Few-cycle-driven XUV laser harmonics: generation and focusing

We have investigated the use of sub-10-fs near-infrared laser pulses to generate high-order harmonic radiation efficiently in the wavelength region between 30 to 10 nm. The ultrashort rise time of the driver pulses allows harmonics to be produced at low ionization levels and hence to grow coherently over propagation lengths becoming comparable to the XUV absorption lengths in the noble gas medium. As a result, absorption-limited harmonic generation has been extended to the 10-nm range for the first time. Harmonic conversion efficiencies of (3-4) x 10(-8) in the range of 10-13 nm in neon and some two orders of magnitude higher at around 30 nm in argon have been obtained in simple gas tube targets under these conditions. Preliminary focusing tests with 13-nm harmonic radiation have been carried out with a specially designed zoneplate and a spherical Mo/Si multilayer mirror and have resulted in spot sizes of about 2 microns. Our experiments aim at paving the way to nonlinear optics in the soft-X-ray regime.