Measurement of the Spatiotemporal Evolution of High-Order Harmonic Radiation Using Chirped Laser Pulse Spectroscopy

We report the first spatially resolved measurements of the time dependence of high-order harmonic radiation. We have measured the time evolution of the harmonic distribution at the exit of the gas jet for the 13th harmonic $(\lambda =60\mathrm{}\mathrm{nm})$ generated in a low-density xenon jet by a 780 nm, 1 ps chirped laser pulse in the ${10}^{13}–{10}^{14}\mathrm{W}{\mathrm{cm}}^{-2\mathrm{}}$ range. We show that the harmonic source can become annular with a radius that increases in time during the laser pulse owing primarily to ionization depletion of the neutral medium. Under certain focusing conditions, we demonstrate that this can lead to significant harmonic pulse shortening.