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Direct measurement of polariton–polariton interaction strength

Abstract

Exciton–polaritons in a microcavity are composite two-dimensional bosonic quasiparticles, arising from the strong coupling between confined light modes in a resonant planar optical cavity and excitonic transitions. Quantum phenomena such as Bose–Einstein condensation, superfluidity, quantized vortices, and macroscopic quantum states have been realized at temperatures from tens of kelvin up to room temperatures. Crucially, many of these effects of exciton–polaritons depend on the polariton–polariton interaction strength. Despite the importance of this parameter, it has been difficult to make an accurate experimental measurement, mostly because of the difficulty in determining the absolute densities of polaritons and bare excitons. Here we report a direct measurement of the polariton–polariton interaction strength in a very high-Q microcavity structure. By allowing polaritons to propagate over 20 μm to the centre of a laser-generated annular trap, we are able to separate the polariton–polariton interactions from polariton–exciton interactions. The interaction strength is deduced from the energy renormalization of the polariton dispersion as the polariton density is increased, using the polariton condensation as a benchmark for the density. We find that the interaction strength is about two orders of magnitude larger than previous theoretical estimates, putting polaritons in the strongly interacting regime.

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Figure 1: Generation of an annular trap in a GaAs microcavity.
Figure 2: Emission spectra at different polariton densities.
Figure 3: Blue shifts at various detunings.
Figure 4: Polariton–polariton interaction strength.
Figure 5: Saturation of blue shift.

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Acknowledgements

We thank A. Daley and D. Pekker for fruitful discussions and J. Beaumarriage for assistance in the calibration of the detuning map of the sample. Y.S., Y.Y. and K.A.N. were supported as part of the Center for Excitonics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001088. M.S., G.L. and D.W.S. were supported by the National Science Foundation under grants PHY-1205762 and DMR-1104383. L.N.P. and K.W. were funded by the Gordon and Betty Moore Foundation through the EPiQS initiative Grant GBMF4420, and by the National Science Foundation MRSEC Grant DMR-1420541.

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Y.S. and D.W.S. designed the experiments; Y.S. performed the experiment; Y.S. and D.W.S. analysed the data; Y.S., Y.Y. and M.S. calibrated the detuning map of the sample; L.N.P. and K.W. grew the microcavity structure; all the authors participated in the discussion of the results and manuscript preparation.

Corresponding authors

Correspondence to Yongbao Sun or David W. Snoke.

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The authors declare no competing financial interests.

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Sun, Y., Yoon, Y., Steger, M. et al. Direct measurement of polariton–polariton interaction strength. Nature Phys 13, 870–875 (2017). https://doi.org/10.1038/nphys4148

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