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Physical Review Letters

Coherent Nonlinear Optical Response of Single Quantum Dots Studied by Ultrafast Near-Field Spectroscopy

Tobias Guenther, Christoph Lienau, Thomas Elsaesser, Markus Glanemann, V. M. Axt, T. Kühn, Soheyla Eshlaghi, Andreas Dirk Wieck

Published Jul 15, 2002• 168 citations
Publisher

Abstract

The nonlinear response of single GaAs quantum dots is studied in femtosecond near-field pump-probe experiments. At negative time delays, transient reflectivity spectra show pronounced oscillatory structure around the quantum dot exciton line, providing the first evidence for a perturbed free induction decay of the excitonic polarization. Phase-disturbing Coulomb interactions between the excitonic polarization and continuum excitations dominate the optical nonlinearity on ultrafast time scales. A theoretical analysis based on the semiconductor Bloch equations accounts for this behavior.

DOI
10.1103/physrevlett.89.057401

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The nonlinear response of single GaAs quantum dots is studied in femtosecond near-field pump-probe experiments. At negative time delays, transient reflectivity spectra show pronounced oscillatory structure around the quantum dot exciton line, providing the first evidence for a perturbed free induction decay of the excitonic polarization. Phase-disturbing Coulomb interactions between the excitonic polarization and continuum excitations dominate the optical nonlinearity on ultrafast time scales. A theoretical analysis based on the semiconductor Bloch equations accounts for this behavior.
We report the first experimental study of the optical Stark effect in single semiconductor quantum dots (QD). For below band gap excitation, two-color pump-probe spectra show dispersive line shapes caused by a light-induced blueshift of the excitonic resonance. The line shape depends strongly on the excitation field strength and is determined by the pump-induced phase shift of the coherent QD polarization. Transient spectral oscillations can be understood as rotations of the QD polarization phase with negligible population change. Ultrafast control of the QD polarization is demonstrated.
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Coherent nonlinear optical spectroscopy of individual excitons in GaAs quantum dots is demonstrated and shows strong similarities with atoms in their coherent optical interaction, unlike higher dimensional heterostructures. The nonlinear response is dominated by an incoherent contribution (saturation) and a coherent contribution (population pulsations) of the single dot and compares well with present theory. The data shows that energy relaxation and dephasing rates are comparable, reflecting the absence of significant pure dephasing, and also demonstrate the presence of interdot energy transfer.
Summary form only given. The optical properties of semiconductor quantum dots have been studied intensively in the past few years. Recently the experiment has demonstrated not only the ability to probe a single QD but also how to coherently control the exciton in a single QD on a time scale that is short compared with the decoherence time. We had proposed a scheme to use the excitons in QD as qubits and to use strong ultra-fast laser pulses to manipulate the qubits via Rabi flopping. As a result a full understanding of the multi-exciton coherent nonlinear spectroscopy with strong pump field is needed. In this work we present a non-perturbative calculation of the nonlinear CW pump-probe spectra.
We report a study of the coherent nonlinear optical response of individual excitons localized in GaAs quantum dots. Our data show that the nonlinear response is comprised of an incoherent contribution dominated by saturation of the oscillator strength and a coherent contribution giving rise to population pulsations. The results compare well with present theory and show the similarities with atoms and quantum dots in their coherent optical interaction with light. The measured energy relaxation and dephasing rates are comparable, reflecting the absence of significant pure dephasing at low temperatures. In addition, we use nonlinear spectroscopy to investigate quantum dot coupling showing the presence of an incoherent coupling leading to fast energy relaxation. The polarization properties as well as the fine structure splitting are also discussed.