  Femtosecond laser pulses interacting with LiNbO3 has 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 wave 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 terahertz wave and laser pulse. We also observed the
excitation of the low frequency phonon polaritons. Experimental results are in
consistent with theoretical calculation.
