  We study the magnon dynamics of an antiferromagnetic NiO single crystal in a
pump-probe experiment with variable pump photon energy. Analysing the amplitude
of the energy-dependent photo-induced ultrafast spin dynamics, we detect a yet
unreported coupling between the material's characteristic THz- and a GHz-magnon
modes. We explain this unexpected coupling between two orthogonal eigenstates
of the corresponding Hamiltonian by modelling the magneto-elastic interaction
between spins in different domains. We find that such interaction, in the
non-linear regime, couples the two different magnon modes via the domain walls
and it can be optically exploited via the exciton-magnon resonance.
