Time-Resolved Femtosecond Degenerate Four-Wave Mixing in LiNbO₃:Fe,Mg Crystal

Forward degenerate four-wave mixing (DFWM) processes are investigated with a femtosecond pulsed laser in lithium niobate crystal doubly-doped with magnesium and iron (LiNbO₃:Fe,Mg). The pulse energy dependence reveals a pure third-order nonlinear response, and the third-order nonlinear susceptibility x⁽³⁾ in the material is evaluated to be 4.96×10^-13esu. The time-resolved DFWM process shows a response time of x⁽³⁾ shorter than 100fs, which is due to the nonresonant electronic nonlinearities. Our results indicate that LiNbO₃ crystals have potentials for ultrafast real-time optical processing systems, which require a large and fast x⁽³⁾ optical nonlinearity.