  Tailored light excitation and nonlinear control of lattice vibrations have
emerged as powerful strategies to manipulate the properties of quantum
materials out of equilibrium. Generalizing the use of coherent phonon-phonon
interactions to nonlinear couplings among other types of collective modes would
open unprecedented opportunities in the design of novel dynamic functionalities
in 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 (THz)
fields can initiate processes of magnon upconversion mediated by an
intermediate magnetic resonance. By using a suite of advanced spectroscopic
tools, including a newly demonstrated two-dimensional (2D) THz polarimetry
technique enabled by single-shot detection, we unveil the unidirectional nature
of coupling between distinct magnon modes of a canted antiferromagnet.
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 inducing desirable energy transfer pathways and
THz-induced coupling between coherent magnons in solids and pave the way for a
new era in the development of ultrafast control of magnetism.
