title: Observation of non-Hermitian degeneracies in a chaotic exciton-polariton billiard.
abstract: Exciton-polaritons are hybrid light-matter quasiparticles formed by strongly interacting photons and excitons (electron-hole pairs) in semiconductor microcavities. They have emerged as a robust solid-state platform for next-generation optoelectronic applications as well as for fundamental studies of quantum many-body physics. Importantly, exciton-polaritons are a profoundly open (that is, non-Hermitian) quantum system, which requires constant pumping of energy and continuously decays, releasing coherent radiation. Thus, the exciton-polaritons always exist in a balanced potential landscape of gain and loss. However, the inherent non-Hermitian nature of this potential has so far been largely ignored in exciton-polariton physics. Here we demonstrate that non-Hermiticity dramatically modifies the structure of modes and spectral degeneracies in exciton-polariton systems, and, therefore, will affect their quantum transport, localization and dynamical properties. Using a spatially structured optical pump, we create a chaotic exciton-polariton billiard--a two-dimensional area enclosed by a curved potential barrier. Eigenmodes of this billiard exhibit multiple non-Hermitian spectral degeneracies, known as exceptional points. Such points can cause remarkable wave phenomena, such as unidirectional transport, anomalous lasing/absorption and chiral modes. By varying parameters of the billiard, we observe crossing and anti-crossing of energy levels and reveal the non-trivial topological modal structure exclusive to non-Hermitian systems. We also observe mode switching and a topological Berry phase for a parameter loop encircling the exceptional point. Our findings pave the way to studies of non-Hermitian quantum dynamics of exciton-polaritons, which may uncover novel operating principles for polariton-based devices.
authors.affiliation: Research School of Physics and Engineering, The Australian National University, Canberra, Australian Capital Territory 2601, Australia.
authors.forename: T
authors.initials: T
authors.lastname: Gao
authors.affiliation: Research School of Physics and Engineering, The Australian National University, Canberra, Australian Capital Territory 2601, Australia.
authors.forename: E
authors.initials: E
authors.lastname: Estrecho
authors.affiliation: Research School of Physics and Engineering, The Australian National University, Canberra, Australian Capital Territory 2601, Australia.|Center for Emergent Matter Science, RIKEN, Wako-shi, Saitama 351-0198, Japan.
authors.forename: K Y
authors.initials: KY
authors.lastname: Bliokh
authors.affiliation: School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore.
authors.forename: T C H
authors.initials: TC
authors.lastname: Liew
authors.affiliation: Center for Emergent Matter Science, RIKEN, Wako-shi, Saitama 351-0198, Japan.
authors.forename: M D
authors.initials: MD
authors.lastname: Fraser
authors.affiliation: Technische Physik and Wilhelm-Conrad-Röntgen Research Center for Complex Material Systems, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Brodbeck
authors.affiliation: Technische Physik and Wilhelm-Conrad-Röntgen Research Center for Complex Material Systems, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Kamp
authors.affiliation: Technische Physik and Wilhelm-Conrad-Röntgen Research Center for Complex Material Systems, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Schneider
authors.affiliation: Technische Physik and Wilhelm-Conrad-Röntgen Research Center for Complex Material Systems, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany.|SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews KY16 9SS, UK.
authors.forename: S
authors.initials: S
authors.lastname: Höfling
authors.affiliation: ImPACT Project, Japan Science and Technology Agency, Chiyoda-ku, Tokyo 102-0076, Japan.|Edward L. Ginzton Laboratory, Stanford University, Stanford, California 94305-4085, USA.
authors.forename: Y
authors.initials: Y
authors.lastname: Yamamoto
authors.affiliation: Center for Emergent Matter Science, RIKEN, Wako-shi, Saitama 351-0198, Japan.|Physics Department, University of Michigan, Ann Arbor, Michigan 48109-1040, USA.
authors.forename: F
authors.initials: F
authors.lastname: Nori
authors.affiliation: Research School of Physics and Engineering, The Australian National University, Canberra, Australian Capital Territory 2601, Australia.
authors.forename: Y S
authors.initials: YS
authors.lastname: Kivshar
authors.affiliation: Research School of Physics and Engineering, The Australian National University, Canberra, Australian Capital Territory 2601, Australia.
authors.forename: A G
authors.initials: AG
authors.lastname: Truscott
authors.affiliation: Research School of Physics and Engineering, The Australian National University, Canberra, Australian Capital Territory 2601, Australia.
authors.forename: R G
authors.initials: RG
authors.lastname: Dall
authors.affiliation: Research School of Physics and Engineering, The Australian National University, Canberra, Australian Capital Territory 2601, Australia.
authors.forename: E A
authors.initials: EA
authors.lastname: Ostrovskaya