Observation of non-Hermitian degeneracies in a chaotic exciton-polariton billiard

• Published in: Nature (London) Vol. 526; no. 7574; pp. 554 - 558
• Main Authors: Gao, T., Estrecho, E., Bliokh, K. Y., Liew, T. C. H., Fraser, M. D., Brodbeck, S., Kamp, M., Schneider, C., Höfling, S., Yamamoto, Y., Nori, F., Kivshar, Y. S., Truscott, A. G., Dall, R. G., Ostrovskaya, E. A.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 22.10.2015; Nature Publishing Group
• Subjects: 140/125; 639/766/119/2791; 639/766/119/999; 639/766/400/2797; 639/766/483/3924; Dynamics of a particle; Eigenvalues; Exciton theory; Geometry; Humanities and Social Sciences; letter; multidisciplinary; Observations; Phase transitions; Physics; Polaritons; Quantum theory; Science
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature15522

In non-Hermitian systems, spectral degeneracies can arise that can cause unusual, counter-intuitive effects; here exciton-polaritons—hybrid light–matter particles—within a semiconductor microcavity are found to display non-trivial topological modal structure exclusive to such systems. Non-Hermitian dynamics in a quantum chaotic exciton-polariton system In non-Hermitian systems, which are open and subject to gain and loss, exceptional points can arise, spectral degeneracies that can cause unusual, counter-intuitive effects. Recent efforts to observe non-Hermitian physics have concentrated on various optical systems, but not yet on exciton-polaritons. These are hybrid light–matter particles, formed by strongly interacting photons and excitons (electron–hole pairs) in semiconductor microcavities. Such systems require constant pumping of energy and continuously decays releasing coherent radiation, so are a profoundly open quantum system. In a striking experiment involving a chaotic exciton-polariton billiard —a two-dimensional area enclosed by a curved potential barrier — these authors demonstrate this non-Hermitian nature for the first time. The experiment reveals the non-trivial topological modal structure exclusive to non-Hermitian systems. These findings open the way for novel types of operating principles for polariton-based optoelectronic devices. Exciton-polaritons are hybrid light–matter quasiparticles formed by strongly interacting photons and excitons (electron–hole pairs) in semiconductor microcavities 1 , 2 , 3 . 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 4 , 5 ) quantum system, which requires constant pumping of energy and continuously decays, releasing coherent radiation 6 . 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 7 , 8 , 9 . Using a spatially structured optical pump 10 , 11 , 12 , 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 13 , 14 . Such points can cause remarkable wave phenomena, such as unidirectional transport 15 , anomalous lasing/absorption 16 , 17 and chiral modes 18 . 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 9 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 . We also observe mode switching and a topological Berry phase for a parameter loop encircling the exceptional point 23 , 24 . 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.