Enhancing the yield of high-order harmonics with an array of gas jets

We report the experimental observation of an enhancement in the yield of high-order harmonics using an array of gas jets as the source medium. By comparing the experimental outcome for jet arrays of different spacings with the predicted harmonic intensity in the case of slit sources of equivalent lengths, we clearly show how the periodic modulation of the gas density can improve the harmonic yield. This behavior may be attributed to a quasi-phase-matching effect which increases the length of coherent harmonic buildup during propagation by partially counteracting the dephasing induced by free electrons.