Phase dependence of third-order harmonic generation in gases induced by two-color laser field

We experimentally demonstrate third-harmonic generation(THG)in gases ionized by a femtosecond laser pulse superimposed on its second-harmonic(SH).The mechanism of THG has been investigated,and it demonstrates that a third-order nonlinear process dominates at low pump intensity.Asymmetric third-harmonic(TH)spectra are observed at different time delays in two color fields,which are attributed to the process of the four-wave mixing(FWM)of the broad spectrum of pump pulses.A joint measurement on the terahertz(THz)and the TH is performed.It reveals that the optimized phase for the THG jumps from 0 to 0.5πas the pump intensity increases,which is different from the THz being a constant,and indicates that the THG arises from the nonlinearity of the third-order bound electrons to the tunnel-ionization current.