Microwave-Induced Optomagnetism in High-Temperature Superconductors

We report the first experimental observation of a steady-state, microwave-driven inverse Faraday effect in a high-temperature superconductor. Circularly polarized microwave radiation generates a helicity-dependent response in an epitaxial $\mathrm{YBa_2Cu_3O_{7-δ}}$ film, detected using homodyne Hall transport. The optomagnetic response emerges exclusively below $T_c$, vanishes in the normal state, and exhibits no power-dependent counterpart under linearly polarized excitation. The effective optomagnetic conversion reaches $1.75\,\mathrm{T}/(\mathrm{W\,cm^{-2}})$, surpassing optical benchmarks by several orders of magnitude. At higher microwave powers, the signal collapses when the self-generated field exceeds $B_{c1}$, marking the onset of a vortex phase-slip regime, and subsequently re-emerges at mode-locked vortex-washboard harmonics. These results establish steady-state microwave optomagnetism as a route to contactless, non-inductive magnetic control and nonequilibrium vortex spectroscopy in superconducting quantum systems.