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Hidden symmetries in the energy levels of excitonic ‘artificial atoms’

Abstract

Quantum dots1,2,3,4,5,6,7 or ‘artificial atoms’ are of fundamental and technological interest—for example, quantum dots8,9 may form the basis of new generations of lasers. The emission in quantum-dot lasers originates from the recombination of excitonic complexes, so it is important to understand the dot's internal electronic structure (and of fundamental interest to compare this to real atomic structure). Here we investigate artificial electronic structure by injecting optically a controlled number of electrons and holes into an isolated single quantum dot. The charge carriers form complexes that are artificial analogues of hydrogen, helium, lithium, beryllium, boron and carbon excitonic atoms. We observe that electrons and holes occupy the confined electronic shells in characteristic numbers according to the Pauli exclusion principle. In each degenerate shell, collective condensation of the electrons and holes into coherent many-exciton ground states takes place; this phenomenon results from hidden symmetries (the analogue of Hund's rules for real atoms) in the energy function that describes the multi-particle system. Breaking of the hidden symmetries leads to unusual quantum interferences in emission involving excited states.

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Figure 1: Scanning electron micrographs illustrating the experimental technique used for studying single self-assembled quantum dots.
Figure 2: State filling spectroscopy on quantum dots.
Figure 3: Contour plot of the variation of the emission of an In0.60Ga 0.40As single quantum dot with excitation power and with energy.
Figure 4: Photoluminescence spectra of an In0.60Ga0.40As single quantum dot for varying excitation powers.
Figure 5: Calculated emission spectra of a quantum dot for different exciton occupations N indicated at each trace.

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Acknowledgements

This work was supported by the State of Bavaria. P.H. thanks the Alexander von Humboldt Stiftung for support.

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Correspondence to M. Bayer.

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Bayer, M., Stern, O., Hawrylak, P. et al. Hidden symmetries in the energy levels of excitonic ‘artificial atoms’. Nature 405, 923–926 (2000). https://doi.org/10.1038/35016020

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