Visualization of Elastic Flat Landau Rainbow.

The Landau rainbow offers a promising way for advanced multiplexed wave processing. However, current Landau rainbows are restricted with narrow k-space range of flat Landau levels. Meanwhile, given the promise of rainbow trapping for developing multiplexed elastic devices in integrated phononic networks, the realization of flat Landau rainbow in elastic systems-particularly in the ultrasonic regime-remains highly desirable. Here, we realize megahertz Landau rainbows in an elastic metamaterial by artificially synthesizing both pseudomagnetic fields (PMFs) and pseudoelectric fields (PEFs). Importantly, nonuniform PMFs are introduced to enhance the k-space range of the flat zeroth Landau levels that form the Landau rainbow. Multiplexed megahertz ultrasonic energy trappings enabled by the Landau rainbow are directly visualized via laser scanning technology, which agree well with the simulation results. Moreover, the unique field distributions of such Landau rainbows are shown to result from the mode profiles of the Landau levels with different group velocities. Our Letter not only offers a new recipe for engineering the Landau rainbow from Landau levels with enhanced flatness, but also paves the way for the development of elastic integrated devices with multiplexed wave preprocessing capacities.