Caustic spin wave beams in an extended thin film excited by a nanoconstriction

The ability to control the directionality of spin waves is important for magnonic logic and computing applications. Here, we demonstrate the emission of caustic-like spin waves in an extended 200 nm thick yttrium iron garnet (YIG) film from a nano-constricted $rf$ waveguide. Using spatially resolved micro-focused Brillouin light spectroscopy in both the backward volume and the Damon-Eshbach geometry, we reveal the propagation of two spin-wave beams directly emitted from the constriction. We find on one hand that these beams are symmetrical in intensity when the $rf$ magnetic field is perpendicular to the applied magnetic field. On the other hand, one beam is more intense than the other one when the $rf$ magnetic field is parallel to the external field. We further study the frequency dependence of the propagation direction of these caustic-like spin wave beams. Our findings have important implications for the development of switchable spin wave splitters, passive spin-wave frequency-division demultiplexers and magnonic interferometry.