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Radiative Processes and Dephasing in Semiconductors

Characterization of the ultrafast resonant secondary emission from GaAs quantum well

Massimo Gurioli, Franco Bogani, Daniele Capanni, M. Colocci, Simone Ceccherini, A. Vinattieri

Published Jan 1, 1998• 0 citations
Publisher

Abstract

The coherent and incoherent components of the resonant secondary emission from GaAs quantum well structures after ultrashort excitation have been studied as a function of the excitation density, lattice temperature, and inhomogeneous broadening by means of interferometric correlation techniques. We find that the coherent emission is thermally quenched and saturates with the excitation power. A good agreement is found within a simple classical model based on the inhomogeneous broadening of the excitonic band, including the experimental exciton-exciton and exciton-phonon induced dephasing.

DOI
10.1364/rpds.1998.rmd1

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The coherent and incoherent components of the resonant secondary emission from GaAs quantum well structures after ultrashort excitation have been studied as a function of the excitation density, lattice temperature, and inhomogeneous broadening by means of interferometric correlation techniques. We find that the coherent emission is thermally quenched and saturates with the excitation power. A good agreement is found within a simple classical model based on the inhomogeneous broadening of the excitonic band, including the experimental exciton-exciton and exciton-phonon induced dephasing.
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We present a comparative study of time-integrated four-wave-mixing and femtosecond emission under resonant excitation on excitons in weakly disordered GaAs quantum wells. At highest exciton densities when dephasing dominates the spectral width (homogeneous broadening), we find that the rise time of the incoherent luminescence signal is given by TIii 2/2. At lowest densities, optical coherence times approach the exciton radiative lifetime (15 to 20 ps). This confirms our previous result that coherent resonant Rayleigh scattering is responsible for the short rise time of the excitonic emission. We also show clear evidence for dephasing due to exciton–phonon interaction, as the rise time of the emission decreases dramatically when the sample temperature is increased.
By degenerate four-wave mixing experiments in a two-beam geometry, we have investigated the initial coherence and dephasing of quasi two-dimensional excitions and biexcitons in GaAs multiple quantum wells. The dephasing has contributions from phonon scattering, interface-roughness scattering and exciton-exciton scattering. Inhomogeneous broadening and generation of coherent wavepackets play a significant role in the coherent exciton dynamics. The incoherent exciton dynamics, diffusion and recombination, is studied by three-beam transient grating experiments. A significant difference in the interface roughness scattering of coherent and incoherent (thermalized) excitons is observed.
Summary form only given. We present the first direct investigation of the coherence properties of resonant secondary emission (RSE) from quantum well excitons. Using ultrafast spectral interferometry, or TADPOLE, we measure spectral fringes that are resonantly enhanced at the exciton transitions. The fringe formation is due to the coherent Rayleigh component of the RSE, and allows us to investigate the dynamics of resonant Rayleigh scattering independent from incoherent luminescence. Our observation of high-contrast spectral fringes is in contradiction with current theories of time resolved resonant Rayleigh scattering from excitons and shows for the first time the non-ergodic nature of the RSE.
Secondary emission from GaAs quantum wells following resonant pulsed excitation is studied in the time domain. Selective excitation of heavy- and light-hole excitons is accomplished in a two-pulse excitation experiment which preserves sub-ps time resolution. After the initial rise the emission decays exponentially at a fast rate and then with another much slower exponential decay at longer times. The variations in the initial decay times with excitation density and sample temperature are similar to the variation in dephasing rates measured in four-wave mixing experiments. The decay in the visibility of heavy- and light-hole exciton beating is consistent with the decay rates measured for both species separately.