In this work, magnon-photon couplings are studied by using a point defect cavity in two
dimensional simple cubic structure photonic crystals. The point defect is introduced by
replacing one of the lattice points (copper cylinder) with a Ca and Ge element doped yttrium-
iron-garnet (YCaGeIG) cylinder. Ultrastrong magnon-photon coupling is detected, with the
coupling strength of 2.10 GHz, which constitutes about 23.4% of the photon energy
compared to the cavity frequency around 8.98 GHz. The ultrastrong coupling might be …