Femtosecond Laser Pulse Driven Caustic Spin Wave Beams

Controlling the directionality of spin waves is a key ingredient in wave-based computing methods such as magnonics. In this Letter, we demonstrate this particular aspect by using an all-optical pointlike source of continuous spin waves based on frequency comb rapid demagnetization. The emitted spin waves contain a range of $k$ vectors and by detuning the applied magnetic field slightly off the ferromagnetic resonance (FMR), we observe $\mathsf{X}$-shaped caustic spin wave patterns at 70\ifmmode^\circ\else\textdegree\fi{} propagation angles as predicted by theory. When the harmonic of the light source approaches the FMR, the caustic pattern gives way to uniaxial spin wave propagation perpendicular to the in-plane component of the applied field. This field-controlled propagation pattern and directionality of optically emitted short-wavelength spin waves provide additional degrees of freedom when designing magnonic devices.