Luận văn Optical time resolved spin dynamics in III-V semiconductor quantum wells

1. Introduction This thesis concerns the optical manipulation of electron spin in III-V semiconductor heterostructures. It presents measurements ofthe time evolution of transient spin polarised carriers on a picosecond timescale. Some of the information contained in the polarisation state of absorbed light is stored in the spin component of the excited state of the absorbing medium, it is lost over time due to processes which decohere or relax the spin polarisation in the medium. How well a material can preserve spin information is represented by the spin relaxation and decoherence rates, quantities which depend on many parameters, the principal determinants are temperature; quantum confinement; and external and internal electromagnetic field configurations, manipulated for example by doping, and excitation intensity. Hysteresis effects are also possible in magnetic-ion doped semiconductors. Mechanisms of light absorption and energy retention in semiconductors canbe described in terms of the photo-creation of transient populations of various quantum quasi-particles; electrons, holes, excitons and phonons being the most basic kind. Holes and excitons are large scale manifestations of electron interactions, whereas phonons represent vibrational (thermal) excitations of the crystal lattice. More exotic wavicles such asthe exciton-photon polariton; the exciton-phonon polariton, bi-, tri- and charged-exciton; and plasmon statesare obtained from various couplings between members of the basic set. It has been found that the basic set of excitations suffice for the work presented in this thesis. Many current semiconductor technologies exploit only the charge or Coulomb driven interactions of induced non-equilibrium electron populations to store, manipulate and transmit information. It has long been recognised that in addition information of a fundamentally different, quantum, nature may also be carried by the electron spin. Many proposals for advances in information processing, the development of quantum computing and spin electronic devices, involve manipulation of spin in semiconductors. Currently, most mass produced semiconductor devices are Silicon based. From an economic viewpoint, since the industrial production infrastructure is already in place, spin manipulation technologies based on Silicon would bemost desirable. Silicon is however an indirect gap semiconductor, it couples only weakly to light, which, in respectof optical spin manipulation, places it at a disadvantage relative to its direct gap counterparts. Many III-V (and II-VI) materials are direct gap semiconductors and couple strongly to light. Interest in research, such as presented here, into the interaction of polarised light with III-V’s for the purpose of manipulating spin information, has thus grown rapidly over recent years. Gallium Arsenide has been the prime focus and other materials such as InAs, InP, and GaN are also under increasingly intense investigation. It is not only potential further technological reward that motivates spin studies in semiconductors, they also provide an ideal physical system in which to test and improve understanding of physical theories. This is because physical parameters, such as alloy concentrations, temperature, quantum confinement lengths, disorder, and strain to name a few, can be systematically varied with reasonable accuracy and effort during experimentation or growth. Theories attempt to relate these parameters to basic physical processes and measurement results, experiments verify (or contradict) the predictions, and through a feedback process fundamental understanding can increase and deepen. The work presented in this thesis is a contribution to this field, the ongoing investigation of the properties and behaviourof electrons in III-V semiconductors, with emphasis on the time-resolved dynamics of optically created transient spin polarisations in quantum confined heterostructures. Laser pulses of ~2 picosecond duration were used to excite a non-equilibrium electron distribution into the conduction band. Optical polarisation of the laser beam is transferred into polarisation of the electron spin. Evolution of this injected spin polarisation was measured using a reflected, weaker, test pulse whose arrival at the sample was delayed. Rotation of the linear polarisation plane of the test pulse revealed some information concerning the state that the spin polarisation had reached after elapse of the delay time. A more detailed description of the measurement method is given in chapter 3. Three pieces of experimental work have been undertaken in this thesis. Measurements in a high mobility modulation n-doped (1.86x10 11 cm -2 ) GaAs/AlGaAs sample were designed to observe the precession of electron spin in the absence of anexternal magnetic field (see chapter 4). The spin vectors are thought to precess around an effective magnetic field related to the conduction band spin-splitting which is caused by the inversion asymmetry of the Zincblende crystal structure. Spin relaxation in an undoped In0.11Ga0.89As/GaAs sample was studied to ascertain whether previously observed fast electron spin relaxation in InGaAs/InP was due the native interface asymmetry present in the structure or ifspin relaxation is generally fast in InGaAs wells (see chapter 5). Finally, quantum beating of exciton spin precession was measured in a GaAs/AlGaAs multiple quantum well sample with a magnetic field applied at various anglesto the growth and excitation direction using optically-induced transient linear birefringence. Previous studies on this sample have shown that some of the excitons experience a low symmetry environment which lifts the degeneracy of the optically active heavy-hole exciton spin states. In this study we attempt to observe the effects of this in time-resolved spectroscopy (chapter 6). In chapter 2 some basic semiconductor physics relating to the behaviour of electrons is outlined in sufficient detail to give some perspective to the work presented in subsequent chapters.

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