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Journal of Physics Condensed Matter

Fano-type interference processes in linear photoluminescence spectra of infrared-driven quantum wells

Seyed M. Sadeghi, Jochen Meyer

Published Sep 8, 1997• 5 citations
Publisher

Abstract

Quantum interference processes are investigated in the diagonal and nondiagonal excitonic emission spectra of quantum well structures coupled by single infrared fields polarized along their growth directions and resonant with the first-to-second-conduction-subband transitions. This investigation predicts generation of the Autler - Townes doublet in the emission spectrum of the coupled ground exciton state (diagonal transition) and development of a dynamic emission hole followed by a dark line in that of the higher coupled exciton states (nondiagonal transition). The characteristic limits determining these processes are determined and reported experimental results are explained successfully.

DOI
10.1088/0953-8984/9/36/016

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Quantum interference processes are investigated in the diagonal and nondiagonal excitonic emission spectra of quantum well structures coupled by single infrared fields polarized along their growth directions and resonant with the first-to-second-conduction-subband transitions. This investigation predicts generation of the Autler - Townes doublet in the emission spectrum of the coupled ground exciton state (diagonal transition) and development of a dynamic emission hole followed by a dark line in that of the higher coupled exciton states (nondiagonal transition). The characteristic limits determining these processes are determined and reported experimental results are explained successfully.
We study quantum interference and nonlinear optical processes in the conduction band of an n-doped quantum well driven by one or two infrared fields. The fields couple a broad upper subband to a lower energy subband and the ground subband, forming a \ensuremath{\Lambda} configuration. We show, including the effect of electron-electron scattering and dynamic screening, that this configuration can lead to coherent control of the intersubband transitions, such as coherent population trapping. We also study quantum interference processes, such as laser-induced transparency and dark-line effects in the linear absorption and gain spectra of the system. The effects on these processes of differing field intensities and of electron-electron scattering are also discussed.
We theoretically and experimentally investigate the non-linear optical properties of undoped and n-doped quantum wells (QW's) in the presence of intense infrared lasers. These lasers are polarized along the QW growth directions and near resonance with the transitions between the conduction subbands. In n-doped QW's by inclusion of electronelectron scattering and dynamic screening we study for the first time: (i) the effects of the frequencies and intensities of the infrared fields on the dephasing rates associated with the intersubband transitions, (ii) the generation of various nonlinear gain processes, and (iii) coherent and optical processes such as coherent population trapping, dark line and laser induced transparency effects. In undoped QW's by presenting a consistent theory we show how one can use infrared coupling of the conduction subbands of a QW to coherently manipulate absorption and emission spectra of excitons. We discuss these effects in.terms of multi-photon coupling of the excitons or electrons, quantum interferences, and by presenting a new model to treat laser-matter interaction. Following these developments we put some of our own results to test by observing the emission dynamics of excitons in the presence of an intense CO2 laser. We report the first observations of infrared enhancement of the effective non-radiative decay rates of excitons and the effects of multi-level transition processes in the emission of QW's. This thesis also includes an extensive analysis of nonlinear effects in atomic systems. While the primary motivation for this analysis was to understand and design nonlinear optics of QW's, in the process some original contributions were made to the study of nonlinear optics of atomic systems.
We present an experimental study of the coherent dynamics in quantum wells for the simultaneous excitation of Lorentzian excitons below the band edge and structured exciton Fano continua corresponding to higher-order subband transitions. Spectrally resolved transient four-wave mixing (FWM) experiments with 100 fs pulses were performed on a 500 \AA{} broad GaAs/${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum well over a wide range of carrier densities. At lower carrier densities, the dephasing times of the different exciton transitions were determined from the width of the narrow resonances in the distinctly structured FWM spectra. These data demonstrate that the decay of the FWM signal from exciton Fano continua in the time-delay domain is not determined by dephasing at the lowest carrier densities. This is indicative of quantum interference which has its origin in many-body exciton-continuum coupling. At higher carrier densities, the FWM spectra are much less structured due to stronger dephasing, quantum interference effects lose their importance, and the structured exciton Fano continua show properties similar to unstructured continua without embedded exciton transitions.
The coherent emission from excitons corresponding to different subband transitions in a 500 Åbroad GaAs/AlGaAs quantum well is studied by spectrally resolved transient four-wave mixing (FWM). For the simultaneous excitation of several subband transitions, the dephasing times of the lowest exciton and higher- order exciton transitions are determined from the width of the resonances in the FWM spectrum. The decay of the spectrally integrated FWM signal as a function of the time delay between the excitation laser pulses does not reflect the dephasing times but is determined by the laser pulse width for multi-subband excitation. This is indicative of quantum interference between exciton and continuum transitions. Many-body Coulomb effects have to be included in a theoretical treatment of these experimental observations.
We study enhancement of the 2s2s2s state emission intensity of the e1-hh1 excitons via generation of dark states. This process is shown for a In0.43Ga0.57As/InP{\mathrm{In}}_{0.43}{\mathrm{Ga}}_{0.57}\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}\mathrm{P}In0.43​Ga0.57​As/InP quantum well structure interacting with a single infrared laser near resonance with the transition between e1 and e2. Via generation of strong nonparabolic hole dispersion, the tensile strain of this structure enhances the dipole moments of the nonallowed transitions between the 2s2s2s states of the e1-hh1 and the 1s1s1s states of e2-hh1 excitons. We show that, due to the simultaneous coupling of these transitions and those between the 1s1s1s states of the e1-hh1 and e2-hh1 excitons (allowed transitions), the infrared field can generate dark states, coherently pumping the 2s2s2s states of e1-hh1 excitons and dramatically increasing their emission intensities. We also show that when the allowed and nonallowed transitions are driven by two resonant infrared field beams, coherent population trapping of excitons of the e1-hh1 excitons occurs.
Optical interband excitonic absorption of semiconductor quantum wells (QW’s) driven by a coherent pump field is investigated on the basis of semiconductor Bloch equations. The pump field has a photon energy close to the intersubband spacing between the first two conduction subbands in the QW’s. An external weak optical field probes the interband transition. The excitonic effects and pump-induced population redistribution within the conduction subbands in the QW system are included. When the density of the electron–hole pairs in the QW structure is low, the pump field induces an Autler–Townes splitting of the exciton absorption spectrum. The split size and the peak positions of the absorption doublet depend not only on the pump frequency and intensity but also on the carrier density. As the density of the electron–hole pairs is increased, the split contrast (the ratio between the maximum and the minimum values) is decreased, because the exciton effect is suppressed at higher densities owing to the many-body screening.