Photon–magnon coupling and transmission nonreciprocity with interfering spoof surface plasmon polaritons

Abstract Photon–magnon coupled systems have established as a versatile platform for investigating hybrid coherent phenomena that bestow important applications in signal communication and transduction potentially extending to the quantum regime. Recently, near-field photon mode couplings leveraging microwave surface plasmon polaritons have emerged as a promising approach towards enhanced coupling strengths and tailored spectral features, primarily owing to their optimized mode profiles and high sensitivity to local dielectric environment. To date, most reports have focused only on a single, isolated surface plasmon mode (either traveling or localized) coupling to magnon modes; however, additional functionalities and novel control knobs can arise via further exploiting the interference between distinct modes. In this work, we investigate the transmission characteristics and the photon–magnon coupling when an interfering spoof surface plasmon polariton (SSPP) mode couples to a magnon mode. We use a Mach–Zehnder interferometer (MZI) structure that transforms the phase difference of the electromagnetic signals propagating on the two arms into amplitude interference, resulting in frequency-selective excitation of even and odd modes. By loading a ferrimagnetic Y 3 Fe 5 O 12 (YIG) disc at selective locations along the MZI structure, we observe strong and tunable photon–magnon couplings between the MZI’s SSPP modes and the YIG’s magnon mode, as well as a pronounced transmission nonreciprocity that is attributed to the coupling between the magnon modes and the rotating rf fields of the photon mode, whose origin can be attributed to the slow-wave propagation along the SSPP structure. Such a nonreciprocity is further tailored by the design of different MZI device symmetries.