Tracing the dynamics of superconducting order via transient third harmonic generation

Ultrafast optical control of quantum systems is an emerging field of physics. In particular, the possibility of light-driven superconductivity with ultrashort laser pulses has attracted much of attention. To identify non-equilibrium superconductivity, it is necessary to measure fingerprints of superconductivity on ultrafast timescales. Recently non-linear THz third harmonic generation (THG) was shown to directly probe the collective degrees of freedoms of the superconducting condensate including particularly the Higgs mode. Here we extend this idea to light-driven non-equilibrium states in superconducting La2-xSrxCuO4 establishing a protocol to access the transient superconducting (SC) order-parameter fluctuations. We perform an optical pump-THz-THG drive experiment and use a two-dimensional spectroscopy approach to disentangle the driven third-harmonic response of optically excited quasiparticles and the pure condensate response. In this way, 2D spectroscopy separately probes both the ultrafast pair breaking dynamics and transient pairing amplitude of the condensate.