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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Two-dimensional coherent spectroscopy of excitons, biexcitons and exciton-polaritons

Alan D. Bristow

Published Oct 3, 2016• 1 citation
Publisher

Abstract

Semiconductors systems exhibiting excitonic properties are discussed in terms of their coherent response, which is extracted using two-dimensional coherent spectroscopy. This control method allows for separation of quantum pathways that comprise the optical response, such as interactions between excitons, their dephasing rates, the effects of many-body interactions and the role of structure on the microscopic electronic environment. Additional controls, such as polarization can be used to further distinguish biexcitons and suppress many-body interactions. These result are compared and contrasted with those from a semiconductor microcavity where the excitons form polaritonic modes due to normal-mode splitting. Rephrasing spectra map the detuning dependence of the exciton-polariton branches. Increasing the detuning shifts all features to higher energy and the expected anti-crossing is observed. An isolated biexciton is seen only at negative detuning, separated by a binding energy. For positive detuning, the spectral weight of the off-diagonal features swap, as the lower polariton branch and biexciton come into resonance. This indicates that the off-diagonal features are sensitive to the interactions of the exciton-polaritons and other resonances in the system.

DOI
10.1117/12.2236697

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Semiconductors systems exhibiting excitonic properties are discussed in terms of their coherent response, which is extracted using two-dimensional coherent spectroscopy. This control method allows for separation of quantum pathways that comprise the optical response, such as interactions between excitons, their dephasing rates, the effects of many-body interactions and the role of structure on the microscopic electronic environment. Additional controls, such as polarization can be used to further distinguish biexcitons and suppress many-body interactions. These result are compared and contrasted with those from a semiconductor microcavity where the excitons form polaritonic modes due to normal-mode splitting. Rephrasing spectra map the detuning dependence of the exciton-polariton branches. Increasing the detuning shifts all features to higher energy and the expected anti-crossing is observed. An isolated biexciton is seen only at negative detuning, separated by a binding energy. For positive detuning, the spectral weight of the off-diagonal features swap, as the lower polariton branch and biexciton come into resonance. This indicates that the off-diagonal features are sensitive to the interactions of the exciton-polaritons and other resonances in the system.
Multidimensional coherent spectroscopy maps the detuning dependence of the upper (UP) and lower (LP) excitonpolariton branches1 in a wedged microcavity with a single InGaAs quantum well at 5 K. Features on the diagonal correspond to intra-action coherences of the UP and LP branches. Off-diagonal peaks are interaction coherences between the UP and LP branches. With increasing detuning (Δ), all peaks move to higher energy, the exciton-like (EEX) and cavity-like (Eγ) modes swap position and have maximum intensity near the anti-crossing at Δ=0. An isolated biexciton (B) is only seen at Δ<0, separated by a binding energy of approximately 2 meV. For Δ>0, the spectral weight of the off-diagonal features swap, as the LP and B come into resonance. This indicates that the off-diagonal features are sensitive to the interactions including two-quantum contributions and that a situation similar to a Feshbach resonance exists.2 Polarization of two-quantum contributions show spin sensitive two-polariton and new biexciton correlations. The latter likely influence the Feshbach resonance between biexcitons and two-polariton states. The two-quantum signatures also demonstate that biexcitons perturb the light-matter coupling in the microcavity to reduce the mixed two-polariton contributions. Detuning dependence of zero-quantum contributions show Raman-like coherences that are enhanced near zero detuning. Asymmetry of the Raman coherences are indicative of many-body interactions, which also grow stronger as the light-matter interactions are enhanced near zero deuning.
Light-matter interactions associated with excitons and exciton related complexes are explored in bulk GaAs and semiconductor microcavities using multidimensional coherent spectroscopy (MDCS). This approach provides rich spectra determining quantum excitation pathways, structural influences on the excitons, and coherence times.;Polarization, excitation density, and temperature-dependent MDCS is performed on excitons in strained bulk GaAs layers, probing the coherent response for differing amounts of strain. Biaxial tensile strain lifts the degeneracy of heavy-hole and light-hole valence states, leading to an observed splitting of the associated excitons at low temperature. Increasing the strain increases the magnitude of the heavy-/light- hole exciton peak splitting, induces an asymmetry in the off-diagonal interaction coherences, increases the difference in the heavy- and light- hole exciton homogenous linewidths, and increases the inhomogeneous broadening of both exciton species. All results arise from strain-induced variations in the local electronic environment, which is not uniform along the growth direction of the thin layers. For cross-linear polarized excitation, wherein excitonic signals give way to biexcitonic signals, the high-strain sample shows evidence of bound light-, heavy- and mixed- hole biexcitons.;2DCS maps the anticrossing associated with normal mode splitting in a semiconductor microcavity. For a detuning range near zero, it is observed that there are two diagonal features related to the intra-action of exciton-polariton branches and two off-diagonal features related to coherent interaction between the polaritons. At negative detuning, the line shape properties of the diagonal intra-action features are distinguishable and can be associated with cavity-like and exciton-like modes. A biexcitonic companion feature is observed, shifted from the exciton feature by the biexciton binding energy. Closer to zero detuning, all features are enhanced and the diagonal intra-action features become nearly equal in amplitude and linewidth. At positive detuning the exciton-like and cavity-like characteristics return to the diagonal intra-action features. Off-diagonal interaction features exhibit asymmetry in their amplitudes throughout the detuning range. The amplitudes are strongly modulated as the lower polariton branch crosses the bound biexciton energy determined from negatively detuned spectra.
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.
Summary form only given. The optical coherent control of quantum mechanical systems enables the direct manipulation of quantum states, and has been demonstrated on excitonic transitions in semiconductors. We have demonstrated the coherent control of the nonlinear optical response of quantum-well excitons in a resonant high-Q microcavity. There are two significant advantages to using excitons in microcavities instead of bare quantum wells: first, the multiple reflections which occur in the cavity strongly enhance the optical nonlinearity, enabling large-signal coherent control, and second, 2-color coherent control may be implemented, where a pump pulse tuned to one of the normal modes of the coupled exciton-microcavity system can control the response of a probe tuned to the other normal mode. The nonlinear response of the normal modes is due to the nonlinearity of the underlying exciton transition.