  For the realization of magnon-based current-free technologies, referred to as
magnonics, all-optical control of magnons is an important technique for both
fundamental research and practical applications. Magnon-polariton is a coupled
state of magnon and photon in a magnetic medium, expected to exhibit
magnon-like controllability and photon-like high-speed propagation. While
recent studies have observed magnon-polaritons as modulation of incident
terahertz waves, the influence of magnon-photon coupling on magnon propagation
properties remains unexplored. This study aimed to observe the spatiotemporal
dynamics of coherent magnon-polaritons through time-resolved imaging
measurements. BiFeO$_3$ was selected as the sample due to its anticipated
strong coupling between magnons and photons. The observed dynamics suggest that
antiferromagnetic magnons can propagate over long distances, up to hundreds of
micrometers, through strong coupling with photons. These results enhance our
understanding of the optical control of magnonic systems, thereby paving the
way for terahertz opto-magnonics.
