Picosecond excitation‐probe measurements using a far‐infrared free‐electron laser (CLIO) have revealed large nonlinearities for indium antimonide at 4.7 &mgr;m. A theoretical model is described to determine the cw third‐order nonlinear susceptibility and the interband relaxation time of the semiconductor which were found to be −8.6×10−11m2 V−2and 0.3 ns, respectively. Furthermore, the observation of the associated coherent transient grating effects allows us to obtain a coherence time of the laser system (2.5 ps) and the &khgr;(3)of the transient grating which was found to be −7.2×10−13m2 V−2. The measurements were performed at room temperature on undoped bulk InSb. ©1995 American Institute of Physics.