Terahertz radiation from oscillating electrons in laser-induced wake fields.

Strong terahertz (1 THz= 10(12) Hz) radiation can be generated by the electron oscillation in fs-laser-induced wake fields. The interaction of a fs-laser pulse with a low-density plasma layer is studied in detail using numerical simulations. The spatial distribution and temporal evolution of terahertz electron current developed in a low-density plasma layer are presented, which enables us to calculate the intensity distribution of THz radiation. It is shown that laser and plasma parameters, such as laser intensity, pulse width, and background plasma density, are of key importance to the process. The optimum condition for wake-field excitation and terahertz emission is discussed upon the simulation results. Radiation peaked at 6.4 THz, with 900 fs duration and 9% bandwidth, can be generated in a plasma of density 5x 10(17) cm(-3) . It turns out that the maximum radiation intensity scales as n(3)(0) a(4)(0) when wake field is resonantly excited, where n(0) and a(0) are, respectively, the plasma density and the normalized field amplitude of the laser pulse.