Terahertz emission from femtosecond laser excited Ge surfaces due to the electrical field-induced optical rectification

We have investigated azimuthal angle dependences of terahertz (THz) emission from differently oriented surfaces of variously doped Ge crystals illuminated by femtosecond Ti:sapphire laser pulses and we have shown the presence of a significant nonlinear optical contribution due to the electric field-induced optical rectification effect. Obtained results have led to the separation of relative contributions of photo-Dember and nonlinear optical effects and to the estimation of the value of the third-order susceptibility in Ge.