Chiral Landau rainbow modulated by in-plane synthetic gauge fields.

Engineering synthetic gauge fields to realize Landau levels for neutral particles has emerged as a pivotal strategy for advanced classical wave manipulation. While out-of-plane pseudomagnetic field (PMF) generates conventional, non-chiral Landau levels with flat-band zeroth modes, their in-plane counterparts unlock valley-locked chiral Landau levels (CLLs) with linear dispersion and topologically protected unidirectional propagation. Here, we propose and demonstrate the chiral Landau rainbow effect in a 2D honeycomb photonic crystal by synergistically engineering an in-plane PMF and a parallel pseudoelectric field (PEF). PMF, from a spatially graded Dirac mass, generates CLLs. PEF, from a graded band structure shifting, induces frequency-dependent spatial dispersion of these CLLs, forming discrete chiral Landau rainbows. Their confinement and trapping position can be independently tuned by controlling the strengths of the PMF and PEF. This work offers a new paradigm for multifunctional topological wave control, with applications in robust slow-light devices, valley-multiplexed routers, and fault-tolerant integrated photonic circuits.