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Physical review. B, Condensed matter

Ultrafast coherent dynamics in quantum wells for multisubband excitation in different density regimes

S. Arlt, U. Siegner, François Morier-Genoud, Ursula Keller

Published Nov 15, 1998• 8 citations
Publisher

Abstract

We present an experimental study of the coherent dynamics in quantum wells for the simultaneous excitation of Lorentzian excitons below the band edge and structured exciton Fano continua corresponding to higher-order subband transitions. Spectrally resolved transient four-wave mixing (FWM) experiments with 100 fs pulses were performed on a 500 \AA{} broad GaAs/${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum well over a wide range of carrier densities. At lower carrier densities, the dephasing times of the different exciton transitions were determined from the width of the narrow resonances in the distinctly structured FWM spectra. These data demonstrate that the decay of the FWM signal from exciton Fano continua in the time-delay domain is not determined by dephasing at the lowest carrier densities. This is indicative of quantum interference which has its origin in many-body exciton-continuum coupling. At higher carrier densities, the FWM spectra are much less structured due to stronger dephasing, quantum interference effects lose their importance, and the structured exciton Fano continua show properties similar to unstructured continua without embedded exciton transitions.

DOI
10.1103/physrevb.58.13073

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We present an experimental study of the coherent dynamics in quantum wells for the simultaneous excitation of Lorentzian excitons below the band edge and structured exciton Fano continua corresponding to higher-order subband transitions. Spectrally resolved transient four-wave mixing (FWM) experiments with 100 fs pulses were performed on a 500 \AA{} broad GaAs/AlxGa1\ensuremath−xAs{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}Alx​Ga1\ensuremath−x​As quantum well over a wide range of carrier densities. At lower carrier densities, the dephasing times of the different exciton transitions were determined from the width of the narrow resonances in the distinctly structured FWM spectra. These data demonstrate that the decay of the FWM signal from exciton Fano continua in the time-delay domain is not determined by dephasing at the lowest carrier densities. This is indicative of quantum interference which has its origin in many-body exciton-continuum coupling. At higher carrier densities, the FWM spectra are much less structured due to stronger dephasing, quantum interference effects lose their importance, and the structured exciton Fano continua show properties similar to unstructured continua without embedded exciton transitions.
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.
We have studied the coherent emission from bulk semiconductors by spectrally resolved and spectrally integrated four-wave mixing (FWM) with broadband 16-fs pulses in the carrier density range from $1\ifmmode\times\else\texttimes\fi{}{10}^{16}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ to $6\ifmmode\times\else\texttimes\fi{}{10}^{17}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}.$ In GaAs, only continuum transitions are excited, while both excitonic and continuum transitions are excited in Al0.06Ga0.94As.{\mathrm{Al}}_{0.06}{\mathrm{Ga}}_{0.94}\mathrm{As}.Al0.06​Ga0.94​As. The decay of the FWM signal for positive time delays unambiguously reflects the dephasing of the continuum, irrespective of the excitation of excitonic transitions. We find identical ultrafast sub-20-fs decay times of the coherent emission from continuum transitions in GaAs and Al0.06Ga0.94As,{\mathrm{Al}}_{0.06}{\mathrm{Ga}}_{0.94}\mathrm{As},Al0.06​Ga0.94​As, independent of the excess excitation energy. These decay times depend only weakly on the carrier density for densities below ${10}^{17}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}.$ The continuum dephasing is analyzed considering carrier-carrier and carrier-LO-phonon scattering in a relaxation rate approach. The excitonic transitions in Al0.06Ga0.94As{\mathrm{Al}}_{0.06}{\mathrm{Ga}}_{0.94}\mathrm{As}Al0.06​Ga0.94​As dominate the FWM signal at negative time delays.
Summary form only given. We performed a comprehensive four-wave-mixing (FWM) study of the optical dephasing in bulk GaAs and AlGaAs with broadband 16-fs laser pulses. In contrast to previous work, spectral resolution has allowed us to unambiguously relate the decay of the coherent emission to either continuum or excitonic transitions. We have extended earlier measurements of continuum dephasing to one order of magnitude lower carrier densities. We find much shorter dephasing times and a much weaker density dependence of the dephasing than expected from the extrapolation of the high-density results. Our data indicate that a quantum kinetics model for both Coulomb and LO-phonon scattering is required to describe continuum dephasing on the 10-fs time scale at lower densities.
We present an experimental and theoretical investigation of ultrafast transient four-wave mixing of GaAs/ Alx{\mathrm{Al}}_{\mathit{x}}Alx​${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells for coherent excitation of exciton and continuum states. The signal appears at the exciton resonance and is shown to consist of two contributions: an intense spectrally narrow part only present for short time delays and a weak exponentially decaying part reflecting the intrinsic coherence properties of the exciton states. The intense part of the signal is due to the excitation of the continuum states mediated by interaction-induced nonlinearities. \textcopyright{} 1996 The American Physical Society.
Summary form only given. Optical interactions in semiconductor microcavities embedded with quantum wells enter a nonperturbative regime when the dipole coupling rate between the exciton and the cavity mode exceeds relevant decay rates. In this regime, coherent energy exchange between the exciton and the cavity mode, i.e. normal mode oscillation (NMO), takes place when the cavity mode is excited impulsively by a short laser pulse. Similar coherent oscillations have also been observed in transient four-wave mixing (FWM) responses. In this paper, we present experimental and theoretical studies on the nature of oscillations in these transient FWM responses. We show that, in contrast to earlier beliefs, oscillations in time-integrated transient FWM do not correspond to NMO of the exciton or photon populations. Theoretical studies further reveal that FWM arising from different physical mechanisms including band filling, excitation-induced dephasing, and renormalized electric fields is characterized by highly distinct coherent dynamics. Oscillations in transient FWM reflect interference between these coherent optical processes and can be used as a unique probe for exciton many-body interactions in the nonperturbative regime.