The microscopic origin of magnon-photon level attraction by traveling waves: Theory and experiment

Abstract

The dissipative light-matter coupling can cause the attraction of two energy levels, i.e., level attraction, when competing with the coherent coupling that induces usual Rabi-level splitting. The level attraction shows attractive potential for topological information processing. However, the underlying microscopic quantum mechanism of dissipative coupling still remains unclear although the behavior has been understood to root in the non-Hermitian physics, which brings difficulties in quantifying and manipulating the competition between coherence and dissipation and thereby the flexible control of level attraction. Here, by coupling a magnon mode to a cavity supporting both standing and traveling waves, we identify the traveling-wave state to be responsible for magnonphoton dissipative coupling. By characterizing the radiative broadening of a magnon linewidth, we quantify the coherent and dissipative coupling strengths and their competition. The effective magnon-photon coupling strength, as a net result of competition, is analytically presented using quantum theory to show good agreement with measurements. In this manner, we extend the control dimension of level attraction by tuning field torque on magnetization or global cavity geometry. Our findings provide insights on engineered coupled harmonic oscillator systems.

Interactions between a magnon mode and a cavity photon mode mediated by traveling photons

“…All these experimental observations well confirm our theoretical predictions. Our work clarifies the mechanism of dissipative coupling mediated by traveling photons, which is useful for us to understand the recently discovered dissipative coupling [20,[29][30][31][32][33][34][35][36][37] and giant nonreciprocity [20] in cavity magnonics systems. Additionally, the physics and technique developed in our work may help us design a new hybrid system based on waveguide magnonics.…”

Dissipative couplings in cavity magnonics

“…In Refs. [48][49][50][51]54,56,57 the dissipative coupling was due to dissipation processes, while in Ref. 61,62 the level attraction was caused by the interference effect between two driven tones.…”

“…Yao et al 56,57 show that quantitatively, the amount of the radiative linewidth changing of the magnon-like hybridized mode can be mapped to the density of photon states. It provides another way for verifying the influence of the traveling wave on the effect of magnon-photon coupling.…”

“…A summary of several theoretical pictures of the dissipative coupling and level attraction. The figure in the third row is adapted from Yao et al56 . The K F term stems from Faraday's law.…”

Circulating cavity magnon polaritons

“…The strong magnon-photon coupling in microwave cavities [1][2][3] allows, e.g., manipulation of spin currents [4][5][6][7], nonreciprocal microwave engineering [8], the design of logic devices [9], data storage [10], and magnon entanglement for quantum information [11]. In closed cavities the coherent coupling hybridizes magnon and photon levels into magnon polaritons, which can be identified in terms of a level repulsion between magnon and photon levels, while a dissipative coupling in open or leaky waveguides causes level attraction [12][13][14][15][16]. Analogous with structures that are coupled by optical resonators [17], metamaterials [18], and dielectric nanostructures [19,20], multiple magnets inside a cavity form new collective modes by the real or virtual exchange of cavity photons [10,21,22].…”