Extreme Nonlinear Opto-Magnonic Effects in a Layered Magnetic Semiconductor*

The nonlinear dynamics of collective excitations offer both intriguing fundamental phenomena and significant practical applications. A prime illustration is the field of nonlinear optics, where diverse frequency mixing processes are central to advancing photonic technology. Demonstration of these frequency mixing processes in magnons holds considerable potential for practical applications in magnonics, an emerging frontier of spintronics and an important platform for developing quantum transducers and wave-based computing beyond traditional paradigms. Here we demonstrate the optical generation and detection of abundant magnonic frequency mixing processes in the antiferromagnetic semiconductor CrSBr. We employ above-gap pump pulses to launch coherent magnons, which we can then optically measure via strong magnon-exciton coupling. We observe a series of magnon sidebands arising from high-harmonic generation and, when breaking the system symmetry, the mixing of discrete magnon modes to produce sum and difference frequency generation (SFG & DFG). Further, we demonstrate control over the DFG in CrSBr by rotating an external magnetic field to tune its frequency over a broad range. This tuning allows us to push the DFG mode into resonance with one of the fundamental magnon modes, where we can controllably induce parametric amplification. These findings herald the opening of a new domain in nonlinear opto-magnonic coupling, offering innovative functionalities for hybrid quantum magnonics.