Linear dichroism in a GaAs microcavity

Linear dichroism experiments show that exciton–polariton ground states are split into a linearly polarized doublet. At normal incidence and zero detuning this splitting does not exceed 10 μeV for the upper polariton branch and 3 μeV for the lower one. For both branches the splitting decreases with positive and negative detuning.     

Exciton warming in III–V semiconductors and microcavities

We present a time resolved experiment in which we dynamically tailor the occupation and temperature of a photogenerated exciton distribution in QWs by excitation with two delayed picosecond pulses. The modification of the excitonic distribution results in ultrafast changes in the PL dynamics. Our experimental results are well accounted by a quasiequilibrium thermodynamical model, which includes the occupation and momentum distribution of the excitons. We use this model and the two-pulse experimental technique to study the polariton dynamics in InGaAs-based microcavities in the strong coupling regime. In particular, we demonstrate that resonantly injected upper polaritons mainly relax to the lower polariton branch via scattering to large momentum polariton states, producing the warming of the polariton reservoir.     

Polaritonic coupling and spin dynamics in GaAs microcavities

We have used time-resolved photoluminescence spectroscopy to study the light emission dynamics in a semiconductor microcavity as a function of excitation density and exciton-cavity detuning. We paid special attention to polariton spin relaxation by using circularly polarized excitation. We have found a striking behavior of the photoluminescence degree of polarization, which reaches its maximum value at a finite time. As the excitation density is increased and the system enters the stimulated emission regime, this maximum is followed by a negative dip, whose depth strongly depends on exciton-cavity detuning.     

Laser theory for semiconductor quantum dots in microcavities

Quantum dots (QDs) used as active material in microresonators are currently of strong topical interest due to breakthroughs in growth and device structuring. From the theory side, however, atomic models are still used to analyse the emission from these semiconductor systems, despite known differences between QDs and atoms. We introduce a semiconductor laser theory based on a microscopic approach with the goal of better describing the characteristic behaviour of QD-based laser devices and to show differences from predictions based on atomic models.     

Exciton lifetime in quantum well with vicinal high-density excitons

Radiative lifetime of an exciton in a GaAs quantum well (QW) is controlled by high-density excitons, which restrict the exciton coherence through scattering. In order to circumvent the phase space filling effect of high-density excitons, we have prepared a QW structure in such a way that a reservoir for high-density excitons is separated from the QW. The lifetime increases (up to 30%) with the exciton density in the reservoir and saturates at 1×10¹⁷/cm³. The upper bound lifetime is determined by the excitonic relative motion.     

Spin relaxation and antiferromagnetic coupling in semiconductor quantum dots

We report carrier spin dynamics in highly uniform self-assembled InAs quantum dots and the observation of antiferromagnetic coupling between semiconductor quantum dots. The spin relaxation times in the ground state and the first excited state were measured to be 1.0 and 0.6 ns, respectively, without the disturbance of inhomogeneous broadening. The measured spin relaxation time decreases rapidly from 1.1 ns at 10 K to 200 ps at 130 K. This large change in the spin relaxation time is well-explained in terms of the mechanism of acoustic phonon emission. In coupled quantum dots, the formation of antiferromagnetic coupling is directly observed. Electron spins are found to flip at 80 ps after photoexcitation via the interdot exchange interaction. The antiferromagnetic coupling exists at temperatures lower than 50–80 K. A model calculation based on the Heitler–London approximation supports the finding that the antiferromagnetic coupling is observable at low temperature. These carrier spin features in quantum dots are suitable for the future quantum computation.     

Optical manipulation of coupled spins in magnetic semiconductor systems: A path toward spin optoelectronics

Experimental results based on the optical excitations in the III–V-based ferromagnetic semiconductors are reviewed. On the bases of results obtained by both cw- and femto-second-pulse optical excitation, we point out the feasibility of magnetization rotation in the hole-mediated ferromagnetic semiconductor (Ga,Mn)As via the angular momentum and photon energy of light. Here, p–d exchange interaction is the effective channel that transmits a small change in spin axis of the valence band to the ferromagnetically coupled Mn spin sub-system. Within the limit of this picture, we also discuss a hole–Mn spin complex for which hole and Mn spins rotate and relax together upon optical excitation. Partial magnetization reversal observed in the experiments of the electrical current injection in (Ga,Mn)As-based magnetic-tunnel-junction devices is also reviewed in view of the effects caused by the spin-polarized holes. Here, we point out that a spin current of 10⁵ A/cm² may be reduced further if spin injection efficiency can be improved by the optimal designs of the device structure.     

Spin–spin interaction in magnetic semiconductor quantum dots

Self-assembled Cd(Mn)Se/Zn(Mn)Se quantum dots have been investigated by means of spatially and time-resolved magneto-optical spectroscopy. In such quasi zero-dimensional diluted magnetic semiconductors, the exchange interaction couples the spins of optically generated charge carriers with localized magnetic ion spins. We demonstrate that this can be used on the one hand to monitor nanoscale magnetization with a resolution of <100 μ_B by a purely optical technique and on the other hand to optically manipulate the magnetization in a semiconductor quantum dot.     

室温下CdSe胶体量子点超快自旋动力学

利用时间分辨法拉第旋转光谱技术研究了室温下CdSe胶体量子点的自旋相干特性.获得了不同磁场下的自旋退相干时间,并分析了自旋退相干的物理机理.零磁场时量子点激子自旋退相干时间为102 ps,主要受电子与核自旋之间的超精细相互作用所影响.当外加横向磁场强度为250 mT时,激子自旋退相干时间为294 ps;增大磁场强度,自旋退相干时间逐渐减小.在较强磁场环境中(≥250mT),量子点激子自旋动力学由非均匀退相干机制所主导.     

Electron spin relaxation in very diluted CdMnTe quantum wells

Electron spin dephasing is studied by time-resolved Kerr rotation in n-type modulation-doped CdMnTe quantum wells with very dilute Mn content. We find good agreement between measured and calculated electron spin relaxation times, considering relaxation induced by fluctuating exchange field created by the Mn spins, and taking into account inhomogeneous heating of the Mn spins by laser pulses.     

Photo-induced magnetic-solitons and spin dynamics in diluted magnetic semiconductor quantum wells

The localization mechanism of transport property in the randomly distributed system of the hole-induced magnetic solitons with the alloy potential fluctuations in diluted magnetic semiconductors has been proposed, by using the effective Lagrangian of diffusion modes. The mechanism of the long relaxation of the spin dynamics below Curie temperature in diluted magnetic semiconductor wells and the bulk system has been discussed.     

Optical generation of spin coherence in single-charged (In,Ga)As/GaAs self-assembled quantum dots

The generation of electron spin coherence has been studied in n-modulation-doped (In,Ga)As/GaAs self-assembled quantum dots (QDs) which contain on average a single electron per dot. The doping has been confirmed by pump–probe Faraday rotation experiments in a magnetic field parallel to the heterostructure growth direction. For studying spin coherence, the magnetic field was rotated by 90° to the Voigt geometry, and the precession of the electron spin about the field was monitored. The coherence is generated by resonant excitation of the QDs with circularly polarized laser pulses, creating a coherent superposition of an electron, and a trion state. The efficiency of the generation can be controlled by the pulse intensity, being most efficient for (2n+1)π pulses.     

The g–r noise in quantum well semiconductor lasers and its relation with device reliability

The g–r noise in quantum well semiconductor lasers is studied theoretically and experimentally. The results indicate that the g–r noise is dependent on bias current, the devices show the g–r a noise only at low bias current, with the bias current increasing, the g–r noise will disappear. The g–r noise has a close relation with defects; the devices with g–r noise degrade rapidly during the electric aging. It is means that measuring the noise at low bias current is very important for estimating device reliability.     

Electron spin resonance of the two-dimensional metallic state and the quantum Hall state in a Si/SiGe quantum well

We report electrically detected electron spin resonance (ESR) measurements of a high mobility two-dimensional (2D) electron system formed in a Si/SiGe quantum well, with millimeter wave in a high magnetic field . The negative ESR signal observed under an in-plane magnetic field gives direct evidence that the spin polarization leads to a resistance increase in the 2D metallic state. Suppression of spin decoherence was observed in the quantum Hall state at the Landau level filling factor ν=2. Strength of the nuclear magnetic field in the resonance is evaluated to be less than , much smaller than that reported for GaAs/AlGaAs heterostructures.     

Aharonov–Bohm effect in Kane-type semiconductor quantum wire

In the present study, the Aharonov–Bohm effect for bound states in Kane-type quantum wire is analysed. It is demonstrated that the wave function and the energy spectra of carriers depend on the magnetic flux. The energy and the g factor of electrons are of a periodic function of the magnetic flux. The statistical property of the orbital magnetism of electrons at low temperatures and strong magnetic fields is presented. It is shown that the magnetisation of electron oscillations depends on the magnetic flux.     

Metal–insulator transition in a quantum well under the influence of magnetic field

Ionization energies of a shallow donor in a quantum well of the Cd_1-xinMn_xinTe/Cd_1-xoutMn_xoutTe superlattice system are obtained. A variational procedure within the effective mass approximation is employed in the presence of magnetic field. Within the one-electron approximation the occurrence of Mott transition is seen when the binding energy of a donor vanishes is observed. The effects of Anderson localization and exchange and correlation in the Hubbard model are included in our model. It is found that the ionization energy (i) decreases as well width increases for a given magnetic field, (ii) decreases when well width increases, (iii) the critical concentration at which the metal–insulator transition occurs is enhanced in the external magnetic field and (iv) spin polaronic shifts not only with the increase in a magnetic field but also with the well width increases. All the calculations have been carried out with finite barriers and the results are compared with available data in the literature.     

Photocurrent response in a double barrier structure with quantum dots–quantum well inserted in central well

We report the photocurrent response in a double barrier structure with quantum dots–quantum well inserted in central well. When this quantum dots–quantum well hybrid heterostructure is biased beyond +1 or −1 V, the photocurrent response manifests itself as a step-like enhancement, increasing linearly with the light intensity. Most probably, at proper bias condition, the pulling down of the X minimum of GaAs at the outgoing interface of the emitter barrier by the photovoltaic effect in GaAs QW will initiate the Γ–X–X tunneling at much lower bias as compared with that in the dark. That gives rise to the observed photocurrent response.     

Recent studies in thermal activation and spin waves

Some recent results in computational approaches to thermally activated fast reversal in magnetic recording media are reviewed. In particular, recent results reported in the simulation of pulsed-field-induced magnetisation reversal and thermal activation of spin waves are described. The short time scale breakdown of the Arrhenius–Néel law for a single moment is demonstrated and explained in terms of the dynamics of the precessional motion. The variation in response as a function of the damping parameter is found to be an important factor determining the remanent magnetisation for a given pulse width. The effects of interactions between moments are described, including the apparent increase in effective damping. It is shown that interactions between moments can be described in terms of thermally excited spin waves. The spectrum of relaxation times for systems consisting of coupled moments is explained in terms of the thermal excitation of spin waves.