Extreme Magnon Multiplication in the van der Waals Magnetic Semiconductor CrSBr

Harnessing nonlinearities in spin wave excitations and their quanta, magnons, is key to positioning spintronics as a viable alternative to traditional electronics. However, the weak nonlinear effects in conventional magnonic systems often necessitate milliwatt-level microwave fields to excite magnons and enhance their nonlinear interactions to detectable ranges. The van der Waals magnetic semiconductor CrSBr overcomes this limitation through strong exciton-magnon coupling where only microwatts of visible optical fields can excite and detect magnons with remarkable sensitivity to nonlinear dynamics. Here, we report the observation of exceptionally high-order nonlinear magnonic effects in CrSBr. Time-resolved optical reflectivity measurements reveal the presence of more than 70 magnon harmonics across a wide range of magnetic fields and frequencies, extending to 2 THz. This represents the highest and most robust harmonic generation observed in magnonic systems to date. Our results open new possibilities for implementing opto-magnonic devices and offer new insights into the nonlinear regime of magnonics, facilitating the study of magnonic frequency combs and other novel magnon states.