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

Abstract: 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 $θ= 0^\circ$, 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.

• The paper shows that controlling the resonator orientation enables selective activation and suppression of distinct photon modes to achieve targeted photon–magnon coupling.
• The study employs both electromagnetic simulation and an analytic equivalent-circuit model with a three-mode Hamiltonian to quantitatively capture the angle-dependent coupling dynamics.
• The results reveal strong hybridization with cooperativities exceeding 20, paving the way for reconfigurable on-chip magnonic filters and quantum information routing.

Polarization-Controlled Photon Mode Switching and Photon–Magnon Coupling in Planar Cavity–Magnonic Systems

Introduction and Motivation

The interaction between microwave photons and magnons in hybrid quantum architectures is fundamental for information transduction, quantum storage, and scalable magnonic devices. However, conventional photon–magnon coupling (PMC) protocols provide limited control over the selectivity and pathway of coupling when multiple photon cavity modes are present. A robust, scalable approach to selectively control mode pathways for PMC within planar, chip-scale devices remains an open challenge.

This work addresses this challenge by introducing a planar electric–LC resonator (ELCR) side-coupled to a microstrip and integrated with a YIG thin film, demonstrating that both mode selectivity and the photon–magnon coupling strength can be controlled by the polarization (orientation) of the resonator relative to the fixed microwave field polarization of the waveguide. The study combines electromagnetic simulation, analytic equivalent circuit modeling, and quantum Hamiltonian formalism to provide both qualitative and quantitative description of the polarization-selective PMC phenomena.

ELCR Design and Polarization-Selective Photon Modes

The ELCR–microstrip hybrid supports two orthogonal photon modes, which, under microwave excitation at different orientation angles $\theta$, can be selectively activated or suppressed. Electromagnetic full-wave simulations (CST Microwave Studio) reveal distinct resonance peaks (mode-1 at $\sim3.93\,\text{GHz}$ and mode-2 at $\sim5.73\,\text{GHz}$), with the excitation efficiency determined by the geometric alignment between the ELCR and the microstrip line.

At $\theta = 0^\circ$, only mode-1 is active due to maximal field overlap. As the orientation increases to $\theta = 90^\circ$, mode-1 is suppressed and mode-2 becomes dominant. Numerical calculation of the surface current distributions elucidates the physical mechanism behind the mode switching: the magnetic near field excites distinct current loops whose mutual inductance contributions add constructively or destructively depending on $\theta$.

This polarization-controlled switching is quantitatively described by an equivalent-circuit model, where the mutual inductances $M_1$ and $M_2$ (corresponding to photonic modes 1 and 2) control the angular dependence of the photon mode visibility in transmission ($S_{21}$) spectra and their extrinsic (radiative) damping rates. The analytic solution for $S_{21}$ based on this circuit model matches the observed spectra across the entire angular range.

Polarization-Driven Radiative Damping Transition

Experimental and simulated results show that for each mode, the extrinsic damping rate ($\sim3.93\,\text{GHz}$0, $\sim3.93\,\text{GHz}$1) varies as $\sim3.93\,\text{GHz}$2, $\sim3.93\,\text{GHz}$3, reflecting the field projection onto the two orthogonal modes. The polarization angle thus directly governs radiative dissipation, allowing a bright-dark state transition.

A phenomenological order parameter $\sim3.93\,\text{GHz}$4 formalizes the continuous, analytic transition from mode-1- to mode-2-dominated dissipation channels as a function of $\sim3.93\,\text{GHz}$5 and normalized damping ratio $\sim3.93\,\text{GHz}$6. This enables systematic, geometry-based radiative switching without any material or circuit reconfiguration.

Hybrid ELCR–YIG Platform and Angle-Selective Photon–Magnon Coupling

Integrating a thin YIG film with the ELCR creates a hybridized magnon–photon system. The YIG magnon mode is tuned via an external magnetic field. As the magnon resonance crosses the ELCR cavity modes, coherent mode splitting (anticrossing) features are observed in angle-resolved $\sim3.93\,\text{GHz}$7 spectra. These anticrossing signatures are strongly angle-dependent:

• At $\sim3.93\,\text{GHz}$8, strong coupling is only observed for photon mode-1 ($\sim3.93\,\text{GHz}$9); mode-2 does not hybridize.
• At $\sim5.73\,\text{GHz}$0, only mode-2 couples ($\sim5.73\,\text{GHz}$1); mode-1 is suppressed.
• At intermediate angles, both channels are active but their effective coupling strengths are redistributed, with $\sim5.73\,\text{GHz}$2 increasing up to $\sim5.73\,\text{GHz}$3 at $\sim5.73\,\text{GHz}$4 and vanishing near $\sim5.73\,\text{GHz}$5, while $\sim5.73\,\text{GHz}$6 grows at finite angles but decreases towards $\sim5.73\,\text{GHz}$7.

These results reveal strong, non-monotonic, and complementary control over the individual photon–magnon hybridization channels with simple in-plane resonator rotation.

Theoretical Modeling: Three-Mode Hamiltonian and Coupling Extraction

The hybrid system is rigorously modeled using a three-mode non-Hermitian Hamiltonian, incorporating both intrinsic (material, conductor, dielectric) losses and extrinsic (radiative) waveguide couplings—each with explicit polarization dependence. The theory captures both coherent and dissipative contributions to hybridization and quantitatively reproduces the angle-dependent splitting and linewidths observed in experiment.

The extracted coupling strengths are robust indicators of near-ideal hybridization: cooperativities for both photon–magnon channels exceed $\sim5.73\,\text{GHz}$8, with $\sim5.73\,\text{GHz}$9 (mode-1) reaching up to $\theta = 0^\circ$0. Notably, the coupling strengths are not strictly correlated with maximum radiative damping but are dictated by geometric mode-field overlap due to polarization projection.

Implications, Prospects, and Theoretical Extensions

The demonstration of robust, geometry-controlled selective hybridization without requiring frequency tuning or in-situ circuit adjustments establishes a powerful strategy for multi-mode, multi-channel cavity magnonics. This approach enables:

• Dynamic pathway selection for quantum state transfer or information routing between different channels without hardware rewiring.
• Reconfigurable on-chip magnonic filters and non-reciprocal devices leveraging selective coupling.
• Integrated, scalable, and programmable multi-mode magnonic–photonic platforms, directly compatible with standard microfabrication and PCB technologies.

From a theoretical perspective, this work lays the foundation for systematic exploration of geometric control of hybridization in a broad class of multimode, planar cavity quantum electrodynamics systems. Further, generalized approaches to Hamiltonian engineering incorporating higher-order photon and magnon modes, as well as chiral or synthetic gauge field effects, can be directly interfaced with the methodology presented here.

Future directions in AI-driven design optimization may leverage such polarization–projection principles for inverse design of optimal magnonic and photonic lattices at both the classical and quantum level.

Conclusion

This work demonstrates a fully planar platform for polarization-controlled, selective photon–magnon hybridization in an ELCR–YIG system, with analytic and experimental validation of orientation-driven mode switching and hybridization channel redistribution. The framework provides a robust control knob—resonator orientation—for deterministic engineering of multi-mode interactions in reconfigurable magnonic–photonic devices, underscoring its potential impact on hybrid quantum information and on-chip signal processing architectures (2605.05018).