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physica status solidi (a)

Biexcitonic Bound and Continuum States of Homogeneously and Inhomogeneously Broadened Exciton Resonances

W. Langbein, J. M. Hvam

Published Mar 1, 2002• 0 citations
Publisher

Abstract

In this paper by Wolfgang Langbein and Jørn M. Hvam [1], selected as Editor's Choice in the present issue of phys. stat. sol. (a), the localization of biexcitonic and excitonic states in GaAs quantum wells is investigated by spectrally resolved four-wave mixing (FWM) experiments. The cover picture is a contour plot of the FWM intensity as a function of temporal pulse separation τ12 for cross-linear and co-polarization, scaled with the exciton binding energy R*y. Data are shown for three single and multi quantum well structures with different thicknesses, causing varying homogeneous and inhomogeneous broadening of the excitonic transitions (for details see [1]). The paper is an invited presentation and part of the Proceedings of the International Workshop on “Physics of Light–Matter Coupling in Nitrides (PLMCN-1)” held in Rome, Italy, September 26–29, 2001.

DOI
10.1002/1521-396x(200203)190:1<1::aid-pssa1>3.3.co;2-o

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In this paper by Wolfgang Langbein and Jørn M. Hvam [1], selected as Editor's Choice in the present issue of phys. stat. sol. (a), the localization of biexcitonic and excitonic states in GaAs quantum wells is investigated by spectrally resolved four-wave mixing (FWM) experiments. The cover picture is a contour plot of the FWM intensity as a function of temporal pulse separation τ12 for cross-linear and co-polarization, scaled with the exciton binding energy R*y. Data are shown for three single and multi quantum well structures with different thicknesses, causing varying homogeneous and inhomogeneous broadening of the excitonic transitions (for details see [1]). The paper is an invited presentation and part of the Proceedings of the International Workshop on “Physics of Light–Matter Coupling in Nitrides (PLMCN-1)” held in Rome, Italy, September 26–29, 2001.
In this paper by Wolfgang Langbein and Jørn M. Hvam [1], selected as Editor's Choice in the present issue of phys. stat. sol. (a), the localization of biexcitonic and excitonic states in GaAs quantum wells is investigated by spectrally resolved four-wave mixing (FWM) experiments. The cover picture is a contour plot of the FWM intensity as a function of temporal pulse separation τ12 for cross-linear and co-polarization, scaled with the exciton binding energy R*y. Data are shown for three single and multi quantum well structures with different thicknesses, causing varying homogeneous and inhomogeneous broadening of the excitonic transitions (for details see [1]). The paper is an invited presentation and part of the Proceedings of the International Workshop on “Physics of Light–Matter Coupling in Nitrides (PLMCN-1)” held in Rome, Italy, September 26–29, 2001.
We investigate the four-wave mixing response of semiconductor quantum wells having a predominantly inhomogeneous broadening of the exciton states. The influence of the biexcitons on the nonlinear emission, in proximity to the lowest exciton resonance, is studied. Using about 1 ps long pulses, we show that biexcitons are created for all excitation energies lying within the broadened exciton absorption line, and that their binding energy does not depend on the exciton energy. Reducing the temporal width of cross-linearly polarized incident laser fields to about 0.1 ps, we observe a drastic changeover from a prompt response to a photon echo, and quite interesting beating phenomena, visible in time-integrated as well as in time-resolved experiments, appear. Our observations compare well with the numerical solutions of the optical Bloch equations for a five-level scheme. The influence of the inhomogeneous broadening on the beating phenomena is discussed.
The coherent response of excitons in semiconductor nanostructures measured in four-wave mixing (FWM) depends strongly on the inhomogeneous broadening of the exciton transition. We investigate GaAs/Al0.3Ga0.7As single quantum wells (SQW) and AlxGa1-xAs mixed crystals. Additional to the usual phase-space filling nonlinearity, excitation-induced dephasing (EID) and biexciton formation (BIF) are important. EID leads to a strong dependence of the signal on the angle between the linear input polarizations. We find that EID persists in inhomogeneous systems, showing that the mutual density-dependent dephasing rate difference between two subsystems within the inhomogeneous distribution is strongly dependent on their energy difference. BIF is strongly affecting the cross-linear polarized FWM response. The signal for positive delay is dominated by the transitions from the one-exciton state X to the two-exciton states. Here, the third-order polarization at X-XX is not at the same resonance as the first-order polarization at 0-X. Consequently, the rephasing of the microscopic third-order polarization to the macroscopic FWM photon echo is blurred by the non-perfect correlation of X and XX energies, leading to a fast and non-exponential signal decay in delay time. For inhomogeneous broadenings larger than the biexciton binding energy, we find an enhanced biexciton binding energy, and a quenching of the oscillator strength of the unbound biexciton.
Summary form only given.The coherent response of excitons in semiconductor nanostructures, as measured in four wave mixing (FWM) experiments, depends strongly on the inhomogeneous broadening of the exciton transition. We investigate GaAs-AlGaAs single quantum wells (SQW) of 4 nm to 25 nm well width. Two main mechanisms are found to be important. First, the excitation-induced dephasing FWM signal (EID), which leads to a strong dependence of the signal on the angle between the linear input polarizations. The presence of the EID in the mainly homogeneously broadened sample (25 nm) is shown by the beating between EID and two-photon coherence (TPC) at the exciton for negative delay and co-linear polarized excitation. It vanishes for cross-polarized excitation, thus excluding the importance of local-field effects.
Current issues in the study of light-matter interactions are coherence and quantum interference. With ultrafast nonlinear spectroscopy such as transient four-wave mixing (TFWM) the imposed coherence on the optical excitations in e.g. GaAs multiple quantum wells has been studied. In this contribution, we study the influence of inhomogeneous broadening on quantum beats observed by spectrally resolved TFWM. In artificially grown semiconductor structures as e.g. quantum wells, inhomogeneities due to interface-disorder has, up till now, been inevitable. In the experiments, the light-hole exciton (lhx) and the heavy-hole exciton (hhx) in a 100 Å GaAs multiple quantum well are excited simultaneously with 150 fs pulses from a self mode-locked Ti:sapphire laser. The spectral profile is shown in the inset of Fig. 1.
The interface roughness on a nanometer scale plays a decisive role in dephasing of excitons in GaAs multiple quantum wells. The excitonic four-wave mixing signal shows a free polarization decay and a corresponding homogeneously broadened line from areas with interface roughness on a scale larger than the exciton diameter. A photon echo and a corresponding inhomogeneously broadened line are observed from areas of interface roughness on a scale less than the exciton diameter. In the present study we observe both mechanisms simultaneously, and are able to clearly distinguish between the two mechanisms by spectrally resolving the transient four-wave-mixing signal.