Abstract: The propagation of ultrafast strong laser pulses in optical media (such as air) can produce a unique nonlinear laser filamentation phenomenon due to the dynamical balance between the optical Kerr self-focusing and plasma defocusing effects. Laser filamentation can induce some unique physical phenomena such as supercontinuum generation via nonlinear frequency conversion, plasmainduced high voltage discharge, and conical emission, which lead to its promising applications in areas such as atmospheric sensing, weather control. In this paper, we present an overview of nonlinear radiation processes induced by the interaction of femtosecond laser pulses in the filamentation with the propagation media. Three types of nonlinear frequency conversion phenomena, i.e., supercontinuum (white light), harmonic generation, and THz radiation, are introduced. We then give a detailed discussion about the physical mechanisms, current status, and promising applications of THz radiation.

Key words: Femtosecond laser, filamentation, nonlinear, frequency conversion.