Anomalous Reflection of Caustic Spin-Wave Beams in a Magnonic Waveguide

Reflection of waves at interfaces is conventionally governed by Snell's law, which follows from conservation of momentum parallel to the interface.

Here we show experimentally that caustic spin-wave beams in anisotropic media obey a fundamentally different reflection mechanism.

Applying time-resolved Kerr microscopy to a yttrium iron garnet waveguide, we observe that reflected beams are selected by transitions between caustic points on the anisotropic iso-frequency contour rather than by momentum conservation.

As a consequence, the reflected carrier wave vector and wavefront orientation exhibit trends opposite to those predicted by Snell's law.

By tuning the magnitude and orientation of an external magnetic field, we continuously control the resulting reflection process and beam routing.

Our results establish caustic-point transitions as a distinct reflection law for anisotropic wave beams and provide a route towards reconfigurable magnonic beam steering.