Magnetic equivalent of electric superradiance: radiative damping in yttrium-iron-garnet films

TL;DR

This paper demonstrates magnetic superradiance in yttrium-iron-garnet films, showing how increasing film thickness enhances radiative damping over intrinsic losses, providing insights into magnetic oscillator coherence.

Contribution

It introduces a method to observe and distinguish superradiance effects in magnetic materials, specifically in yttrium iron garnet films, highlighting the dominance of radiative damping with increased thickness.

Findings

Superradiance can be observed in magnetic yttrium iron garnet films.

Increasing film thickness leads to radiative damping surpassing intrinsic losses.

The experimental setup allows clear separation of intrinsic damping and superradiance effects.

Abstract

A dense system of independent oscillators, connected only by their interaction with the same cavity excitation mode, will radiate coherently, which effect is termed superradiance. In several cases, especially if the density of oscillators is high, the superradiance may dominate the intrinsic relaxation processes. This limit can be achieved, e.g., with cyclotron resonance in two-dimensional electron gases. In those experiments, the cyclotron resonance is coupled to the electric field of light, while the oscillator density can be easily controlled by varying the gate voltage. However, in the case of magnetic oscillators, to achieve the dominance of superradiance is more tricky, as material parameters limit the oscillator density, and the magnetic coupling to the light wave is rather small. Here we present quasi-optical magnetic resonance experiments on thin films of yttrium iron garnet. Due to the simplicity of experimental geometry, the intrinsic damping and the superradiance can be easily separated in the transmission spectra. We show that with increasing film thickness, the losses due to coherent radiation prevail the system's internal broadening.