Observation of Long-Lifetime Magnon Pairs by Fano Resonance of Photons

Abstract

Mode fluctuations with a long lifetime are essential for quantum information and logic operations in magnonic devices. We probe the broadband nonlinear magnetization dynamics of a high-quality ferromagnet under a strong microwave drive using microwave spectroscopy. We observe an \textit{unexpected} Fano resonance in the microwave transmission when the driven amplitude of the magnetization is large and the drive frequency $\omega_d$ is close to but not at the ferromagnetic resonance. We interpret this Fano resonance by a scattering theory of photons considering the three-magnon interaction between the Kittel magnon and magnon pairs with opposite wave vectors of frequency $\omega_d/2$. The theoretical model suggests that the microwave spectroscopy measures the dynamics of the fluctuation $\delta \hat{\alpha}$ of the Kittel magnon and $\delta\hat{\beta}_{\pm k}$ of the magnon pairs over the driven steady states, which are coupled coherently by the steady-state amplitudes. With the damping of $\delta\hat{\beta}_{\pm k}$ much smaller than that of $\delta \hat{\alpha}$, the theoretical calculation well reproduces the observed Fano resonance, indicating the magnon pairs hold a recorded long lifetime.