| We study high-order harmonic generation in excited media using a four-wave-mixing-like configuration. We analyze the spatial profile of high harmonics emitted by a grating of rotationally excited molecules as a function of the pump-probe delay. This scheme allows a dramatic improvement in contrast of the diffracted signal relative to the total high harmonic signal. The diffraction pattern is sensitive to phase modulations of the harmonic emission as a function of molecular alignment, which allows us to detect phase jumps due to the recombination process in CO2. By decreasing the number of interfering sources to 2, we perform accurate measurements of the harmonic phase. |
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