Landau Rainbow Induced by Artificial Gauge Fields

The ability to generate Landau levels using a pseudomagnetic field (PMF), also called an artificial gauge field, opens up new pathways for exploring fundamental physics and developing novel applications based on topological protection. In this Letter, we simultaneously realize a PMF and a pseudoelectric field (PEF) on a photonic crystal platform and observe a rainbow effect of the Landau zeroth modes. While a PMF induces a series of discretized Landau levels of photons in a similar way as the quantum Hall effect for electrons, a PEF breaks the degeneracy of the flat band of Landau levels over a broad range. Thus, different frequencies of the Landau modes can be dispersed into different spatial positions, leading us to term this phenomenon the "Landau rainbow." The Landau rainbow, induced by the joint action of PMFs and PEFs, features robustness, broadband capability, and scalability, and may find applications in areas such as slow-light effect, multifrequency divider, and optical information multiplexing.