How to generate whispering gallery magnons*

One of the most fascinating topics in current quantum physics are hybridized systems, in which resonators of different quantum systems are strongly coupled. Prominent examples are circular resonators with high quality factors that allow the coupling of optical whispering gallery modes to microwave cavities or magnon resonances. However, the coupling to magnons with finite wave vectors has not yet been achieved due to the lack of efficient excitation schemes.
Here, we present the generation of whispering gallery magnons with unprecedented high azimuthal wave vectors via nonlinear 3-magnon scattering in a µm-sized magnetic disk exhibiting a vortex state. These modes show a strong localization at the perimeter of the disk and practically zero amplitude in an extended area around the vortex core. They originate from the splitting of the fundamental radial magnon modes, which can be resonantly excited in a vortex state by an out-of-plane microwave field. We will shed light on the basics of this non-linear scattering mechanism from experimental and theoretical point of view. Using Brillouin light scattering (BLS) microscopy, we investigated the frequency and power dependence of this nonlinear mechanism. The spatially resolved mode profiles give evidence for the localization at the boundaries of the disk and allow for a direct determination of the modes’ wavenumbers. Furthermore, time resolved BLS in combination with pulsed microwave excitation revealed the temporal evolution of the 3-magnon splitting and its dependence on the applied microwave power.