  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.
