Optical sensing of magnons via the magnetoelastic displacement

We show how to measure a steady-state magnon population in the magnetostatic mode of ferromagnet or ferrimagnet, such as yttrium iron garnet. adopt an optomechanical approach and utilize magnetoelasticity ferromagnet. The magnetostrictive force dispersively couples magnons deformation displacement ferromagnet, which is proportional population. By further coupling mechanical optical cavity that resonantly driven by weak laser, magnetostrictively induced can be sensed measuring phase quadrature field. shows excellent linear dependence on for not very large thus used ``magnometer'' study effect thermal noises find high signal-to-noise ratio even at room temperature. At cryogenic temperatures, resolution excitation numbers essentially limited vacuum fluctuations phase, significantly improved using squeezed light.