Magnon polaron formed by selectively coupled coherent magnon and phonon modes of a surface patterned ferromagnet

In a metallic ferromagnet, fast dephasing of phonons and magnons typically prevents their strong coupling. Here, the authors overcome this limitation and demonstrate a hybridized coherent state of phonon and magnon known as a ``magnon polaron'' in a metallic ferromagnetic film under femtosecond optical excitation. Surface patterning provides magnons and phonons with long lifetimes in the ferromagnetic film, while spatial matching of their wave distributions enhances their coupling strength and selects particular modes in the broad phonon and magnon spectra for hybridization.