Experimental Signature of Transient Symmetry Breaking in a Cavity Dirty-Limit Superconductor

Transient states of matter far from equilibrium may exhibit physical properties beyond those allowed by the equilibrium-state crystalline symmetries. We explore ultrafast and direct electronic excitations of transient states in a dirty-limit superconductor with time-resolved terahertz-pump terahertz-probe spectroscopy. By embedding the superconductor into a designed microcavity structure of metallic split-ring resonators, we can sensitively probe the pump field-induced transient variations of the superconducting states around the cavity’s resonant frequency. Our results show that the strong terahertz field can transiently modify the symmetries of the electronic subsystems via the injection of a picosecond supercurrent, leading to high-order nonlinear dynamical responses that are not compatible with the equilibrium-state symmetries, which is evidence for transient symmetry breaking on the picosecond time scale.