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Physical review. B, Condensed matter

Biexcitonic four-wave-mixing signal in quantum wells: Photon-echo versus free-induction decay

H. Nickolaus, F. Henneberger

Published Apr 15, 1998• 22 citations
Publisher

Abstract

We have performed time-integrated as well as time-resolved femtosecond degenerate four-wave-mixing measurements on inhomogeneously broadened (Zn,Cd)Se/ZnSe quantum-well structures. Potential fluctuations due to alloy disorder in the ternary wells give rise to localized quasi-zero-dimensional excitons and biexcitons. For time-integrated detection, they manifest themselves by distinct (disorder-renormalized) quantum beats. Time resolving these transients reveals a free-induction-decay-like emission nature of the biexciton in contrast to the photon-echo exciton contribution. Using a five-level scheme, containing both biexcitons and antibound two-exciton states, this striking emission-type change is explained by fluctuation and correlation effects upon the biexciton binding energy. A combination of time-resolved and time-integrated four-wave mixing is thus a powerful tool to gain information about the disorder in a given kind of quantum structure.

DOI
10.1103/physrevb.57.8774

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Recent transient four-wave-mixing experiments in the two-pulse photon-echo configuration performed on GaAs quantum wells have yielded the surprising result that in some samples the T2{\mathit{T}}_{2}T2​ time obtained from the polarization decay can be shorter for cross-polarized input fields than for parallel input fields. A concomitant change from photon-echo to free-induction-decay behavior has also been observed. A phenomenological model is presented which explains these observations by means of a disorder-induced coupling of the ${\mathrm{\ensuremath{\sigma}}}^{+}$ and ${\mathrm{\ensuremath{\sigma}}}^{\mathrm{\ensuremath{-}}}$ exciton transitions which is inhomogeneous in coupling strength. Such coupling also leads to a remarkable change in the light-hole--heavy-hole exciton quantum beat phase as a function of pulse delay for a critical relative orientation of the polarization of the incident pulses. Experimental observations of this effect are presented to support the validity of the model.
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.