  Uncovering pathways to optically drive magnetic order-disorder transitions on
ultrashort timescales can lead to the realization of novel out-of-equilibrium
quantum phenomena. A long-sought pathway is to directly excite a highly
non-thermal energy-momentum distribution of magnons, bypassing both charge and
lattice degrees of freedom. However, this remains elusive owing to the weak
coupling and large momentum mismatch between photons and magnons. Here we
demonstrate strong parametric excitation of magnons across the entire Brillouin
zone of the antiferromagnetic insulator Sr$_2$Cu$_3$O$_4$Cl$_2$ by periodically
modulating the superexchange interaction with the electric field of light. The
excitation efficiency is greatly enhanced by tuning to the van Hove singularity
in the magnon spectrum, sufficient to transiently collapse the
antiferromagnetic state using a pulsed laser field of 10$^9$ V/m. The order
parameter recovery timescale increases by over 1000 times as a function of
excitation density, reflecting a crossover from high- to low-energy magnon
dominated decay dynamics. This electric-field induced parametric magnon pumping
mechanism is applicable to a broad range of magnetic insulators and opens up
the possibility of dynamically engineering magnon distributions by design.
