Spectral characterization of third-order harmonic generation assisted by two-dimensional plasma grating in air

A dramatic spectral modulation of third-order harmonic (TH) emission generated in a near in- frared femtosecond (fs) pulse filamentation, assisted by a two-dimensional plasma grating formed by two pump femtosecond pulses, is experimentally demonstrated when their spatiotemporal overlap is achieved. It is mainly attributed to strong cross-phase modulation induced by the fundamental wave of the probe pulse and two pump ones. The delay dynamic of TH spectra indicates the influence of two retarded nonlinear responses on the TH generation. The dependences of TH generation on the energies of probe and pump pulses, relative field polarization angle are also studied.