Thermal mechanism of laser induced THz generation from a metal surface

We propose a new theoretical model of terahertz (THz) generation from a metal surface irradiated by femtosecond laser pulses. The mechanism is based on the inducing of the low-frequency polarization by the inhomogeneous temperature distribution of the electron gas near the surface. We present analytical and numerical results for the characteristic experimental parameters of a laser pulse. The experimentally observed duration of the THz signal (similar to 1 ps) is interpreted as a time of the thermal energy relaxation after the optical radiation action. The total THz energy as a function of the initial laser pulse characteristics and metal parameters is obtained. The most important experimental features are discussed and interpreted on the basis of the thermal mechanism.