High-harmonic pulses driven by a flying focus*

An intense laser pulse can accelerate a photoionized electron back into its parent ion unleashing a broad spectrum of high-harmonic radiation. Conventional techniques for driving coherent pulses of this radiation rely on a delicate balance of dispersion and geometry to achieve phase matching and minimize absorption. Here we discover that spatiotemporally shaped laser pulses enable a novel technique for driving high-harmonic pulses that allows the radiation to accumulate without precise phase matching or significant absorption. By travelling faster than the phase velocity of the radiation, the intensity peak of a spatiotemporally shaped pulse can ionize and drive electron collisions ahead of the existing phase fronts. This prevents the transfer of energy back to the laser pulse and eliminates single-photon absorption in the downstream medium, increasing the energy radiated into broadband, extreme ultraviolet pulses by orders of magnitude.