Spin Ensemble Interference with Its Mirror Image in Waveguide

Bang-Yao Wu^1*, Yu-Ting Cheng¹, Kuan-Ting Lin², Io-Chun, Hoi¹

¹Department of Physics, City University of Hong Kong, Hong Kong SAR, China
²Department of Physics, National Taiwan University, Taipei, Taiwan

* Presenter:Bang-Yao Wu, email:bangyaowu2-c@my.cityu.edu.hk

We investigate the magnetic dipole-dipole interaction between a magnon mode and its mirror image, mediated by traveling photons. The magnon mode of interest is the Kittel mode (KM), representing a spatially uniform precession of a collective spin excitation within a Yttrium Iron Garnet (YIG) sphere. Away from the mirror, the resonance frequencies of the KM have sinusoidal shifts of ±5.8 MHz in a YIG sphere with diameter 1.2 mm compared to the setup without the mirror. The oscillation of the magnon radiative decay into the waveguide follows a cosine-squared pattern, with its enhancement doubling when the KM is positioned at the magnetic antinode of the corresponding eigenmode. These observations are attributed to the coherent radiation emitted by the KM interferes with its mirrored counterpart. Moreover, we can finely tune the KM to maximize the absorption of input photons at the critical coupling point. When the KM is placed near the resonance field node, its lifetime extends to over eight times that observed when positioned close to the antinode.

Keywords: Mangnon, Spin ensemble, Resonant dipole-dipole interaction, Waveguide magnonics