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Journal of the Optical Society of America B

Coherent dynamics of excitons and excitonic complexes in ZnSe epilayers

U. Neukirch, D. Weckendrup, Kai Wundke, J. Gutowski, D. Hommel

Published Jun 1, 1996• 13 citations
Publisher

Abstract

The contribution of bound excitons and biexcitons to degenerate four-wave mixing signals is investigated in high-quality biaxially strained ZnSe layers. For the former complexes signatures of polarization interference with free excitons are found in time-resolved four-wave mixing, whereas spectrally resolved data point to the occurrence of quantum beating. Thus this system behaves as a mixture of independent two-level systems on the one hand and three-level systems on the other. Biexcitons contribute to the signal mainly at negative delay between the exciting pulses and are identified by means of spectrally resolved measurements and by use of different configurations of the pulse polarization. Complete agreement is found between the selection rules for biexcitons (and excitons) and the experimental data.

DOI
10.1364/josab.13.001256

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•Jan 1, 2006•0 citations
The contribution of bound excitons and biexcitons to degenerate four-wave mixing signals is investigated in high-quality biaxially strained ZnSe layers. For the former complexes signatures of polarization interference with free excitons are found in time-resolved four-wave mixing, whereas spectrally resolved data point to the occurrence of quantum beating. Thus this system behaves as a mixture of independent two-level systems on the one hand and three-level systems on the other. Biexcitons contribute to the signal mainly at negative delay between the exciting pulses and are identified by means of spectrally resolved measurements and by use of different configurations of the pulse polarization. Complete agreement is found between the selection rules for biexcitons (and excitons) and the experimental data.
A systematic analysis of excitonic and biexcitonic contributions to four-wave mixing in dependence on the polarization state and on the intensity of the exciting beams is presented for ZnSe/Zn1—xMgxSySe1—iiy quantum wells. The relative signals of excitons and biexcitons for different polarization configurations are discussed in terms of a microscopic density matrix model revealing excellent correspondence to the complex experimental data. In configurations which do not allow for a third-order contribution of the biexciton of zero angular momentum higher-order correlations are shown to become important already at densities as low as 1010 excitons cm—2/pulse.
Biexcitonic polarization excited in a ZnSe quantum well is coherently controlled with a pair of phase-locked laser pulses. The coherent control of this four-particle correlation is directly verified by observing a spectrally resolved four-wave-mixing signal in direction of the first order of diffraction of the pulse pair. By using polarization selection rules for the generation of biexcitons and changing the interpulse delay of the pulse pair the oscillations of the four-wave-mixing signal are studied at the energetic position of the excitonic as well as the biexcitonic resonance. The excitonic and biexcitonic signals exhibit a phase shift in dependence on the interpulse delay which clearly indicates the separate coherent control of the excitonic and biexcitonic polarization.
We report time-resolved degenerate four-wave-mixing experiments on a high-quality single GaAs quantum well. The signals for parallel and cross-polarized exciting pulses show pronounced quantum beats with different frequencies corresponding to the heavy/light-hole splitting and the biexciton binding energy, respectively. Comparison of the experimental data with solutions of the optical Bloch equations for a phenomenological model system shows that these features result from strong contributions of the biexcitonic state. The importance of biexcitons to the nonlinear coherent response of the two-dimension exciton is further confirmed by the frequency dependence of the time-integrated four-wave-mixing signal amplitudes measured for the two polarization geometries.
Biexcitonic complexes in a ZnSe single quantum well are investigated by spectrally resolved four-wave mixing (FWM). The formation of heavy-heavy-hole XXh{\mathrm{XX}}_{h}XXh​ and of mixed heavy-light-hole XXm{\mathrm{XX}}_{m}XXm​ biexcitons showing binding energies of ${\ensuremath{\Delta}}_{h}=4.8\mathrm{meV}$ and ${\ensuremath{\Delta}}_{m}=2.8\mathrm{meV}$ is identified by polarization selection rules. The coherent dynamics of the FWM response and the observed FWM intensity ratio between the XXh{\mathrm{XX}}_{h}XXh​ and XXm{\mathrm{XX}}_{m}XXm​ biexciton-induced nonlinear signals are in agreement with the solution of an extended optical Bloch equation.
The complex polarization behavior of quantum beating between heavy- and light-hole excitons in nonlinear pump-and-probe spectroscopy is experimentally investigated. The strain-split free excitons in thin high-quality ZnSe layers embedded in ternary ZnSx{\mathrm{ZnS}}_{\mathit{x}}ZnSx​${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ barriers served as the three-level system involved. Spectrally integrated pump-and-probe experiments confirm the theoretically predicted dependence of the quantum-beat phase on the relative polarization of the exciting fields. These observations are discussed in comparison with results from four-wave mixing obtained when using the same experimental setup. Beyond that, spectrally resolved transients measured on the light-hole resonance exhibit pronounced, weakly damped beating phenomena even at negative delay, i.e., with the probe pulses preceding the pump pulses. Additionally, for cross-polarized beams the quantum-beat phase is strongly dependent on the energy of detection. It turns out that the phase observed in the spectrally integrated mode is determined by the off-resonant contributions. \textcopyright{} 1996 The American Physical Society.
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 ∑ -+