LAUSR

A laser-fabricated waveguide array creates a nonlinear medium that supports solitons The intense coherent emission from lasers enabled the study of light propagation in nonlinear media, which spurred many important applications. More recently, the study of electromagnetic wave propagation in periodic media, where linear band structures play an important role, has advanced in new directions. By breaking certain symmetries, such as time reversal, the medium can support so-called “topologically protected” modes that possess uncommon robustness to material defects. Theory has suggested that certain nonlinear waves can inherit the topology of associated linear waves. On page 856 of this issue, Mukherjee and Rechtsman (1) describe experiments where such nonlinear waves, called solitons, can now be observed in the bulk of photonic topological media. These localized waves exhibit cyclotronic motion as the light propagates down a specifically engineered waveguide. When a different mode is considered—one with trivial topology—the waves no longer circulate but remain essentially fixed in their initial spatial distribution.