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Physical review. B./Physical review. B

Observation of bound and antibound states of two excitons in GaAs single quantum well by two-dimensional coherent spectroscopy

Yoshihiro Ogawa

Published May 23, 2017• 4 citations
Publisher

Abstract

We study the bound and antibound states of two excitons in a GaAs single quantum well by two-dimensional coherent spectroscopy. We also propose a method to directly observe the amplitude and phase (real and imaginary parts) of optical fields by using a heterodyne interference technique and through synchronized detection of the interference signals. In this scheme, the mechanical fluctuation of an unstabilized interferometer does not affect the measurement. In addition, the amplitude and phase of the optical fields, resulting from the coherent interactions between light and matter, are directly obtained by comparing a reference heterodyne beating signal by using a two-phase lock-in amplifier. By using this method, the bound and antibound states of two excitons are observed. It is expected that the excitons are localized to the quantum islands and the lateral confinement induces the large repulsive energy between the two excitons.

DOI
10.1103/physrevb.95.201113

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We study the bound and antibound states of two excitons in a GaAs single quantum well by two-dimensional coherent spectroscopy. We also propose a method to directly observe the amplitude and phase (real and imaginary parts) of optical fields by using a heterodyne interference technique and through synchronized detection of the interference signals. In this scheme, the mechanical fluctuation of an unstabilized interferometer does not affect the measurement. In addition, the amplitude and phase of the optical fields, resulting from the coherent interactions between light and matter, are directly obtained by comparing a reference heterodyne beating signal by using a two-phase lock-in amplifier. By using this method, the bound and antibound states of two excitons are observed. It is expected that the excitons are localized to the quantum islands and the lateral confinement induces the large repulsive energy between the two excitons.
We investigate the coherent optical response of individual localized exciton–biexciton (X–XX) systems formed at interface fluctuations of a growth-interrupted GaAs/AlAs quantum well. We apply heterodyne spectral interferometry to perform two-dimensional four-wave mixing (FWM) spectroscopy. We retrieve the binding energy of bound and unbound XXs, as well as characterize the system in terms of biexciton–exciton dipole moment ratio and mutual FWM phase. Polarization selection rules of the FWM are determined. FWM hyperspectral imaging and autocorrelation analysis reveal the expected spatial colocalization of XXs with respect to their Xs. A value for the biexciton renormalization in a coherently coupled pair of Xs is retrieved. Our study gives insight into the coherent optical properties of an exciton–biexciton system with a confinement energy comparable to the biexciton binding energy.
We demonstrate three-dimensional (3D) electronic Fourier transform spectroscopy of GaAs quantum wells using four fully phase-coherent, noncollinear optical fields. Since the full complex signal field is measured as a function of all three time intervals, nearly every peak in the resulting 3D spectral solid arises from a distinguishable sequence of transitions represented by a single Feynman pathway. We use the 3D spectral peaks to separate two pathways involving weakly bound mixed biexcitons generated in different time orders. In the process, we reveal a peak that was previously obscured by a correlated but unbound exciton pair coherence. We also demonstrate a calibration procedure for the carrier frequency which yields biexciton binding energy values with high accuracy.
We review our recent advances in four-wave mixing spectroscopy of single semiconductor quantum dots using heterodyne spectral interferometry, a novel implementation of transient nonlinear spectroscopy allowing the study of the transient nonlinear polarization emitted from individual electronic transitions in both amplitude and phase. We present experiments on individual excitonic transitions localized in monolayer islands of GaAs/AlAs quantum wells and in self-assembled CdTe/ZnTe quantum dots. We investigate the formation of the photon echo from individual transitions, both with increasing number of transitions in the ensemble, and in the presence of temporal jitter of the energy of a single transition. The detection of amplitude and phase of the signal allows the implementation of a two-dimensional femtosecond spectroscopy, in which mutual coherent coupling of single quantum dot states can observed and quantified.
Correlated electron-hole pairs, or excitons, in semiconductor nanostructures have been studied extensively over the past few decades. The optical response of excitons is complicated due to inhomogeneous broadening, presence of multiple states, and exciton-exciton interactions. In this work we bring new perspectives to exciton physics in semiconductor quantum wells (QWs) through two-dimensional coherent spectroscopy (2DCS).\nThe effect of QW growth direction on the optical properties of excitons is explored by studying (110)-oriented GaAs QWs. The homogeneous and inhomogeneous linewidths of the heavy-hole exciton resonance are measured. By probing the optical nonlinear response for polarization along the in-plane crystal axes [1-10] and [001], we measure different homogeneous linewidths for the two orthogonal directions. This difference is found to be due to anisotropic excitation-induced dephasing, caused by a crystal-axis-dependent absorption coefficient. The extrapolated zero-excitation density homogeneous linewidth exhibits an activation-like temperature dependence.\nSpectral diffusion of excitons in (001)-oriented QWs has been studied. We show that the spectral diffusion characteristics depend strongly on the sample temperature. Spectral diffusion is generally assumed to follow the strong-redistribution approximation, partly because of lack of any evidence to the contrary. We find that this assumption is violated at low sample temperatures for excitons in QWs; high-energy excitons preferentially relax due to a negligible phonon population at low temperatures. The frequency-frequency correlation function is measured through a numerical fitting procedure to quantify spectral diffusion for sample temperatures >20>20>20 K.\nExciton-exciton interactions affect the light-matter interactions in QWs significantly. We present an intuitive and simple model for these interactions by treating excitons as interacting bosons. We show that the polarization-dependent exciton dephasing rate in GaAs quantum wells is due to the bosonic character of excitons. We fit slices from simulated spectra to those from the experimentally measured spectra and show that interference between two different quantum mechanical pathways results in a slower dephasing rate for co-circular and co-linear polarization of optical excitation pulses. This interference does not exist for cross-linearly polarized excitation pulses resulting in a faster dephasing rate. Additionally, we were able to separately quantify inter- and intra-mode interactions between excitons through exciton-density-dependent measurements.
The degenerate four-wave mixing technique in the self-diffraction geometry is employed to investigate a novel GaAs/AlAs quantum well microstructure. Time-integrated transient measurements are carried out to study the coherent dynamics of excitons in a GaAs quantum well with AlAs islands inserted at the center of the well (IIQW) in excitation densities from 6×109 to 1.5×1011 cm-2. The decay of the time-integrated self-diffracted signal shows slower dephasing for the IIQW compared to that of a normal quantum well (NQW) without island insertion. The dephasing rate is found to increase slightly in the IIQW, while it is found to increase sharply in the NQW with the excitation density. A deep beat with a period of 1.45 ps, which disappears for co-circular polarization configuration, is observed in the decay of the self-diffracted signal in the IIQW. The detection-energy-resolved measurement reveals that the modulation of the beat increases up to full depth at low detection energy with respect to the peak energy of the self-diffracted signal. The dependence on detection energy and on polarization configuration shows that the beat is a quantum beat due to biexcitonic effect, which is enhanced by localization due to island insertion.