Two color laser self focusing and terahertz generation in multi-ion species plasma

An analytical formalism of self focusing of two co-propagating laser beams, fundamental ω₁ and close to second harmonic ω₂≈2ω₁, in a multi-ion species plasma and resonant terahertz radiation generation has been developed. The nonlinearity arises due to ponderomotive force on electrons by both the beams and subsequent ambipolar redistribution of the plasma. The ions of higher charged state are strongly depleted from the axial region due to space charge field effects and electron charge is largely compensated by the ions of lower charge. When the power of the strong beam equals the critical power for its self-guiding, the plasma provides an oscillatory waveguide for the weak beam; the weak beam shrinks and rebounds when its frequency is lower than that of the strong beam, and it swallows and rebounds when the frequency is higher. When both beams are strong the nonlinearities have cumulative effect on both the beams. The cross coupling of the beams gives rise to nonlinear current giving rise to terahertz radiation at ω=ω₂−2ω₁ frequency. An axial density ripple aids phase matching and gives rise to higher power conversion efficiency. Self focusing leads to enhancement in conversion efficiency.