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Applied Physics Letters

Ultrafast spectral hole burning spectroscopy of exciton spin flip processes in InAs∕GaAs quantum dots

Thomas G. Muller, G. Strasser, K. Unterrainer

Published May 8, 2006• 12 citations
Publisher

Abstract

The exciton spin relaxation within the radiative doublet of the exciton ground state in InAs∕GaAs self-assembled quantum dots is studied via an ultrafast spectral hole burning technique. In the case of cross-polarized pump and probe pulses a spectral “antihole” emerges due to relaxation of the exciton spin. The measured relaxation time decreases rapidly from 1.15ns at T=5Kto90ps at 90K, suggesting exciton-acoustic phonon interaction as the underlying spin relaxation mechanism.

DOI
10.1063/1.2202721

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Applied Physics Letters•Jul 5, 2010•1 citation
The exciton spin relaxation within the radiative doublet of the exciton ground state in InAs∕GaAs self-assembled quantum dots is studied via an ultrafast spectral hole burning technique. In the case of cross-polarized pump and probe pulses a spectral “antihole” emerges due to relaxation of the exciton spin. The measured relaxation time decreases rapidly from 1.15ns at T=5Kto90ps at 90K, suggesting exciton-acoustic phonon interaction as the underlying spin relaxation mechanism.
The three-dimensional confinement created by the ultrasmall semiconductor structures known as quantum dots greatly modifies the optical properties of the spatially localized carriers. These systems are presently of great interest to the research community, both for the improved understanding of the physics of confined structures, and for the potential applications of these systems to areas such as optoelectronics and quantum computing. Here, the techniques of transient coherent nonlinear optical spectroscopy are applied to the investigation of excitons confined within InAs/GaAs self-assembled quantum dots. These methods provide a powerful tool for the study of dynamic processes in these novel materials. The nonlinear measurements are based on the differential transmission experimental geometry, and utilize ultrafast pulsed lasers in order to temporally resolve the carrier dynamics after excitation. The experiments performed here are resonant with the ground state absorption of the quantum dots, as evidenced by comparison with the photoluminescence measurements which are used to characterize the sample. The incoherent relaxation dynamics of the population of photoexcited excitons are examined, allowing for a determination of both the lifetime of the excitons and the time scale for relaxation between the two polarization eigenstates of the system. The three-dimensional quantum confinement results in the suppression of these relaxation processes, as predicted. The use of coherent spectroscopic techniques allows for the first unambiguous demonstration of an optically-induced nonradiative Raman coherence in these quantum dots. Simultaneous excitation of both polarization states of the exciton results in a coherent superposition state, whose temporal evolution is observed through the homodyne-detection scheme. In these transient experiments, this is manifested as a beating in the nonlinear response, which allows us to directly measure the fine-structure splitting between the polarization states. Utilizing optical pulse shaping techniques, transient spectral hole burning is performed, which allows for the nonlinear response to be spectrally resolved. As expected for the strongly confined quantum dot excitons, the results show that diffusion among the localized states is not a significant effect. Temperature dependent measurements show that at elevated temperatures, extra dephasing processes, potentially related to acoustic phonon scattering, become important. Finally, this technique allows for the first direct observation of the induced absorption from the exciton to biexciton transition in these InAs quantum dots, and a measurement of the associated binding energy. These results lay the foundation for future studies in this material system, and for the possible development of quantum logic devices utilizing quantum dots.
Femtosecond spectral hole burning experiments were performed on InAs/GaAs quantum dots to investigate the influence of high excitation intensities. The observation of additional spectral holes, as well as distinct absorption line broadening are both attributed to two-photon absorption processes. These spectral holes can be unambiguously assigned to degenerate two-photon biexciton creation, whereas the absorption line broadening is a direct result of Coulomb renormalization, initiated by carriers created through two-photon absorption in the surrounding GaAs crystal matrix and the InAs wetting layer.