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Semiconductor Science and Technology

Microscopic theory of exciton coherent control and Rayleigh scattering in semiconductor quantum wells

Joaquín Fernández-Rossier, C. Tejedor, Roberto D. Merlin

Published Nov 15, 2000• 3 citations
Publisher

Abstract

We present a bosonic description of excitons created by means of resonant optical pumping in a quantum well in the presence of a weak disorder potential. The excitonic collective state is a Glauber coherent state, closely related to that of photons but substantially different from that of few-level models. The theory, applying to the low-density regime, is used to explain various transient linear optical experiments involving resonantly excited excitons in GaAs quantum wells. In particular, we consider coherent control of the exciton density and spin, resonant Rayleigh scattering and scattering coherent control. Our quantum mechanical approach for light emission shows that excitonic secondary emission has a coherent component, in agreement with experiments.

DOI
10.1088/0268-1242/15/12/201

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We present a bosonic description of excitons created by means of resonant optical pumping in a quantum well in the presence of a weak disorder potential. The excitonic collective state is a Glauber coherent state, closely related to that of photons but substantially different from that of few-level models. The theory, applying to the low-density regime, is used to explain various transient linear optical experiments involving resonantly excited excitons in GaAs quantum wells. In particular, we consider coherent control of the exciton density and spin, resonant Rayleigh scattering and scattering coherent control. Our quantum mechanical approach for light emission shows that excitonic secondary emission has a coherent component, in agreement with experiments.
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.
Summary form only given. The light emission of optically excited quantum well excitons can be divided into two parts: the nonscattered contribution (primary emission, where the momentum of the photon is conserved) and the scattered one (secondary emission). Recent theoretical work was focused on isolated mechanisms for the generation of secondary emission, such as exciton-disorder (Rayleigh signal), exciton-phonon and exciton-photon interactions. In extension of these studies, we treat a more general situation by studying the simultaneous action of several emission generating mechanisms for the ease of 1s-exciton excitation in the low density case. The theoretical approach is based on a coupled set of Heisenberg equations of motion for quantum correlation functions of excitons, photons, phonons and their assisted density matrices in a disordered system using 2nd order Born and Markovian approximation. First, we illustrate the procedure and numerical results for a coupled system of excitons, phonons and photons in an ordered quantum well.
Phase-locked pulse pairs are used for resonant excitation of excitons confined within GaAs/AlGaAs quantum wells. The time-resolved secondary emission displays strong dependence on the relative phases of the two pulses for inter-pulse delays of the order of 1ps or less. Constructive interference is combined with an increase in the rise time of the emission following the second pulse. For longer inter-pulse delays the different phases of inhomogeneous excitons complicate the effect of the second pulse, with the apparent effect changing with time after the second pulse. To model this, the optical Bloch equations for a line of three-level systems are integrated over time and the resulting polarisations summed over a range of angles. The importance of the inhomogeneity in descriptions of coherent control of secondary emission is shown.
Summary form only given. The coherence decay of optically excited electronic systems provides one of the most powerful tools to investigate interaction processes of excited states. In semiconductors, the optical coherence phenomena are usually investigated by nonlinear techniques such as four wave mixing (FWM), photon echo, or reflectivity experiments. Exciton-phonon, exciton-exciton, or exciton-free carriers scattering processes have been explored in recent years with these techniques. As demonstrated recently, a direct measurement of the optical dephasing time can be obtained through the temporal coherent control of electronic excitations. In these experiments, the emission following the excitation by an ultrashort optical pulse sequence, is detected in a non-specular direction (secondary emission). This provides an alternative approach to FWM techniques, which allows us to analyse the scattering processes either with phonons or due to mutual collisions. Using the coherent control technique monitored through the secondary emission (SE), we report on the coherent dynamics of excitons in bare quantum wells (QW) and compare it to that of exciton-polaritons in microcavity (MC) embedded quantum wells.
Summary form only given. The optical properties of semiconductor quantum wells (QW) and, in particular, the coherent dynamics of excitons following resonant excitation with ultrafast laser pulses have attracted much attention in recent years. It is generally accepted that there is a transfer of coherence between the optical field and the QW that disappears in a characteristic time T/sub 2/ (picoseconds for GaAs) after the laser is turned off. However, the questions as to how the coherence is actually induced and that of the nature of the coherent state of the solid are poorly understood. In this work we address these points by re-examining the longstanding problem of the (classical vs. quantum) nature of the ubiquitous beats associated with the light-hole (LX) heavy-hole (HX) excitons, which are observed in transient optical experiments on QW. To this end, we consider the coherent behavior of excitons using the simplest albeit non-trivial model where they are treated as non-interacting bosons. Our results bear directly on studies for which nonlinear effects are not important and are not aimed directly at explaining the fourwave-mixing (FWM) experiments that dominate the field. Nevertheless, since nonlinear effects are, typically, weak compared with harmonic contributions, the free boson picture provides in all cases the correct lowest order wavefunction of the photoexcited solid.