  Strong coupling between two quanta of different excitations leads to the
formation of a hybridized state which paves a way for exploiting new degrees of
freedom to control phenomena with high efficiency and precision. A magnon
polaron is the hybridized state of a phonon and a magnon, the elementary quanta
of lattice vibrations and spin waves in a magnetically-ordered material. A
magnon polaron can be formed at the intersection of the magnon and phonon
dispersions, where their frequencies coincide. The observation of magnon
polarons in the time domain has remained extremely challenging because the weak
interaction of magnons and phonons and their short lifetimes jeopardize the
strong coupling required for the formation of a hybridized state. Here, we
overcome these limitations by spatial matching of magnons and phonons in a
metallic ferromagnet with a nanoscale periodic surface pattern. The spatial
overlap of the selected phonon and magnon modes formed in the periodic
ferromagnetic structure results in a high coupling strength which, in
combination with their long lifetimes allows us to find clear evidence of an
optically excited magnon polaron. We show that the symmetries of the localized
magnon and phonon states play a crucial role in the magnon polaron formation
and its manifestation in the optically excited magnetic transients.
