Observation of Kerr nonlinearity and Kerr-like nonlinearity induced by terahertz generation in LiNbO₃

Femtosecond laser pulses interacting with LiNbO₃ have been widely applied in intense terahertz generation, excitation and imaging of phonon polaritons, and modulating light. It is of great importance to study the nonlinearities induced by femtosecond laser pulses in LiNbO₃. Here, we demonstrate that both Kerr nonlinearity and Kerr-like nonlinearity induced by terahertz waves occur under the interaction between laser pulses and LiNbO₃ using optical pump-probe techniques. We show that the contribution of Kerr-like nonlinearity to pump-probe spectra varies with the interaction length because of the phase mismatching between the terahertz wave and the laser pulse. We also observed the excitation of the low frequency phonon polaritons. Experimental results are consistent with theoretical calculations.