Ultra-strong magnon-photon coupling induced in the photonic crystals with an YGaGeIG defect

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 derived from the large spin numbers of the ferrite cylinder. This work may open a way to tune the coupling induced in the photonic crystals with a magnetic defect.