Microwave interference between a spin ensemble and its mirror image
Phys. Rev. A 113, 053720 – Published 26 May, 2026
DOI: https://doi.org/10.1103/swf5-ynd5
Abstract
We investigate microwave interference between a spin ensemble and its mirror image in a one-dimensional (1D) waveguide. Away from the mirror, the resonance frequency of the Kittel mode (KM) inside a ferrimagnetic spin ensemble displays sinusoidal shifts (compared to a setup without the mirror) as a function of the distance to the mirror. Furthermore, the radiative decay of the KM into the waveguide shows a cosine-squared oscillation as the distance to the mirror is varied. These shifting frequencies and decay rates are a consequence of the KM's interaction with its own image, mediated by the waveguide. The decay is enhanced twofold when the spin ensemble sits at the magnetic antinode of the corresponding eigenmode in the waveguide; conversely, we measure an extension of the KM lifetime by a factor of 8 when the spin ensemble is approximately at a node. We can finely tune the KM decay rate to achieve maximal absorption of input photons at a critical coupling point. Moreover, by pumping the spin ensemble to generate a pump-induced magnon mode in a two-tone experiment, we achieve a pump-induced phase shift of up to about 90° for a coherent microwave probe field.
