Excitation and nonlinear control of lattice vibrations with light has become a powerful method to manipulate the properties of quantum materials out of equilibrium. Generalizing from coherent phonon–phonon interactions to nonlinear couplings among other types of collective mode would open additional opportunities to design the dynamic properties of solids. For example, the collective excitations of magnetic order—magnons—can carry information with little energy dissipation, and their coherent and nonlinear control would provide an attractive route to achieve collective-mode-based information processing and storage in forthcoming spintronics and magnonics. Here we discover that intense terahertz fields can initiate the processes of magnon upconversion mediated by an intermediate magnetic resonance. By utilizing two-dimensional terahertz polarimetry, we demonstrate the unidirectional nature of coupling between distinct magnon modes of a canted antiferromagnet. The calculations of spin dynamics further suggest that this coupling is a universal feature of antiferromagnets with canted magnetic moments. These results demonstrate a route to induce desirable energy transfer pathways and a terahertz-induced coupling between coherent magnons in solids.
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