  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.
