Polarization-Controlled Photon Mode Switching and Photon--Magnon Coupling in a Planar Cavity--Magnonic System

This work presents polarization-selective photon-magnon coupling (PMC) in a planar cavity-magnonic platform consisting of an electric-LC resonator (ELCR) side-coupled to a microstrip transmission line and integrated with a yttrium iron garnet (YIG) thin film. The ELCR supports two orthogonal photon modes at $\sim 3.93$ GHz and $\sim 5.73$ GHz, whose excitation and radiative damping are governed by the resonator orientation relative to the microwave-field polarization. Rotating the resonator enables controlled switching between these modes and tunable photon-magnon hybridization. An equivalent circuit model including intrinsic and extrinsic damping successfully reproduces the polarization-driven mode switching, while an effective three-mode Hamiltonian accurately captures the coupled-mode evolution. The results reveal strong angular tunability of the PMC strength through redistribution between two competing interaction channels. At $\theta = 0^\circ$, only the lower-frequency photon mode is excited, yielding $g_{31}=56.5$ MHz, while the higher-frequency mode remains inactive. As the angle increases, both channels become active: $g_{31}$ increases from $56.5$ to $98$ MHz over $0^\circ$-$60^\circ$ before vanishing at $90^\circ$, whereas $g_{23}$ decreases from $76$ to $30$ MHz over $30^\circ$-$90^\circ$. The observed evolution yields a measured transition near $25.7^\circ$ and a symmetry-related model-predicted transition near $154.3^\circ$. These findings establish resonator-orientation--driven polarization selectivity as a versatile mechanism for controllable photon--magnon interactions in planar architectures.