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AIP conference proceedings

Temperature Dependence of the Dephasing of Excitonic and Biexcitonic Polarization in a ZnSe Single Quantum Well

T. Voss

Published Jan 1, 2005• 0 citations
Publisher

Abstract

The polarization decay of the exciton and bound‐biexciton states in a ZnSe single quantum well was studied as a function of the temperature in real‐time‐resolved pulse‐transmission and spectrally resolved four‐wave‐mixing experiments. Optical and acoustic‐phonon scattering coefficients for both resonances were determined and found to be comparable. Additionally, the impact of the dephasing of the coherent excitonic polarization on coherent‐control experiments was analyzed as a function of the sample temperature.

DOI
10.1063/1.1994561

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The polarization decay of the exciton and bound‐biexciton states in a ZnSe single quantum well was studied as a function of the temperature in real‐time‐resolved pulse‐transmission and spectrally resolved four‐wave‐mixing experiments. Optical and acoustic‐phonon scattering coefficients for both resonances were determined and found to be comparable. Additionally, the impact of the dephasing of the coherent excitonic polarization on coherent‐control experiments was analyzed as a function of the sample temperature.
The coherent control of excitonic and biexcitonic polarization in pulse-transmission and four-wave-mixing (FWM) experiments is analysed with real time resolution. As a model system a ZnSe single quantum well is exclusively excited at the heavy-hole exciton and exciton–biexciton resonance. In pulse-transmission experiments the coherent control of excitonic polarization is shown for low excitation intensities. At elevated excitation intensities the dephasing time of the polarization is found to be strongly dependent on the relative phase of two phase-locked laser pulses which perform the coherent manipulation of the polarization. In FWM experiments the separate coherent control of the excitonic and biexcitonic FWM polarization is demonstrated which enables a coherent manipulation of the amplitude and phase of exciton–biexciton beats which are observed on the FWM polarization transients.
Optical coherent manipulation of excitonic polarization is studied in pulse-transmission experiments at medium to high excitation intensities. As a well-defined model system a ZnSe single-quantum well is used. It is found that for constant excitation intensities the dephasing time of the polarization in the sample changes significantly if the cases of coherent amplification and destruction are compared. This effect is explained in terms of an excitation-induced dephasing. The change of the dephasing time of the polarization is quantified. Furthermore, the simultaneous excitation of the exciton and exciton-biexciton resonances is shown to strongly influence the signal of the polarization in the coherent-control experiments. Both the excitation-induced dephasing and the biexcitonic effects are simulated by use of phenomenological models which give a good agreement with the experiments.
Abstract We investigate coherences between σ+ and σ− exciton states in a 10 nm thick ZnSe/ZnMgSe single quantum well by spectrally resolved three‐pulse degenerate four‐wave mixing using circularly polarized 100 fs pulses. Intensity dependent measurements are performed to study the influence of exciton–exciton scattering on their dephasing rates. The experiments reveal nearly equal zero‐density dephasing rates for coherences between σ+ and σ− heavy‐ and/or light‐hole exciton states as compared to the dephasing rate of the interband coherence between these states and the ground state. The measurements also demonstrate that exciton‐exciton scattering destroys the coherence between heavy‐ and light‐hole excitons about three times more effectively than the coherence between σ+ and σ− heavy‐hole excitons. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The optical coherent-control technique is used to study biexcitonic effects in the four-wave-mixing signal of a ZnSe single quantum well. The signal is analyzed in both directions 2k1 − k2 and 2k2 − k1 which are not equivalent if a pulse pair is applied from direction k1 to achieve coherent control of the induced polarization. It is shown that the coherent control enables a selective enhancement or suppression of the contribution at the exciton and biexciton resonance to the signal, respectively, but only for certain sequences of the excitation pulses. Further, the suppression of exciton-biexciton beats in the signal as a function of tdel by a selective destruction of the biexciton polarization is demonstrated.
Spectrally resolved and time-integrated degenerate four-wave mixing (FWM) experiments with ultra-short pulses have been used to investigate the relaxation times and coherent interactions of excitons in the Boltzmann and in the quantum kinetic regime in ZnSe single quantum wells.Three-pulse FWM experiments using 100 fs pulses in a 10 nm thick ZnSe/ZnMgSe single quantum well reveal nearly equal zero-density dephasing rates for coherences between + σ and - σ heavy-and/or light-hole exciton states as compared to the dephasing rate of the interband coherence between these states and the ground state.The measurements also demonstrate that exciton-exciton scattering destroys the phase coherence between heavy-and light-hole excitons about three times more effectively than the coherence between + σ and + σ heavy-hole excitons.The different coherent nonlinear signals obtained in a three-beam FWM experiment are explained and reproduced by numerical calculations of the optical Bloch equations for a ten-level model including excitation induced dephasing (EID).Two-beam FWM experiments using 30 fs pulses on a 10 nm ZnSe quantum well reveal marked quantum beats and a pronounced non-exponential decay for delay times smaller ~500 fs.The pronounced peak at pulse overlap is attributed to rapid destructive interference of the created polarization and to spin dependent quantum kinetic effects caused by the transient formation of an electron-hole (eh) pair grating.The three-beam FWM trace shows a bi-exponential decay which is attributed to a nonlinear polarization that is caused by the interaction of polarization 3 k with both an exciton grating and an eh-pair grating.Model calculations using the OBEs of a two level system that includes ∑ -+
Abstract This paper studies coherent excitons in the presence of alloy disorder by femtosecond degenerate four‐wave mixing on ternary (ZnCd)Se/ZnSe quantum wells. The use of very low excitation densities enables to uncover an impurity‐mediated exciton dephasing time of (16 ± 2) ps and a related homogeneous exciton width of only 80 μeV. At higher densities, the coherence loss speeds up dramatically due to exciton‐exciton interaction. Separating the contributions from inhomogeneous (photon echo) and homogeneous broadening to the spectrally resolved decay transients allows to deduce the energy dependence of the dephasing time across the disorder broadened exciton ensemble. Further, pronounced biexciton resonances are observed in the signal spectrum which exhibit characteristic polarization properties. For cross‐polarized pulses, the biexciton signal shows a striking stimulation when the excitation density is increased.