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Quantum Electronics and Laser Science Conference

A new method for study of coherent exciton spin dynamics in GaAs quantum wells

Treena Grevatt, Nicholas Traynor, R.E. Worsley, Richard T. Harley

Published Jun 7, 1996• 0 citations
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Abstract

Summary form only given. Ultrafast dephasing of excitons in semiconductor quantum wells is important because it presents insight into fundamental interactions and possible coherent control of optical properties. Standard measurement techniques include time- and frequency-domain four-wave mixing, resonant Rayleigh scattering, and Faraday rotation. We report on a new time-resolved technique based on reflective probing of pump-induced transient linear birefringence. This has various advantages, including sensitivity, applicability to opaque as-grown samples, ease of implementation, and freedom from superimposed signals.

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IEEE Lasers and Electro-Optics Society Annual Meeting (LEOS)•Dec 30, 2002•0 citations
Summary form only given. Ultrafast dephasing of excitons in semiconductor quantum wells is important because it presents insight into fundamental interactions and possible coherent control of optical properties. Standard measurement techniques include time- and frequency-domain four-wave mixing, resonant Rayleigh scattering, and Faraday rotation. We report on a new time-resolved technique based on reflective probing of pump-induced transient linear birefringence. This has various advantages, including sensitivity, applicability to opaque as-grown samples, ease of implementation, and freedom from superimposed signals.
Coherent optical spectroscopy in the form of nonlinear transient four-wave mixing (TFWM) and linear resonant Rayleigh scattering (RRS) has been applied to investigate the exciton dynamics of low-dimensional semiconductor heterostructures. The dephasing times of excitons are determined from the decay of the spectrally resolved non- linear signal as a function of the delay between the incident pulses in a two-beam TFWM experiment, and from the real time analysis of single speckles in RRS experiments (pure dephasing). From the density- and temperature- dependence of the dephasing times the exciton-exciton and the exciton-phonon interactions are determined. The degree of coherence of the secondary emission is determined from the speckle analysis.
Summary form only given. The coherence decay of optically excited electronic systems provides one of the most powerful tools to investigate interaction processes of excited states. In semiconductors, the optical coherence phenomena are usually investigated by nonlinear techniques such as four wave mixing (FWM), photon echo, or reflectivity experiments. Exciton-phonon, exciton-exciton, or exciton-free carriers scattering processes have been explored in recent years with these techniques. As demonstrated recently, a direct measurement of the optical dephasing time can be obtained through the temporal coherent control of electronic excitations. In these experiments, the emission following the excitation by an ultrashort optical pulse sequence, is detected in a non-specular direction (secondary emission). This provides an alternative approach to FWM techniques, which allows us to analyse the scattering processes either with phonons or due to mutual collisions. Using the coherent control technique monitored through the secondary emission (SE), we report on the coherent dynamics of excitons in bare quantum wells (QW) and compare it to that of exciton-polaritons in microcavity (MC) embedded quantum wells.
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
We use reflective probing of transient linear birefringence to study coherent exciton dynamics in GaAs quantum wells as a function of applied magnetic field at low temperatures. The results show the significance of exchange-enhanced exciton spin relaxation for dephasing at zero field. In applied field the signals show background-free quantum beating which gives the exciton g factor and field-dependent dephasing time. The latter is dominated by spin relaxation which shows for the first time the dramatic slowdown predicted in earlier work.
The coherent emission from excitons corresponding to different subband transitions in a 500 Åbroad GaAs/AlGaAs quantum well is studied by spectrally resolved transient four-wave mixing (FWM). For the simultaneous excitation of several subband transitions, the dephasing times of the lowest exciton and higher- order exciton transitions are determined from the width of the resonances in the FWM spectrum. The decay of the spectrally integrated FWM signal as a function of the time delay between the excitation laser pulses does not reflect the dephasing times but is determined by the laser pulse width for multi-subband excitation. This is indicative of quantum interference between exciton and continuum transitions. Many-body Coulomb effects have to be included in a theoretical treatment of these experimental observations.