We report large polarization state rotations of a weak pulsed beam in CdTe/CdMnTe quantum wells photoinduced by intense circularly polarized pulsed pump beam. Time-resolved analysis of the effect shows that, at moderate pump fluences, the polarization state rotation is due to the combined effect of many-body interactions and spectral hole burning at early times and to photoinduced magnetization at longer ones. At higher pump fluences, space filling and degenerate four-wave mixing become important. At low pump fluences (few μJ/cm2) already the polarization rotation photoinduced at early times can be as large as a few degrees making these multiple quantum wells potential candidates for applications.
We report large polarization state rotations of a weak pulsed beam in CdTe/CdMnTe quantum wells photoinduced by intense circularly polarized pulsed pump beam. Time-resolved analysis of the effect shows that, at moderate pump fluences, the polarization state rotation is due to the combined effect of many-body interactions and spectral hole burning at early times and to photoinduced magnetization at longer ones. At higher pump fluences, space filling and degenerate four-wave mixing become important. At low pump fluences (few μJ/cm2) already the polarization rotation photoinduced at early times can be as large as a few degrees making these multiple quantum wells potential candidates for applications.
Semiconductor quantum wells exhibit numerous nonlinear optical effects that are potentially useful in electro-optical or all-optical devices. In general, the underlying physical mechanisms are complex many-particle processes and can be viewed as interesting research subjects in their own right. One example of such a device is the all-optical polarization switch that has been demonstrated [1]. In that device, conventional (i.e. non-strained) GaAs quantum wells are used in conjunction with two polarizers to switch the throughput of a weak probe pulse with a strong pump pulse. The switching is due to an effective polarization rotation of the probe light by the pump light. The basic idea underlying this effective polarization rotation can be based on a simplified picture of two spin-degenerate heavy-hole exciton transitions. A linearly polarized probe pulse, which consists of right ("+") and left ("-") circularly polarized components, would not undergo polarization rotation, because it couples to both transitions in the same way. If, however, a circularly polarized pump beam modifies, for example, the transition frequency of the "+" transition, the absorption of the "+" component of the probe light differs from that of the "-" component. This leads to an effective polarization rotation.
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 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. 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.
By using a circularly polarized pulse that is tuned well below resonance, we create a virtual population of spin-polarized coherent excitons in a GaAs/AlGaAs multiple quantum well, and we study its effects on the polarization state of a weak signal pulse using a combination of time-resolved and spectrally resolved ellipsometric techniques. From these data the polarization state contributions to an all-optical polarization switch, which exhibits a ~415 fs switching time and large contrast ratios, are identified and the performance of the switch is demonstrated.
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.
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
Using laser pulses of duration comparable to semiconductor relaxation and dephasing times, the coherent phenomena common to two-level systems have been demonstrated in II-VI and III-V semiconductors. These light-matter interactions are characterized by electrons and holes in their initially excited states and/or a macroscopic polarization that persists after the excitation. A nonthermal carrier distribution was observed in bulk CdSe, demonstrating the extremely fast energy relaxation possible when carrier-LO-phonon scattering is included. Quantum-confined CdSe microcrystallites were then employed to limit the available decay avenues. The one and two-pair transitions were characterized, and using spectral hole-burning, energy relaxation was shown to be substantially slower than in bulk. Dipole dephasing remained very rapid. Persistent macroscopic polarization was demonstrated in semiconductors through pump and probe experiments conducted on time scales faster than the inverse linewidth of the exciton. In the first set of experiments, it was found that the exciton optical Stark shift deviates from that predicted by a steady-state theory. The shift is smaller than expected and lasts longer than the pump pulse. Both effects are explained by free evolution of the exciton's polarization. Transient exciton bleaching is observed and shown to be a result of adiabatic following. In a second study of coherent polarization, spectral oscillations at negative delay times were studied. Oscillations were measured in a wide variety of semiconductors, temporally preceding several different pump-probe phenomena. By comparison with a semiclassical theory, it was shown that spectral oscillations in the spectral region of the exciton are evidence of persistent exciton polarization. Oscillations in the band are due to four wave mixing of the pump and probe beams.