  We observe a nonreciprocal ultrastrong magnon-photon coupling in the bandgap
of photonic crystals by introducing a single crystal YIG cylinder into copper
photonic crystals cavity as a point defect. The coupling strength reaches up to
1.18 GHz, which constitutes about 10.9% of the photon energy compared to the
photon frequency around 10.8 GHz. It is fascinating that the coupling achieves
unidirectional signal transmission in the whole bandgap. This study
demonstrates the possibility of controlling nonreciprocal magnon-photon
coupling by manipulating the structure of photonic crystals, providing new
methods to investigate the influence of magnetic point defects on microwave
signal transmission.
