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Continuous transition between weak and ultrastrong coupling through exceptional points in carbon nanotube microcavity exciton–polaritons

Abstract

Non-perturbative coupling of photons and excitons produces hybrid particles, exciton–polaritons, which have exhibited a variety of many-body phenomena in various microcavity systems. However, the vacuum Rabi splitting (VRS), which defines the strength of photon–exciton coupling, is usually a single constant for a given system. Here, we have developed a unique architecture in which excitons in an aligned single-chirality carbon nanotube film interact with cavity photons in polarization-dependent manners. The system reveals ultrastrong coupling (VRS up to 329 meV or a coupling-strength-to-transition-energy ratio of 13.3%) for polarization parallel to the nanotube axis, whereas VRS is absent for perpendicular polarization. Between these two extremes, VRS is continuously tunable through polarization rotation with exceptional points separating crossing and anticrossing. The points between exceptional points form equienergy arcs onto which the upper and lower polaritons coalesce. The demonstrated on-demand ultrastrong coupling provides ways to explore topological properties of polaritons and quantum technology applications.

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Fig. 1: Characterization of macroscopically aligned (6,5) SWCNTs and fabrication of SWCNT microcavity exciton–polariton devices.
Fig. 2: Anisotropic dispersions of microcavity exciton–polaritons in the E11 and E22 regions.
Fig. 3: Dispersions of SWCNT microcavity exciton–polaritons showing a continuous transition from the weak coupling to strong coupling regime with EPs.
Fig. 4: Evidence for collective ultrastrong light–matter coupling in aligned (6,5) films and determination of the E11 exciton oscillator strength and dipole moment.

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Acknowledgements

We thank D. Hagenmüller and J. Yuen-Zhou for useful discussion. This work was supported by the Department of Energy Basic Energy Sciences through grant no. DE-FG02-06ER46308 (optical spectroscopy experiments), the National Science Foundation through award no. ECCS-1708315 (device fabrication) and the Robert A. Welch Foundation through grant no. C-1509 (sample preparation). M.B. was supported by JST PRESTO (grant no. JPMJPR1767), KAKENHI (grant no. 26287087) and ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan).

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W.G. prepared the aligned SWCNT films, fabricated the devices and performed all measurements under the guidance of J.K. X.L. performed the transfer-matrix method simulations. M.B. developed the comprehensive theoretical model to explain the angular dependence of the coupling strength. All authors discussed the results and wrote the manuscript.

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Correspondence to Junichiro Kono.

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Gao, W., Li, X., Bamba, M. et al. Continuous transition between weak and ultrastrong coupling through exceptional points in carbon nanotube microcavity exciton–polaritons. Nature Photon 12, 362–367 (2018). https://doi.org/10.1038/s41566-018-0157-9

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