Complex Third Order Nonlinear Optical Susceptibility in the Terahertz Region Evaluated by Free-Electron Laser

We experimentally studied the third harmonic generation (THG) from indium arsenide induced by terahertz free-electron laser. The azimuthal angle dependence of THG generation was measured, and we obtained the real and imaginary parts of the susceptibility in the third order. This paves the way for the deeper understanding of nonlinear optics in the high-field region.