P. Agostini

Cross-correlation measurements of femtosecond extreme-ultraviolet high-order harmonics

He atoms have been ionized by a combination of two pulses the fundamental (800 nm, 150 fs) from a Ti:sapphire laser, and its 21st harmonic (38 nm). The resulting above-threshold ionization spectra comprise peaks that scale as the cross-correlation function, which can be mapped out by variation of the time delay between the two pulses. Using this technique, we have carried out the first measurement, to our knowledge, of the duration of high-order harmonic pulses with subpicosecond resolution. The result indicates a duration of 40 fs. This duration is much shorter than the duration of the fundamental pulse, in agreement with existing model calculations.

Cross-correlation measurement of femtosecond noncollinear high-order harmonics

The durations of extreme-ultraviolet femtosecond pulses (harmonics 21 and 27 of a Ti:sapphire femtosecond laser) have been measured by a cross-correlation technique. Photoelectron energy spectra resulting from ionization of helium atoms by one harmonic photon in the presence of the infrared laser field show sidebands that provide a cross-correlation signal from which the durations of the harmonic pulses can be obtained. The measurements yield a mean value of 130 fs for both harmonics. The accuracy of the method depends on the actual intensity dependence of the sidebands.

Generation of ultrashort XUV pulses by harmonic generation from solid surfaces

We describe the generation of optical harmonics of high order during the interaction of intense femtosecond laser pulses with the surface of insulating and metallic solid targets. With a p- polarized fundamental beam from a titanium sapphire, chirped pulse amplification laser system ((lambda) equals 800 nm) we have observed even and odd harmonics up to the eighteenth order (lambda approximately equals 45 nm) for fundamental laser intensities of about 1017 to 1018 W/cm2.

Harmonic Generation from a Sharp Plasma-Vacuum Boundary

Generation of high order optical harmonics from solid targets by intense laser pulses is interpreted as phase modulation from an oscillating reflecting surface. Using femtosecond laser pulses of 1017 W/cm2 coherent beams of XUV pulses up to harmonic order 18 corresponding to a wavelength of 45 nm were observed.

Femtosecond pulses as probe of the atomic structure under strong irradiation

Huge dynamic Stark shifts of atomic energy levels in xenon induced by high-intensity laser light have been mesured by a new method based upon multiphoton ionization by a tunable femtosecond laser source. Stark-induced multiphoton resonances appear as identifiable structures in the photoelectron energy spectra1. The change in the structures positions, when the photon energy is tuned, is directly related to the energy change of the atomic levels as a function of intensity. Shifts of the order of the electron-volt have thus been measured in xenon and found to vary linearly with intensity.