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.

除了增强约束之外，将光学系统限制在二维范围内还会产生新的光子状态，改进的传输和明显的非线性效应。在这里，我们结合探索这些特性，并通过实验证明了非线性光子晶格中的Berezinskii–Kosterlitz–Thouless相变。在此拓扑转换中，成对创建涡流，然后解除绑定，将动力学从光子流体的动力学更改为自由（拓扑）电荷的等离子气体的动力学。我们明确地测量了排斥和吸引力相互作用（铁磁和反铁磁条件的光子当量）时自由涡的数量和相关特性，并确定了Berezinskii–Kosterlitz–Thouless过渡的传统热力学。我们还提出了一种纯粹的流体解释，其中涡流由不均匀的流动形核，并由种子的不稳定性驱动。如果温度和相互作用控制不当，则结果对于光学流体力学至关重要，并且会影响二维光子器件。