Terahertz-field-induced second-harmonic generation measurements of liquid dynamics

A new third-order non-linear THz pump–optical probe measurement of liquid dynamics is reported. Terahertz-field-induced second-harmonic generation (TFISH) measures the correlations between the hyperpolarizability and dipole moment fluctuations at equilibrium. This method is analyzed in the Born–Oppenheimer approximation, and is shown to measure diffusive orientational dynamics as 〈P1(cos θ)〉. Measurements of water and acetonitrile yield relaxation time constants of 13 and 9.6 ps, respectively. Signals were also obtained from thin polymer films and surfaces, suggesting a number of applications for this method.