Direct measurement of a non-Hermitian topological invariant in a hybrid light-matter system

Rui Su*, Eliezer Estrecho*, Dąbrówka Biegańska, Yuqing Huang, Matthias Wurdack, Maciej Pieczarka, Andrew G. Truscott, Timothy C.H. Liew, Elena A. Ostrovskaya*, Qihua Xiong*

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    63 Citations (Scopus)

    Abstract

    Topology is central to understanding and engineering materials that display robust physical phenomena immune to imperfections. Different topological phases of matter are characterized by topological invariants. In energy-conserving (Hermitian) systems, these invariants are determined by the winding of eigenstates in momentum space. In non-Hermitian systems, a topological invariant is predicted to emerge from the winding of the complex eigenenergies. Here, we directly measure the non-Hermitian topological invariant arising from exceptional points in the momentum-resolved spectrum of exciton polaritons. These are hybrid light-matter quasiparticles formed by photons strongly coupled to electron-hole pairs (excitons) in a halide perovskite semiconductor at room temperature. We experimentally map out both the real (energy) and imaginary (linewidth) parts of the spectrum near the exceptional points and extract the novel topological invariant—fractional spectral winding. Our work represents an essential step toward realization of non-Hermitian topological phases in a condensed matter system.

    Original languageEnglish
    Article numbereabj8905
    JournalScience advances
    Volume7
    Issue number45
    DOIs
    Publication statusPublished - Nov 2021

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