Strong Terahertz Third-Harmonic Generation by Kinetic Heavy Quasiparticles in CaRuO 3

We report on time -resolved nonlinear terahertz spectroscopy of a strongly correlated ruthenate, CaRuO 3 , as a function of temperature, frequency, and terahertz field strength. Third -harmonic radiation for frequencies up to 2.1 THz is observed evidently at low temperatures below 80 K, where the low -frequency linear dynamical response deviates from the Drude model and a coherent heavy quasiparticle band emerges by strong correlations associated with the Hund 's coupling. Phenomenologically, by taking an experimentally observed frequency -dependent scattering rate, the deviation of the field driven kinetics from the Drude behavior is reconciled in a time -dependent Boltzmann description, which allows an attribution of the observed third -harmonic generation to the terahertz field driven nonlinear kinetics of the heavy quasiparticles.