Investigations on terahertz radiation generated by two superposed femtosecond laser pulses

The generation of terahertz (THz) radiation based on tunnel ionization of a gas jet is analytically investigated when two superposed short pulse lasers with finite initial phase difference are focused on to it after passing through an axicon. The phase difference between these two lasers plays an important role for the optimization of rate of ionization, evolution of plasma density, and subsequently the residual current due to dipole oscillations. The directionality of the emitted THz radiation can be controlled by tuning initial phase difference between the two laser pulses. Since a nonuniform plasma is produced during the tunnel ionization, the effect of radial variation in the electron density in the plasma channel is studied on the frequency of the emitted THz radiation and on its power. Higher power THz radiation is obtained for the higher fields of the lasers. With optimum initial phase of the laser envelope and the channel width, the mechanism seems to be much more efficient than some of the other mechanisms.