Dynamics of the Berezinskii–Kosterlitz–Thouless transition in a photon fluid

• Published in: Nature photonics Vol. 14; no. 8; pp. 517 - 522
• Main Authors: Situ, Guohai, Fleischer, Jason W.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.08.2020; Nature Publishing Group
• Subjects: 639/624/399; 639/766/119; 639/766/400; Antiferromagnetism; Applied and Technical Physics; Correlation analysis; Ferromagnetism; Flow stability; Fluid dynamics; Fluid flow; Hydrodynamics; Phase transitions; Photonics; Physics; Physics and Astronomy; Quantum Physics; Topology; Vortices
• ISSN: 1749-4885; 1749-4893
• DOI: 10.1038/s41566-020-0636-7

In addition to enhancing confinement, restricting optical systems to two dimensions gives rise to new photonic states, modified transport and distinct nonlinear effects. Here we explore these properties in combination and experimentally demonstrate a Berezinskii–Kosterlitz–Thouless phase transition in a nonlinear photonic lattice. In this topological transition, vortices are created in pairs and then unbind, changing the dynamics from that of a photonic fluid to that of a plasma-like gas of free (topological) charges. We explicitly measure the number and correlation properties of free vortices, for both repulsive and attractive interactions (the photonic equivalent of ferromagnetic and antiferromagnetic conditions), and confirm the traditional thermodynamics of the Berezinskii–Kosterlitz–Thouless transition. We also suggest a purely fluid interpretation, in which vortices are nucleated by inhomogeneous flow and driven by seeded instability. The results are fundamental to optical hydrodynamics and can impact two-dimensional photonic devices if temperature and interactions are not controlled properly. A topological transition in a nonlinear photonic lattice results in new vortex dynamics and a change from photonic fluid behaviour to that of a plasma-like gas.