Terahertz-frequency magnon upconversion unveiled by two-dimensional magnetic resonance spectroscopy

The dynamic manipulation of nonlinear phonon-phonon interactions has arisen as a potent scheme to functionalize the properties of materials. Extensions of such a strategy to collective spin-waves could provide an attractive avenue for high-speed information processing based upon nonlinear coupled magnonics. Here, we discover that intense terahertz (THz) fields can initiate coherent magnon upconversion. By using a suite of single-shot multidimensional THz spectroscopy measurements on the canted antiferromagnet ErFeO₃, we unveil the unidirectional nature of the coupling between distinct magnon modes. Analysis of the temperature-dependent spin dynamics demonstrates that this energy transfer originates from a complex interplay of competing nonlinear pathways. Our results demonstrate a route to inducing directional energy transfer phenomena between coherent magnons in magnetic materials and pave the way for a new generation of magnonic signal processing devices.