons. As will be seen later, exciton states are shallow two-particle states rather close to the nanostructure gap, i.e. their spatial extension is relatively large with respect to the crystal lattice, so that the envelope function approximation can be used to describe these states.The problem of exciton states in bulk crystals or nanostructures is in fact a N −electron problem, in which we seek for the stationary states of a crystal where one electron has been removed from the valence states and set in the conduction band, thus leaving N − 1 valence band electrons. Due to electron indiscernability, the latter states should be antisymmetrized. At this point it is more convenient to use the electron-hole pair states basis, ψ s,ke (r e )ψ obtained by applying the time reversal operatorK to a corresponding electron valence state ψ mv ,kv (r). This description offers the advantage to give the possibility to solve the exciton problem as a two-body problem. This is done usually in two steps: first treat the Hartree type problem between a conduction electron and a hole with direct Coulomb attractive potential, thus yielding the so-called "mechanical" exciton, then solve by perturbation the corrections due to electron-hole exchange terms. Note these terms appear due to the non vanishing coulomb exchange terms arising between the conduction electron 2 Thierry Amand and Xavier Marie and the remaining N − 1 valence band electrons. A description of the exciton fine structure in bulk semiconductors can be found in [3]. In quantum wells structures, as in bulk material, a conduction electron and a valence hole can bind into an exciton, due to the coulomb attraction. However, the exciton states are strongly modified due to confinement of the carriers in one direction. As we have seen, this confinement leads to the quantization the single electron and hole states into subbands [1,73], and to the splitting of the heavy-and light-hole band states. The description of excitons is obtained, through the envelope function approach, and the fine exciton structure is then deduced by a perturbation calculation performed on the bound electron-hole states without electron-hole exchange. However, this approach becomes then more complex in the context of two dimensional structures, and is summarized in Appendix I. The full electron-hole wave function can finally be approximated by:Ψ α (r e , r h ) = χ c,νe (z e )χ j,ν h (z h ) ej,nl (r ⊥ )u s (r e )u m h (r h ) (1.1)where, α represents the fu...

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“…It should be noted that circular polarization of luminescence under excitation with linearly polarized light in zero magnetic field was observed under study of excitons localized on anisotropic islands in QWs [15]. This effect is caused by optical alignment of exciton dipoles followed by dipole oscillations in anisotropic media and, generally speaking, can be observed in spinless systems.…”

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confidence: 99%

Optical orientation of electron spins by linearly polarized light

Tarasenko

¹

2005

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Absorption of circularly polarized light in semiconductors is known to result in optical orientation of electron and hole spins. It has been shown here that in semiconductor quantum well structures spin orientation of carriers can be achieved by linearly or even unpolarized light. Moreover, the sign and magnitude of the spin orientation can be varied by rotating the polarization plane of incidence light. The effect under study is related to reduced symmetry of the quantum wells as compared to bulk materials and, microscopically, caused by zero-field spin splitting of electron and hole states. PACS numbers: 72.25.Fe, 72.25.Rb, 72.25.Dc, 78.67.De Spin-dependent phenomena in semiconductor structures are the subject of extensive ongoing research. One of the most widespread and powerful methods for creating spin polarization and investigating kinetics of spinpolarized carriers is optical orientation of electron and nuclear spins by circularly polarized light [1,2,3,4,5]. This effect can be interpreted as a transfer of the photon angular momenta to free carriers. Under interband excitation by circularly polarized light, direct optical transitions from the valence band to the conduction band can occur only if the electron angular momentum is changed by ±1. These selection rules lead to the spin orientation of photoexcited carriers, with degree and sign of spin orientation depending on the light helicity.In the present paper we show that in low-dimensional semiconductor systems spin orientation of carriers can be achieved by linearly or even unpolarized light. The effect under consideration is related to reduced symmetry of the low-dimensional structures as compared to bulk compounds and is forbidden in bulk cubic semiconductors. Microscopically, it is caused by asymmetrical photoexcitation of carriers in spin subbands followed by spin precession in an effective magnetic field induced by the Rashba or Dresselhaus spin-orbit coupling [6].The effect is most easily conceivable for direct transitions between the heavy-hole valence subband hh1 and the conduction subband e1 in quantum well (QW) structures of the C s point symmetry, e.g. in (113)-or (110)-grown QWs based on zinc-blende-lattice compounds. In such structures the spin component along the QW normal z is coupled with the in-plane electron wave vector. This leads to k-linear spin-orbit splitting of the energy spectrum as sketched in Fig. 1, where the heavyhole subband hh1 is split into two spin branches ±3/2 shifted relative to each other in the k space. Due to the selection rules the allowed optical transitions from the valence subband hh1 to the conduction subband e1 are | + 3/2 → | + 1/2 and | − 3/2 → | − 1/2 , as illustrated in Fig. 1 by dashed vertical lines. Under excitation with linearly polarized or unpolarized light the rates of both transitions coincide. In the presence of the spin splitting, the optical transitions induced by photons of the fixed energyhω occur in the opposite points of the k space for the electron spin states ±1/2. Such an asymmetry o...

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“…As the holes are still confined in the GaAs well, the recombination takes therefore place between the hole in the well and the electrons in the barrier (type II quantum well). This accounts for the long exciton PL lifetime, of the order of a few microseconds [8].The spin dynamics in these type II quantum well systems has been extensively studied by optical orientation experiments in stationary or time-resolved regime [1,51,57,58,63]. Because of the very small overlap between the electron and hole wavefunction, the spin relaxation mechanism induced by the exchange interaction between the electron and the hole does not play a significant role in contrast to type I QWs (see sections 1.1.2, 1.3.4).…”

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Exciton Spin Dynamics in Semiconductor Quantum Wells

Amand

¹

,

Marie

²

2017

Springer Series in Solid-State Sciences

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Two-Dimensional exciton fine structureThe spin properties of excitons in nanostructures are determined by their fine structure. Before analysing the exciton spin dynamics, we give first a brief description of the exciton states in quantum wells. We will mainly focus in this review on GaAs or InGaAs quantum well which are model systems. For more details, the reader is referred to the reviews in ref. [1,2]. As in bulk material, exciton states in II-VI and III-V quantum wells (QW) correspond to bound states between valence band holes and conduction band electrons. As will be seen later, exciton states are shallow two-particle states rather close to the nanostructure gap, i.e. their spatial extension is relatively large with respect to the crystal lattice, so that the envelope function approximation can be used to describe these states.The problem of exciton states in bulk crystals or nanostructures is in fact a N −electron problem, in which we seek for the stationary states of a crystal where one electron has been removed from the valence states and set in the conduction band, thus leaving N − 1 valence band electrons. Due to electron indiscernability, the latter states should be antisymmetrized. At this point it is more convenient to use the electron-hole pair states basis, ψ s,ke (r e )ψ obtained by applying the time reversal operatorK to a corresponding electron valence state ψ mv ,kv (r). This description offers the advantage to give the possibility to solve the exciton problem as a two-body problem. This is done usually in two steps: first treat the Hartree type problem between a conduction electron and a hole with direct Coulomb attractive potential, thus yielding the so-called "mechanical" exciton, then solve by perturbation the corrections due to electron-hole exchange terms. Note these terms appear due to the non vanishing coulomb exchange terms arising between the conduction electron 2 Thierry Amand and Xavier Marie and the remaining N − 1 valence band electrons. A description of the exciton fine structure in bulk semiconductors can be found in [3]. In quantum wells structures, as in bulk material, a conduction electron and a valence hole can bind into an exciton, due to the coulomb attraction. However, the exciton states are strongly modified due to confinement of the carriers in one direction. As we have seen, this confinement leads to the quantization the single electron and hole states into subbands [1,73], and to the splitting of the heavy-and light-hole band states. The description of excitons is obtained, through the envelope function approach, and the fine exciton structure is then deduced by a perturbation calculation performed on the bound electron-hole states without electron-hole exchange. However, this approach becomes then more complex in the context of two dimensional structures, and is summarized in Appendix I. The full electron-hole wave function can finally be approximated by:Ψ α (r e , r h ) = χ c,νe (z e )χ j,ν h (z h ) ej,nl (r ⊥ )u s (r e )u m h (r h ) (1.1)where, α represents the fu...

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Quantum beats in the exciton emission of type II GaAs/AlAs quantum wells

Cited by 39 publications

References 9 publications

Exciton Spin Dynamics in Semiconductor Quantum Wells

Exciton Spin Dynamics in Semiconductor Quantum Wells

Optical orientation of electron spins by linearly polarized light

Exciton Spin Dynamics in Semiconductor Quantum Wells

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