Terahertz radiation from plasma filament generated by two-color laser gas–plasma interaction

We develop a theoretical model for terahertz (THz) radiation generation, when an intense short laser pulse (ω1, k1) is mixed with its frequency shifted second harmonic (ω2, k2), where ω2 = 2ω1 + ωT and ωT is in the THz range in the plasma. The lasers exert a ponderomotive force on the electrons and drive density perturbations at (2ω1, 2k1) and (ω2 − ω1, k2 − k1). These density perturbations couple with the oscillatory velocities of the electron due to the lasers and produce a nonlinear current at (ω2 − 2ω1, k2 − 2k1). This current acts as an antenna to produce the THz radiation. The THz power depends upon the square of plasma density and $I_1^2 {I_2}$, where I1 and I2 are the intensities of fundamental and second harmonic laser. The radiation is mainly along the forward direction. Two-dimensional particle-in-cell simulations are used to study the near-field radiation properties.