Zeptosecond High Harmonic keV X-Ray Waveforms Driven by Midinfrared Laser Pulses

We demonstrate theoretically that the temporal structure of high harmonic x-ray pulses generated with midinfrared lasers differs substantially from those generated with near-infrared pulses, especially at high photon energies. In particular, we show that, although the total width of the x-ray bursts spans femto- second time scales, the pulse exhibits a zeptosecond structure due to the interference of high harmonic emission from multiple reencounters of the electron wave packet with the ion. Properly filtered and without any compensation of the chirp, regular subattosecond keV waveforms can be producedC. H.-G. acknowledges support by a Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007–2013), under REA grant agreement No. 328334. C. H.-G., J. A. P.-H., and L. P. acknowledge sup- port from Spanish MINECO (CSD 2007-00013 and FIS 2009-09522). A. B., M. M. M. and H. C. K. acknowledge sup- port from the United States Air Force Office of Scientific Research through the MURI ‘‘Mathematical Modeling and Experimental Validation of Ultrafast Nonlinear Light-Matter Coupling associated with Filamentation in Transparent Media’’ (Grant No. FA9550-10-1-0561) and by the National Science Foundation (Grant No. 1125844). A.J.-B. was supported by the National Science Foundation under Grant Nos. 1125844 and 1068706