Abstract
Einstein’s 1925 paper predicted the occurrence of Bose–Einstein condensation (BEC) in an ideal gas of non-interacting bosonic particles1. However, particle–particle interaction and peculiar excitation spectra are keys for understanding BEC and superfluidity physics. A quantum field-theoretical formulation for a weakly interacting Bose condensed system was developed by Bogoliubov in 1947, which predicted the phonon-like excitation spectrum2 in the low-momentum regime. The experimental verification of the Bogoliubov theory on the quantitative level was carried out for atomic BEC3 using the two-photon Bragg scattering technique4. Exciton-polaritons in a semiconductor microcavity, which are elementary excitations created by strong coupling between quantum-well excitons and microcavity photons, were proposed as a new BEC candidate in solid-state systems5. Recent experiments with exciton-polaritons have demonstrated several interesting signatures from the viewpoint of polariton condensation, such as quantum degeneracy at non-equilibrium conditions6,7,8, the polariton-bunching effect at the condensation threshold9, long spatial coherence10,11,12 and quantum degeneracy at equilibrium conditions13. The particle–particle interaction and the Bogoliubov excitation spectrum are at the heart of BEC and superfluidity physics, but have only been studied theoretically for exciton-polaritons14,15. In this letter, we report the first observation of interaction effects on the exciton-polariton condensate and the excitation spectra, which are in quantitative agreement with the Bogoliubov theory.
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Acknowledgements
This work was supported by the JST/SORST programme and Special Coordination Funds for Promoting Science and Technology in Japan. We thank T. Maruyama for support and S. Sasaki for device fabrication.
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S.U. carried out the experiments, analysed the data and wrote the paper, L.T. theoretically studied the data, G.R. carried out the experiments and analysed the data, C.W.L. conceived, designed and carried out the experiments, N.K., T.F., M.G., A.L., S.H. and A.F. prepared materials and experimental tools and Y.Y. conceived the project in this paper.
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Utsunomiya, S., Tian, L., Roumpos, G. et al. Observation of Bogoliubov excitations in exciton-polariton condensates. Nature Phys 4, 700–705 (2008). https://doi.org/10.1038/nphys1034
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DOI: https://doi.org/10.1038/nphys1034
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