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Conference on Lasers and Electro-Optics

Direct observation of the decay of a coherent polarization into an incoherent exciton population in quantum wells

Stefan Haacke, Gary R. Hayes, M. Kauer, Robert Anthony Taylor

Published Nov 27, 2002• 0 citations
Publisher

Abstract

Summary form only given. We experimentally demonstrate the validity of the concept that separates coherent polarization and incoherent exciton population. We observed directly the transfer of polarization into population. The decay of the microscopic polarization builds up the population, but the macroscopic polarization in form of Rayleigh scattering decays faster when inhomogeneous broadening dominates.

DOI
10.1109/iqec.1998.680473

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Summary form only given. We experimentally demonstrate the validity of the concept that separates coherent polarization and incoherent exciton population. We observed directly the transfer of polarization into population. The decay of the microscopic polarization builds up the population, but the macroscopic polarization in form of Rayleigh scattering decays faster when inhomogeneous broadening dominates.
Coherent Rayleigh scattering and incoherent luminescence are studied by femtosecond upconversion under resonant excitation as a function of exciton density in GaAs quantum wells. Comparing samples with different well widths allows to establish the relation between inhomogeneous broadening and the shape of resonant Rayleigh scattering. We find an excellent agreement between experiment and a classical model. In samples with moderate inhomogeneous broadening we provide evidence for the transition from coherent to incoherent emission. The comparison with excitonic dephasing times T2 obtained from four-wave-mixing experiments demonstrates that the incoherent exciton population rises within T2/2, at any exciton density when changed by more than an orders of magnitude. The experimental results confirm very clearly the concept predicting that the coherent polarization made up by localized excitons decays into an incoherent exciton population under conservation of the number of excited quanta.
Temporal coherent control experiment have been performed on semiconductor quantum well structures, using a sequence of polarized excitation pulses. The oscillatory behaviour of the populations, spin and alignment of the exciton states are observed through the time resolved secondary emission when the time separation between the two excitation pulses is varied. We show that, in specular detection direction, for bare quantum wells at 10 K and low exciton densities, the oscillations amplitude decay is determined by the exciton inhomogeneous broadening. In non-specular detection directions however, when the SE is dominated by resonant Rayleigh scattering, the decay is longer and becomes temperature dependent, which strongly supports that it is now determined by the exciton dephasing time.
We have investigated the coherent dynamics of excitons in a single, a double, and a quintuple quantum-well structure under resonant excitation. In pump-and-probe experiments on the quintuple quantum well we observe a fast decay of the excitonic population within the initial coherent regime. This fast initial population decay is less pronounced for the double quantum well and is almost absent for the single quantum well. The energy linked to the initially lost coherent population is found in the coherent electromagnetic emission induced by the first-order polarization. More specifically, the intensity of this directional light emission increases quadratically with the number of quantum wells. Interference effects and electromagnetic coupling effects between polarizations located in distinct quantum wells probably play a crucial role in the superlinear coherent emission process.
When a system is excited by an ultrashort pulse of light, a macroscopic polarization is created. After the excitation, the coherently prepared dipoles radiate in the forward direction while losing coherence because of dephasing by scattering, diffusion, etc., resulting in an exponential decay in time of the macroscopic polarization. Thus, the signal in the forward direction, the free polarization decay (FPD), has a decay time related to this polarization decay time (T2), and in the limit of delta-function pulses it has no rise time.