[en] Complete text of publication follows. Since the first experimental demonstration of the generation of isolated attosecond pulses, the attosecond technology has become an important branch of ultrafast science. So far, the reported applications of isolated attosecond pulses have been limited by the low photon flux of the available sources. We demonstrate a technique for the generation of isolated attosecond pulses with energy up to 2.1 nJ. The key elements are: the use of few-optical-cycle driving pulses with stable carrier-envelope phase (CEP), linear polarization and peak intensity beyond the saturation intensity of the gas use for HHG; and the optimization of the interaction geometry in terms of gas pressure, position and thickness of the gas cell. We used 5-fs driving pulses with stable CEP to generate XUV radiation by HHG in a 2.5-mm-long cell filled with xenon at static pressure (2.5-3 torr) at a peak intensity I = (2.3±0.3) x 10¹⁵ W/cm². The XUV spectra display an evolution from a continuous behavior to a modulated one by changing the CEP value. The energy of the XUV pulses in the case of continuous spectra was 2.1 nJ, after a 100-nm-thick aluminium filter used to block the fundamental radiation and the low order harmonics. We have also used argon, krypton and neon as generating media: also in such cases clear transition between modulated and continuous XUV spectra were observed upon changing the CEP of the driving pulses. We have measured the temporal characteristics of the attosecond pulses by using the FROGCRAB method. Figure 1 a shows a portion of the FROGCRAB trace; in the reconstructed temporal intensity profile of the XUV pulses, the pulse duration was 155±5 as (the transform limit was ∼ 120 as). The physical mechanism at the basis of this method is related to the ionization dynamics in the generating medium. We used a nonadiabatic three-dimensional numerical model. In agreement with experimental results, the calculated XUV spectra display an evolution from a continuous behavior to a modulated one by changing the CEP value. We have then calculated the propagation of the XUV beam from the gas cell to the target position by taking into account the effects of the toroidal mirror and of a 200-μm-diameter pinhole used for spatial filtering of the XUV radiation. With a proper choice of the CEP, isolated attosecond pulses are generated, with a constant pulse duration across the transverse profile of the beam, thus demonstrating the excellent spatial characteristics of the generated attosecond pulses.

[en] Complete test of publication follows. In the last few years the field of attosecond science has shown impressive and rapid progress, mainly due to the introduction of novel experimental methods for the characterization of extreme ultraviolet (XUV) pulses and attosecond electron wave packets. This development has been also triggered by significant improvements in the control of the electric field of the driving infrared pulses. Particularly interesting for the applications is the generation of isolated attosecond XUV pulses using few-cycle driving pulses. In this case significant progresses have been achieved thanks to the stabilization of the carrier-envelope phase (CEP) of amplified light pulses. In this work we demonstrate that the polarization gating (PG) method with few-cycle phase-stabilized driving pulses allows one to generate few-cycle isolated attosecond pulses tunable on a very broad spectral region. The PG method is based on temporal modulation of the ellipticity of a light pulse, which confines the XUV emission in the temporal gate where the polarization is close to linear. The time-dependent polarization of phase-stabilized sub-6-fs pulses, generated by the hollow fiber technique, has been obtained using two birefringent plates. It is possible to create a linear polarization gate, whose position is imposed by the intensity profile of the pulse whilst the emission time is linked to the CEP of the electric field. The pulses have been analyzed by using a flat-field spectrometer. Continuous XUV spectra, corresponding to the production of isolated attosecond pulses, have been generated for particular CEP values. Upon changing the rotation of the first plate it was possible to tune the XUV emission in a broad spectra range. We have then achieved a complete temporal characterization of the generated isolated attosecond pulses using frequency-resolved optical gating for complete reconstruction of attosecond bursts (FROG CRAB). The measured parabolic phase indicates the presence of a predominant second order dispersion (positive chirp), which is intrinsic to the XUV generation process. As recently demonstrated in the case of trains of attosecond pulses, the positive chirp of the radiation produced by high-order harmonic generation can be compensated for by the negative group delay dispersion of thin aluminum foils. Upon increasing the thickness of an aluminum plate we have obtained XUV pulses with duration shorter than 300 as (at 37 eV), thus corresponding to few cycles of the electric field.

[en] Complete test of publication follows. Control of the carrier-envelope phase (CEP) of light pulses is one of the most advanced frontiers of ultrafast optics. The generation of optical waveforms with reproducible electric field profile is important in the case of few-optical-cycle pulses for which a CEP variation produces a strong change in the waveform. CEP-dependent phenomena are observable in high-intensity interactions, such as above-threshold ionization and high-order harmonic generation (HHG), moreover CEP stabilization is a fundamental prerequisite for the production of isolated attosecond pulses through HHG. In this work we propose a novel scheme for the generation of high energy ultrabroadband near-IR pulses with passive CEP stabilization, starting from an amplified Ti:sapphire laser system. A phase stable seed is generated by difference frequency generation (DFG) between the frequency components of a hollow-fiber-broadened supercontinuum. This configuration intrinsically provides a high CEP stability due to the absence of mechanical delays between the frequency components undergoing the DFG process. The seed is then boosted in energy through a multistage near-IR optical parametric amplifier (OPA), exploiting the broad gain bandwidths available around degeneracy. We generated ∼ 300 μJ phase-stable pulses at 1.6 μm and, after compensation of the negative dispersion by propagation through bulk media, we have obtained nearly transform-limited sub-25 fs pulse duration, which corresponds to less than 4 optical cycles at the considered carrier frequency. To verify that CEP-stabilization is preserved after the OPA stages and that amplified parametric superfluorescence is negligible, we used an f-to-2f interferometer. The appearance of a stable, high-contrast fringe pattern upon averaging is a clear proof of the shot-to-shot stability of the CEP due to the self-phase-stabilization mechanism. Once compressed, these pulses could be ideal drivers for HHG experiments; indeed taking into account that the ponderomotive energy of the recolliding electron scales as the square of the laser carrier wavelength, the use of near-IR pulses will enable the generation of higher order harmonics. Even more interesting is the possibility of using this system as a front-end for high energy OPA pumped by a 100 mJ, 10 hZ Ti:sapphire laser system, enabling multi-mJ-level self-phase-stabilized pulses.

[en] It is shown that, in the framework of classical electrodynamics and in some peculiar cases, an exhaustive description of rotational evolution of molecules driven by intense few-optical-cycle laser pulses should consider the electric field of the pulse rather than its intensity envelope. We show that, at moderate pulse intensities, nonlinear effects driven by the molecular hyperpolarizability play a significant role. These findings are illustrated by numerical simulations concerning the classical motion of several molecules exposed to few-cycle light pulses

[en] The polarization gating method in combination with few-optical-cycle driving pulses with controlled waveform is a powerful technique for the generation of isolated few-cycle attosecond pulses. We show that such a technique allows one to generate attosecond pulses tunable in a broad spectral region, corresponding to more than 26 eV. Complete temporal characterization of the attosecond pulses has been obtained by using the frequency resolved optical gating for complete reconstruction of attosecond bursts technique. The physical processes which determine the temporal confinement of the extreme ultraviolet radiation and the effects of various experimental parameters on the electric field of the attosecond pulses have been investigated using numerical simulations based on the nonadiabatic saddle-point method

[en] We demonstrate the generation of broadband extreme ultraviolet (XUV) continua by high order harmonic generation in neon using carrier-envelope phase (CEP) stabilized, few-cycle pulses with a time-dependent polarization. The XUV spectra can support isolated attosecond pulses with a duration of about 50 as, opening the possibility of achieving time resolution close to the atomic unit of time. A theoretical model that calculates the spectral and temporal characteristics of the XUV radiation emitted by a single atom is used to interpret the CEP dependence of the harmonic structures.

[en] In this work we report on the first experimental demonstration of selection of the long electron quantum paths in the process of high-order harmonic generation by phase-stabilized multiple-cycle light pulses. A complete experimental investigation of the role of intensity and carrier-envelope phase of the driving pulses on the spectral characteristics of the long quantum paths is performed. Simulations based on the nonadiabatic saddle-point method and on a complete nonadiabatic three-dimensional model reproduce the main features of the experimental results. The use of phase-stabilized driving pulses allows one to control, on an attosecond temporal scale, the spectral and temporal characteristics associated with the electron quantum paths involved in the harmonic generation process

[en] High-order harmonic generation process in the few- and multiple-optical-cycle regime is theoretically investigated, using the saddle-point method generalized to account for nonadiabatic effects. The influence of the carrier-envelope phase of the driving pulses on the various electron quantum paths is analyzed. We demonstrate that the short and long quantum paths are influenced in different ways by the carrier-envelope phase. In particular, we show that clear phase effects are visible on the long quantum paths even in the multiple-optical-cycle regime, while the short quantum paths are significantly influenced by the carrier-envelope phase only in the few-optical-cycle regime

[en] The development of novel tools for the generation of atto second light pulses is continuously triggering the introduction of new spectroscopic and measurement methods, which will offer the opportunity to investigate unexplored research areas with unprecedented time resolution. In this work we will report on recent advances in atto second science, with particular emphasis on the generation of isolated atto second pulses produced by laser-driven high-order harmonic generation in gases. We will consider several approaches to atto second metrology and we will present some applications of atto second pulses. Future perspectives in atto second technology will then be discussed.

[en] In this review we report on recent advances in laser technology, which have contributed to the fast development of attosecond science. In particular we will concentrate on two experimental methods for the generation of high-peak-power, few-optical-cycle laser pulses with controlled electric field, which are crucial for the generation of isolated attosecond pulses. The first method is the hollow-fiber compression technique, introduced in 1996 and now routinely used in several laboratories. So far, isolated attosecond pulses have been generated by using few-cycle pulses produced by such compression technique, in combination with active stabilization of the carrier-envelope phase. More recently, few-cycle pulses tunable in the infrared region have been generated by optical parametric amplification with passive stabilization of the carrier-envelope phase. Such parametric sources represent excellent drivers for the generation of harmonic radiation with an extended cutoff, and offer the possibility to extend attosecond science towards the soft-X rays region. Finally, we will briefly discuss the basic elements of attosecond metrology