MEASUREMENT OF THE SPATIOTEMPORAL EVOLUTION OF HIGH-ORDER HARMONIC RADIATION USING CHIRPED LASER-PULSE SPECTROSCOPY

We report the first spatially resolved measurements of the rime depend ence of high-order harmonic radiation. We have measured the time evolu tion of the harmonic distribution at the exit of the gas jet for the 1 3th harmonic (lambda = 60 nm) generated in a low-density xenon jet by a 780 nm, 1 ps chirped laser pulse in the 10(13)-10(14) W cm(-2) range . We show that the harmonic source can become annular with a radius th at increases in time during the laser pulse owing primarily to ionizat ion depletion of the neutral medium. Under certain focusing conditions , we demonstrate that this can lead to significant harmonic pulse shor tening. [S0031-9007(97)05277-0].