Search

Among non-linear optical processes, Four Wave Mixing (FWM) can be exploited for the generation of tunable light pulses in several spectral regions. Usually this process is driven in bulk materials [1], which are prone to optical damage at high pump laser intensities. Here we report on the implementation of a FWM experiment based on a two-colour ultrafast field interaction inside a gas-filled Hollow Core Fiber (HCF). A FWM-based tunable source of new spectral components is thus obtained. The high-damage threshold and the long interaction distance provided by the HCF-based configuration are suitable for high energy applications.

We investigated the giant resonance in Xenon by high-order harmonic generation spectroscopy driven by a two-color field. The addition of a non-perturbative second harmonic component parallel to the driving field breaks the symmetry between neighboring sub-cycles resulting in the appearance of spectral caustics at two distinct cut-off energies. By controlling the phase delay between the two color components it is possible to tailor the harmonic emission in order to amplify and isolate the spectral feature of interest. In this paper we demonstrate how this control scheme can be used to investigate the role of electron correlations that give birth to the giant resonance in Xenon. The collective excitations of the giant dipole resonance in Xenon combined with the spectral manipulation associated with the two color driving field allow to see features that are normally not accessible and to obtain a quantitative good agreement between the experimental results and the theoretical predictions.

We studied the dependence of harmonic signal on the ellipticity of the driving field in xenon. We were able to observe trajectories beyond the classical cutoff and interpret the results in the strong-field approximation picture.

High harmonic generation (HHG) spectroscopy has opened up a new frontier in ultrafast science, where electronic dynamics can be measured on an attosecond time scale. The strong laser field that triggers the high harmonic response also opens multiple quantum pathways for multielectron dynamics in molecules, resulting in a complex process of multielectron rearrangement during ionization. Using combined experimental and theoretical approaches, we show how multi-dimensional HHG spectroscopy can be used to detect and follow electronic dynamics of core rearrangement on sub-laser cycle time scales. We detect the signatures of laser-driven hole dynamics upon ionization and reconstruct the relative phases and amplitudes for relevant ionization channels in a CO2 molecule on a sub-cycle time scale. Reconstruction of channel-resolved complex ionization amplitudes on attosecond time scales has been a long-standing goal of high harmonic spectroscopy. Our study brings us one step closer to fulfilling this initial promise and developing robust schemes for sub-femtosecond imaging of multielectron rearrangement in complex molecular systems.

We probed the electronic structure of xenon by two-color high-order harmonic spectroscopy. We were able to identify the contribution of electron correlations in the spectral region corresponding to the giant resonance.

The development of ultrafast and intense laser sources has paved the way to the study of laser–matter interaction in the strong-field regime, where the amplitude of the laser field becomes comparable to the Coulomb field seen by electrons in the proximity of their parent ion. This realm embraces numerous phenomena, such as high-order harmonic generation, tunnel photoionization, laser-induced electron diffraction and holography, ponderomotive electron streaking, and so forth. For a long time, the Ti:Sapphire laser has been the sole workhorse of strong-field scientists; the recent development of intense and tunable ultrafast sources operating in the mid-infrared added an extra dimension to strong-field investigations, with valuable benefits in the understanding of several phenomena and in the development of high-photon-energy coherent sources. In this work we review the recent achievements in strong-field science which were allowed by the exploitation of mid-infrared ultrafast lasers.

Raman excitation of molecular gases can be accomplished and detected by optical filamentation of ultrashort laser pulses in the investigated sample. Exploiting this technique, we could clearly resolve the full Raman response of H2 as well as the major vibrational modes in several molecules (N2, O2, CO2, N2O, CH4, C2H6). The experimental results show that the temporal requirements for efficient time-resolved Raman spectroscopy, namely pump and probe pulses shorter than the investigated dynamics, can be relaxed by the occurrence of self-compression during pulse filamentation and by the exploitation of spectrally-resolved detection.

As femtosecond laser pulses increase in intensity and decrease in duration, interaction between the focused laser radiation and a substance is followed by a sharp increase in the intensity of fast-electron, ion, and electromagnetic noise generation. In turn, the noise signals of X-ray detectors grow in amplitude and the signal-to-noise ratio in recording X-ray spectra of multiply charged ions approaches unity. A significant excess of the noise level over the useful signal is observed in plasmas generated by laser pulses with a power density of >= 10(17) W/cm(2). The most powerful effect of the above factors is exerted on X-ray spectra recorded by such electromagnetic equipment as CCD-based detectors, photoelectron amplifiers, etc. A new '' mean-median '' algorithm is described, with which it is possible to considerably increase the signal-to-noise ratio of CCD detectors used to measure X-ray spectra of femtosecond laser-produced plasma.

Ultrafast transient reflectivity measurements were performed in epitaxial 4x4 silicene grown on Ag(111). Comparison with bulk silicon and silver response highlighted the occurrence of peculiar photo-physical mechanisms, suggesting a metallic-like behavior in silicene. © 2014 OSA.

We report a combined experimental and theoretical study of the optical response of epitaxial silicene on silver. The silicene/Ag(111) ultraviolet-visible absorption spectra, which turn out to be strongly nonadditive, are analyzed in the framework of ab initio calculations. Electronic transitions involving silver states are found to provide huge contributions to the optical absorption of silicene, compatible with a strong Si-Ag hybridization. The results are independent of the specific silicene configuration and are also worked out for thin amorphous silicon. This points to a dimensionality-driven peculiar dielectric response of the two-dimensional-silicon/silver interface, which is confirmed by means of transient-reflectance spectroscopy. The latter shows a metalliclike relaxation time, hence demonstrating the effects of the strong hybridization arising in silicene/Ag(111) systems on charge carriers dynamical properties.

The impressive technological advances in the generation of ultrashort XUV and soft X-ray light pulses have triggered the quest for novel techniques, able to characterize these pulses in the temporal domain. In this chapter we will present and discuss the most important methods for temporal pulse characterization, with particular emphasis on attosecond pulses generated by high-order harmonics and on femtosecond soft X-ray pulses emitted by free electron lasers.

We review the main research directions ongoing in the development of extreme ultraviolet sources based on high-harmonic generation for the synthesization and application of trains and isolated attosecond pulses to time-resolved spectroscopy. A few experimental and theoretical works will be discussed in connection to well-established attosecond techniques. In this context, we present the unique possibilities offered for time-resolved investigations on the attosecond timescale by the new Extreme Light Infrastructure Attosecond Light Pulse Source, which is currently under construction. (C) 2015 Published by Elsevier B.V.

Extending the dimensionality of high harmonic generation (HHG) measurements has the potential to reconstruct structural features in molecules and resolve multielectron dynamics on attosecond time scales. We demonstrate that structural and dynamical effects in molecules can be unambiguously distinguished using multidimensional HHG techniques.

X-ray spectra from Teflon targets irradiated by laser pulses with a duration of 60 fs to 1 ps have been investigated experimentally. It is shown that, when the contrast of the laser pulse is sufficiently low, the effect of self-focusing of the main laser pulse in the plasma produced by the prepulse can significantly enhance the generation efficiency of fast particles. In this case, ions with energies as high as similar to 1 MeV are observed at relatively low laser intensities, q(las) approximate to (4-6) x 10(16) W/cm(2).

Two-centre interference in the recombination step of molecular high harmonic generation (HHG) has been probed in CO2 and O-2. We report the order dependence of characteristic enhancements or suppressions of high harmonic production in aligned samples of both molecules. In CO2, a robust destructive interference was seen consistent with the known separation of the oxygen atoms that are active in HHG. In O-2, a harmonic enhancement was found indicating constructive interference. A good agreement was found with a simple two-centre interference model that includes the angular distribution function of the sample. The effective momentum of the electron wave was determined from the spectral position of these interferences. Ellipticity-dependent studies in CO2 clearly show how the destructive interference can be 'switched off' by increasing the degree of ellipticity and thus shifting the effective resonance condition.

Attosecond electron wavepackets are produced when an intense laser field ionizes an atom or a molecule(1). When the laser field drives the wavepackets back to the parent ion, they interfere with the bound wavefunction, producing coherent subfemtosecond extreme-ultraviolet light bursts. When only a single return is possible(2,3), an isolated attosecond pulse is generated. Here we demonstrate that by modulating the polarization of a carrier-envelope phase-stabilized short laser pulse(4), we can finely control the electron-wavepacket dynamics. We use high-order harmonic generation to probe these dynamics. Under optimized conditions, we observe the signature of a single return of the electron wavepacket over a large range of energies. This temporally confines the extreme-ultraviolet emission to an isolated attosecond pulse with a broad and tunable bandwidth. Our approach is very general, and extends the bandwidth of attosecond isolated pulses in such a way that pulses of a few attoseconds seem achievable. Similar temporal resolution could also be achieved by directly using the broadband electron wavepacket. This opens up a new regime for time-resolved tomography of atomic or molecular wavefunctions(5,6) and ultrafast dynamics.

A high-order harmonic generation process using carrier-envelope phase stabilized few-optical-cycle light pulses is investigated. A time-frequency analysis of generated spectra is used to retrieve direct information about the temporal structure of the generated extreme ultraviolet radiation.

Supercontinuum generation in gas-filled hollow fibers is investigated using numerical simulation of the nonlinear propagation of light pulses in hollow waveguides. The use of the cascading hollow-fiber configuration allows one to significantly enhance the achievable spectral broadening, particularly in the high energy regime. General design criteria for a single- and a double-fiber configuration are presented, which allow the generation of high-energy supercontinua.

The dependence of the high-order harmonic cutoff spectrum from the absolute phase of the laser pulse is here experimentally investigated. Noticeable shifts are observed. Their interpretation is proposed with a model based on the absolute-phase dependence of the trajectories of the photoelectrons that contribute to a given harmonic order.

Ultrabroadband chirped mirrors with a bandwidth of 270 THz have been manufactured using the BASIC design approach. These mirrors were used to compress the supercontinuum of cascaded hollow fibers down to 4.6 fs. The pulse duration was measured with spectral phase interferometry for direct electric-field reconstruction (SPIDER).

We investigated the sub-picosecond relaxation dynamics of two alkoxy-substitued poly-p-phenylenevinylenes that differ only for the structure of the alkoxy side chains. Photoinduced absorption, bleaching, and stimulated emission were observed in distinct spectral regions. Our results show that the temporal dynamics of stimulated emission strongly depends on excitation density. We ascribe such behavior to on-chain ultrafast nonlinear processes, which are almost independent on the substituents grafted onto the polymeric backbone.

Few-optical-cycle light pulses, produced by the hollow-fiber compression technique, are used to generate high-order harmonics with excellent spatial characteristics at wavelengths as short as 10 nm. We show that the spatial features of the harmonic radiation are strongly affected by the spatial profile of the excitation beam. The role of the fundamental beam shape has been carefully investigated by means of numerical simulations. The use of a fundamental beam with truncated Bessel radial intensity profile, produced upon propagation along a hollow fiber, allows us to obtain low-divergence, high-brightness harmonic radiation.

Journal of physics / B 50(13), 132002 - (2017). doi:10.1088/1361-6455/aa6ee8, This review presents the technological infrastructure that will be available at the Extreme Light Infrastructure Attosecond Light Pulse Source (ELI-ALPS) international facility. ELI-ALPS will offer to the international scientific community ultrashort pulses in the femtosecond and attosecond domain for time-resolved investigations with unprecedented levels of high quality characteristics. The laser sources and the attosecond beamlines available at the facility will make attosecond technology accessible for scientists lacking access to these novel tools. Time-resolved investigation of systems of increasing complexity is envisaged using the end stations that will be provided at the facility., Published by IOP Publ., Bristol

We studied the excitation dynamics and the photoluminescence spectra in polymeric matrix implanted with metallic nanoparticles. A supersonic beam of gold nanoparticles (sizes 2-7 nm) was generated by using a Pulsed Microplasma Cluster Source (PMCS). The nanoparticles were implanted in a thin polyfluorene film (thickness around 100 nm) spin coated on a PMMA (poly methyl methacrilate) substrate. We performed photoluminescence and pump-probe measurements using a Ti:Sapphire amplified laser system, the excitation pump pulse was centered at 400 nm with a 150 fs time duration. The experimental results show a suppression of the amplified spontaneous emission from the polyfluorene (PFO) film due to the presence of the implanted nanoparticles. The transient transmission spectra revealed the presence of a photoinduced absorption band due to the nanoparticles which overlaps with the stimulated emission band from the PFO, hindering the possibility of having amplified spontaneous emission. These results demonstrate the possibility of a local nanotailoring of the polymeric photophysics. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High-harmonics generation spectroscopy is a promising tool for resolving electron dynamics and structure in atomic and molecular systems. This scheme, commonly described by the strong field approximation, requires a deep insight into the basic mechanism that leads to the harmonics generation. Recently, we have demonstrated the ability to resolve the first stage of the process -- field induced tunnel ionization -- by adding a weak perturbation to the strong fundamental field. Here we generalize this approach and show that the assumptions behind the strong field approximation are valid over a wide range of tunnel ionization conditions. Performing a systematic study -- modifying the fundamental wavelength, intensity and atomic system -- we observed a good agreement with quantum path analysis over a range of Keldysh parameters. The generality of this scheme opens new perspectives in high harmonics spectroscopy, holding the potential of probing large, complex molecular systems., 11 pages, 5 figures

High order harmonic generation (HHG) has shown its impact on several applications in Attosecond Science and Atomic and Molecular Physics. Owing to the complexity of the experimental setup for the generation and characterization of harmonics, as well as to the large computational costs of numerical modelling, HHG is generally performed and modelled in collinear geometry. Recently, several experiments have been performed exploiting non-collinear geometry, such as HHG in a grating of excited molecules created by crossing beams. In such studies, harmonics were observed at propagation directions different from those of the driving pulses; moreover the scattered harmonics were angularly dispersed. In this work we report on a new regime of HHG driven by multiple beams, where the harmonics are generated by three synchronized, intense laser pulses organized in a non-planar geometry. Although the configuration we explore is well within the strong-field regime, the scattered harmonics we observe are not angularly dispersed.

A portable and compact photon spectrometer to be used for photon in-photon out experiments, in particular x-ray emission spectroscopy, is presented. The instrument operates in the 25-800 eV energy range to cover the full emissions of the FEL1 and FEL2 stages of FERMI. The optical design consists of two interchangeable spherical varied-lined-spaced gratings and a CCD detector. Different input sections can be accommodated, with/without an entrance slit and with/without an additional relay mirror, that allow to mount the spectrometer in different end-stations and at variable distances from the target area both at synchrotron and at free-electron-laser beamlines. The characterization on the Gas Phase beamline at ELETTRA Synchrotron (Italy) is presented. (C) 2014 AIP Publishing LLC.

We studied the two-color HHG emission from xenon in the giant resonance spectral region. We found a substantial departure from the behavior expected for the single-active-electron picture which could be ascribed to electron correlation effects.

High-order harmonic generation is a powerful and sensitive tool for probing atomic and molecular structures, combining in the same measurement an unprecedented attosecond temporal resolution with a high spatial resolution of the order of an angstrom. Imaging of the outermost molecular orbital by high-order harmonic generation has been limited for a long time to very simple molecules, like nitrogen. Recently we demonstrated a technique that overcame several of the issues that have prevented the extension of molecular orbital tomography to more complex species, showing that molecular imaging can be applied to a triatomic molecule like carbon dioxide. Here we report on the application of such a technique to nitrous oxide (N2O) and acetylene (C2H2). This result represents a first step towards the imaging of fragile compounds, a category which includes most of the fundamental biological molecules.

Recent advances in ultrafast laser technology have led to the generation of light pulses comprising few optical cycles employing different compression techniques. In particular, two techniques have been developed, which allow addressing the issues of high peak power or frequency tunability in a wide spectral range, namely: the hollow-fiber compression technique and the optical parametric amplification. The paper analyzes the general scheme of pulse compression and reports on the most interesting results obtained using the above-mentioned techniques. The combination of spectral broadening in a gas-filled hollow fiber with ultrabroad-band dispersion control, has led to the generation of pulses with duration of /spl sim/5 fs with peak powers up to 0.11 TW. Using optical parametric amplifiers with different configurations sub-15-fs laser pulses have been generated tunable in the visible and in the near-infrared.

We report on a sub-10-fs optical parametric amplifier in the visible, using noncollinear phase matching in β-barium borate, with a pulse compressor made exclusively of chirped dielectric mirrors. We employ mirrors of novel design, providing negative group delay dispersion for bandwidths of over 200 THz, up to the blue-green spectral region. The resulting setup, compact, easy to operate and reliable, is an ideal tool for ultrafast spectroscopy with extremely high time resolution. We use these pulses to observe in the time domain coherent oscillations in organic molecules with period as short as 16 fs.

We here report on an investigation of the differential transmittance of Ge quantum dots, with average radius between 4 and 16 nm, in the subpicosecond time domain. Pump pulses of 390 nm have been used, and the response has been probed in the spectral range 450 to 750 nm, including in this way the E1 and E1 +Δ1 spectral structures, which have a partially excitonic nature. It is shown that the observed features can be essentially explained in terms of (i) occupancy of the conduction states by the photoexcited carriers and (ii) their correlation–exchange interactions. A comparison with the ultrafast response of Ge bulk is also carried out. Finally, the screening of the excitonic interaction and the presence of quantum confinement effect are discussed.

The nature and relaxation channels of photoexcitations in \ensuremath{\beta},${\mathrm{\ensuremath{\beta}}}^{\ensuremath{'}}$dihexylsexithiophene disordered films are investigated by femtosecond transmission difference spectroscopy. We found that the initial excited state is localized and migrates incoherently within the amorphous structure. During this process a fraction of the initial population relax in the ps time domain by dissociation into correlated charge pairs (associated to absorption bands at 500 nm and 800 nm). The charge pairs recombine geminately with a time constant of 30 ps. The surviving excitations behave as isolated molecules giving rise to long-lived triplet states.

Powerful techniques for spectral broadening and ultrabroad-band dispersion control, which allow compression of high-energy femtosecond pulses to a duration of a few optical cycles, are analyzed. Spectral broadening in a gas-filled hollow fiber and compression by chirped mirrors with high-energy 20-fs input pulses are presented. Using 1-mJ seed pulses we have demonstrated the generation of 0.5-mJ 5-fs pulses at 0.8-/spl mu/m and 1-kHz repetition rate. General design criteria to scale the compression technique toward the terawatt level are presented.

We present the design and the characteristics of a portable and compact photon spectrometer to be installed in free-electron-laser (FEL) beamlines for photon in - photon out experiments, in particular single-shot X-ray emission spectroscopy. The instrument is operated in the 30 - 800 eV energy range with two channels and is designed to be initially used in the LDM (Low-Density Matter) and TIMEX (TIme-resolved studies of Matter under EXtreme conditions) beamlines of the FERMI@ELETTRA FEL, covering the whole spectral range of FERMI-1 and FERMI-2 emissions. The design of the instrument is tailored to achieve high spectral resolution in the whole interval of operation, high acceptance angle and high dynamic range. These characteristics are achieved in a compact environment to give a portable instrument that may be easily installed in different experimental chambers. The design consists of an entrance slit, a grazing-incidence diffraction grating and a detector. The number of elements within the optical path is kept to a single component, to minimize the losses due to reflectivity. The grating is spherical with variable line spacing along its surface, to provide an almost flat spectral focal plane that is perpendicular to the direction of the diffracted light. The detector is a back-illuminated CCD. The spectral resolution is better than 0.2% in the 30 - 800 eV region and the acceptance angles are 10 x 17 mrad in the 30-250 eV and 5 x 17 mrad in the at 250-800 eV.

Probing the ionization times and the recollision times by adding a weak perturbation in simple atomic systems was discussed. The results deviated from the simple classical model were found to be in agreement with the quantum path analysis. The multiple channel ionization and the differences between two ionization channels were probed in aligned CO2 molecules.

This chapter reviews several experimental methods to measure the temporal duration of sub-femtosecond pulses, either isolated or in the form of trains. Two classes of techniques, developed over the last few years, are presented in this chapter. The first is based on cross-correlation measurements between the extreme ultraviolet (XUV) pulse and the infrared (IR) driving pulse, the other is based on nonlinear effects induced by the attosecond XUV pulses.

Summary form only given. We investigated experimentally and theoretically the photo-physical properties of a class of linear sp-carbon chains, the α,ω-dinaphthylpolyyne Ar-C2n-Ar, where Ar is the naphthyl group and n is the number of triple bonds. The dynamics of the excited states has been investigated by ultrafast transient absorption spectroscopy and DFT//TDDFT calculations.We found that the role of molecular conformers, in which end-capped naphthalene rings are planar or perpendicular to the polyyne plane, is fundamental for understanding the steady state properties, like the UV-Vis absorption spectra and vibronic transitions, and the ultrafast transient absorption features. Owing to DFT//TD-DFT analysis, we were able to quantify the contributions of each conformer in the UV-Vis absorption spectra (not shown in the figure). Accordingly ultrafast transient absorption measurements revealed the excited states spectral features and dynamics. Figure 1(a) shows the 2D map of the pump-probe signal. The excitation pulse was 150-fs long and centered at 390 nm while the probe pulse was supercontinuum white light from 450 to 750 nm. Figure 1(b) shows two cuts of the 2D map for two different wavelengths. Over the whole spectral bandwidth it is possible to observe a broad photo-induced absorption, which appears instantaneously upon impulsive photo-excitation. A photo-induced absorption signal forms at 600 nm with a time constant of 30 ps and a narrow-band spectral shape.In light of DFT//TD-DFT, these features can be assigned to different transitions. The photo-induced absorption with a fast formation time can be assigned to optical transitions from the excited singlet state to higher molecular singlet levels. The narrow feature centered at 600 nm can be related to an intersystem crossing event bringing to the formation of a triplet state followed by transition between triplet states. In this picture the conformers play a prominent role: we observed a selective pathway of triplet formation, determined by the peculiar conformational structure of dinaphthylpolyyne in the ground state. Understanding the role of end-capped groups opens the way for other possible chemical functionalizations of sp-carbon chain systems in order to govern their chemical stability and allow optical control of their photo-excited properties.

We present the design, realization and characterization of active deformable gratings for extreme-ultraviolet monochromators for ultrashort pulses. The core device consists of a bimorph deformable mirror on the top of which a diffraction grating with laminar profile is realized by UV lithography. The curvature radius of the grating substrate can be varied changing the voltage applied to an underlying piezo-actuator. The advantage of this technology is to provide gratings with high optical quality, robust, compatible with any coating deposition and realized with only vacuum-compatible materials. We present the characterization of a time-delay compensated monochromator realized with these devices, showing that the active grating can optimize the beam focusing through its rotation and deformation. Two equal active gratings have been mounted in a compensated configuration to realize a grazing-incidence double-grating monochromator for the spectral selection of ultrashort pulses and the simultaneous compensation of the pulse front-tilt given by the diffraction. The wavelength scanning is performed by the first grating through rotation. The radiation is focused on the intermediate plane, where a slit carries out the spectral selection. Finally, the second grating compensates for the pulse front-tilt given by the first one. The spectral focusing of both gratings is maintained at the different wavelengths through the variation of the radii of curvature. The instrument has been tested with a Ti:Sa laser operated at 800 nm. We have been able to demonstrate that the double-grating configuration with active gratings compensates for the pulse front-tilt, that is reduced from 1 ps at the intermediate plane to 100 fs at the output. The final value is limited by the group delay dispersion of the monochromator within the 10-nm bandwidth of the laser. A configuration for the selection on XUV ultrashort pulses has been theoretically studied and the expected performances presented. Active gratings may be considered as a cheaper and more flexible alternative to standard gratings for the realization of extreme-ultraviolet monochromators for ultrafast pulses, such as free-electron lasers and high-order laser harmonics.

Active gratings have been used to realize a grazing-incidence double-grating monochromator for the spectral selection of ultrashort pulses while preserving the temporal duration by compensating for the pulse-front tilt. The active grating consists of a bimorph deformable mirror on the top of which a diffraction grating with laminar profile is realized by UV lithography. The time-delay compensated configuration has been tested with ultrashort pulses at 800 nm. The feasibility of this configuration for the extreme-ultraviolet spectral region has been demonstrated by ray tracing studies. (C) 2013 Optical Society of America

The generation of harmonics from the interaction of an ultra-intense laser pulse and a gas jet is strongly influenced by the intensity and shape of the laser beam. In this work we present a sensor-less optimization procedure of the harmonics yield based on the use of an adaptive mirror. Be cause of the limited repetition rate of the laser source, we used a resistive actuator-deformable mirror which allows a rapid convergence to the optimal laser beam correction with out a wave-front measurement. Our approach demonstrated that this kind of deformable mirror was able to enhance the harmonic yield by a factor of two in about 40 acquisitions.

Imaging of the outermost molecular orbital by high-order harmonic generation has been limited so far to very simple molecules like nitrogen, since several difficulties had to be overcome in order to extend it to more complex species. In this work we show that molecular imaging can be applied to a triatomic molecule like carbon dioxide, hence paving the way to the exploitation of such technique to the imaging of polyatomic species.

We demonstrate experimentally the full tunability of a coherent femtosecond source in the whole ultraviolet spectral region. The experiment relies on the technique of high-order harmonic generation driven by a near-infrared parametric laser source in krypton gas. By tuning the drive wavelength in the range between 1100 to 1900 nm, we generated intense harmonics from near to extreme ultraviolet. A number of photons per shot of the order of 10^7 has been measured for the first harmonic orders. Many novel scientific prospects are expected to benefit from the use of such a table-top tunable source.

High-order harmonic generation is as a powerful tool for the study of molecular properties. Up to now this investigation tool has been confined to simple molecules, with a relatively high ionization potential, since ionization saturation hindered its exploitation to fragile molecules. In this work we show that such limitation can be overcome by using mid-IR ultrashort driving pulses; as prototypical molecules we considered hydrocarbons. Clear signatures of the highest occupied molecular orbital were found in the harmonic spectra generated in unsaturated aligned hydrocarbons like acetylene, ethylene, allene and 1,3-butadiene. Our findings demonstrate that high-order harmonic generation spectroscopy can be extended to complex molecular species.

The production of isolated attosecond pulses is nowadays based on the generation of high harmonics driven by ultrafast and intense laser sources in gases. The generation of such pulses requires the confinement of the harmonic emission within a single event, thus obtaining a continuous XUV spectrum. Here we report on the generation of continuous XUV spectra produced by an ultrabroadband optical parametric amplifier operating in the mid-IR and generating multi-cycle pulses, thus avoiding the use of few-cycle Ti:sapphire lasers and, thanks to the favorable cutoff scaling law for mid-IR drivers, paving the way to the production of isolated attosecond pulses well above the 100-eV photon energy.

In this work we present a procedure for the reconstruction of the Highest Occupied Molecular Orbital in carbon dioxide by exploiting the generation of high order harmonics in a sample of impulsively aligned molecules. The procedure allows to extract the single molecule contribution to the harmonic spectrum in both amplitude and phase as a function of angular orientation and photon energy and to exploit this information in the reconstruction of the orbital. The role of multielectron contributions and of the Coulomb field of the ion on the reconstruction are discussed.

We present the imaging of carbon dioxide by high-order harmonic tomography, overcoming some critical obstacles that have prevented this result up to now by exploiting a few-cycle, high-energy, mid-IR parametric source as driving field.