Non-Hermitian Control between Absorption and Transparency in Reflectionless Magnonics

Abstract Photon propagation with controllable absorption/transmission and meanwhile vanishing reflection is highly desired for many practical applications. Recent works in metamaterials and transformation optics have demonstrated promising progresses including coherent perfect absorption/transparency (PA/PT), electromagnetically induced transparency, cloaking or invisibility etc. However, most of these reported works eluded the non-Hermicity nature of the open systems. Meanwhile, reported works in non-Hermitian physics field focused mainly on the eigenstate properties and less attention has been paid to the reflection condition despite that perfect zero-reflection (PZR) is a prerequisite condition to realize actual applications like PA/PT. Here we demonstrate that the coupled system of two subwavelength-sized magnon resonators on a same microwave transmission line exhibits remarkable non-Hermitian properties such as level repulsion (LR) and level attraction (LA) interactions and meanwhile allows the reflection to be tuned to perfect zero. The observed PZR condition is found to exhibit an infinitely narrow bandwidth and meanwhile an infinite discontinuity in the group delay. This singularity in reflection distinguishes from the eigenstates of magnonic polaritons but can be adjusted on or off resonance with the eigenstates. Accordingly, the absorption and transmission can be flexibly tuned from nearly PA to nearly PT regimes. Our work not only enriches non-Hermitian physics in magnon-photon coupled systems but also brings important implications to practical applications such as microwave circuits, photonic chips and quantum information.