Α 10-gigawatt attosecond source for non-linear XUV optics and XUV-pump-XUV-probe studies

The quantum mechanical motion of electrons and nuclei in systems spatially confined to the molecular dimensions occurs on the sub-femtosecond to the femtosecond timescales respectively. Consequently, the study of ultrafast electronic and, in specific cases, nuclear dynamics requires the availability of light pulses with attosecond (asec) duration and of sufficient intensity to induce two-photon processes, essential for probing the intrinsic system dynamics. The majority of atoms, molecules and solids absorb in the extreme-ultraviolet (XUV) spectral region, in which the synthesis of the required attosecond pulses is feasible. Therefore, the XUV spectral region optimally serves the study of such ultrafast phenomena. Here, we present a detailed review of the first 10-GW class XUV attosecond source based on laser driven high harmonic generation in rare gases. The pulse energy of this source largely exceeds other laser driven attosecond sources and is comparable to the pulse energy of femtosecond Free-Electron-Laser (FEL) XUV sources. The measured pulse duration in the attosecond pulse train is 650 ± 80 asec. The uniqueness of the combined high intensity and short pulse duration of the source is evidenced in non-linear XUV-optics experiments. It further advances the implementation of XUV-pump-XUV-probe experiments and enables the investigation of strong field effects in the XUV spectral region.

Tracking autoionizing-wave-packet dynamics at the 1-fs temporal scale

We present time-resolved studies and Fourier transform spectroscopy of inner-shell excited states undergoing Auger decay and doubly excited autoionizing states, utilizing coherent extreme-ultraviolet (XUV) radiation continua. Series of states spanning a range of ∼4  eV are excited simultaneously. An XUV probe pulse tracks the oscillatory and decaying evolution of the formed wave packet. The Fourier transform of the measured trace reproduces the spectrum of the series. The present work paves the way for ultrabroadband XUV spectroscopy and studies of ultrafast dynamics in all states of matter.

Coherent continuum extreme ultraviolet radiation in the sub-100-nJ range generated by a high-power many-cycle laser field

High-energy coherent continuum radiation is generated by the interaction of rare gases with a high-power many-cycle laser field, utilizing the interferometric polarization gating technique. A narrow temporal gate is formed in the laser pulse within which the extreme ultraviolet (XUV) emission is restricted. An analytical expression for the gate function is derived. A super-XUV continuum down to 15 nm, broad enough to support synthesis of single pulses of 260 as duration and a few tens of nanojoule energy, has been measured. These results directly challenge the perspectives of single-attosecond pulse XUV-pump-XUV-probe applications.

Spectral phase distribution retrieval through coherent control of harmonic generation

The temporal intensity distribution of the third harmonic of a Ti:sapphire laser generated in Xe gas is fully reconstructed from its spectral phase and amplitude distributions. The spectral phases are retrieved by cross correlating the fundamental laser frequency field with that of the third harmonic, in a three laser versus one harmonic photon coupling scheme. The third harmonic spectral amplitude distribution is extracted from its field autocorrelation. The measured pulse duration is found to be in agreement with that expected from lowest order perturbation theory both for unstretched and chirped pulses.

Effect of diffraction on early-arriving photons during femtosecond laser transillumination of highly scattering media of biological significance

Early-arriving photons of 100-fs laser pulses transmitted through highly scattering media have been detected by a streak camera. Because of their partial spatial coherence, they are affected by diffraction from small hidden discontinuities. The experimental data of the patterns are analyzed with Fresnel diffraction theory and then corrected accordingly. Submillimeter hidden objects were scanned and imaged. Diffraction correction resulted in a significantly improved contrast in the hidden object's image.

Subpicosecond laser spectroscopy: strong field effects on molecular photophysics and dynamics

The possible role of strong field effects is discussed in the context of picosecond relaxation and reaction in polyatomic systems. The picosecond pulse diagnostics from an argon-ion synchronously pumped dye laser are then outlined in terms of cavity detuning and laser pumping parameters. A profile of auto-correlation data on ensemble-averaged pulses and streak camera data on single pulses is presented, from which the domain of stable, single pulse operation can be deduced. Finally data are presented on the photoionization and energy relaxation of pyrene in alcohols and micelles, and for the reorientational dynamics of liquid chlorobenzene, which illustrate different consequences of strong field phenomena in dense media.