In this work, we use attosecond time-resolved techniques to investigate photoionization dynamics on its natural timescale, employing both high harmonic generation and seeded free-electron lasers to generate extreme ultraviolet attosecond pulse trains for our studies. With the former approach, we examine the role of nuclear motion in molecular photoionization dynamics, while with the latter we introduce a novel attosecond timing tool for single-shot characterization of the relative phase between the XUV and the infrared field.

Two XUV-photon double ionization of Ne, induced by an intense few-pulse attosecond train with a 4 fs envelope duration is investigated experimentally and theoretically. The experiment is performed at ELI-ALPS utilizing the recently constructed 10 Hz gas phase high-order harmonic generation SYLOS GHHG-COMPACT beamline. A total pulse energy up to 1 μJ generated in Argon in conjunction with high reflectivity optics in the XUV region, allowed the observation of the doubly charged state of Ne induced by 40 eV central XUV photon energies. The interaction of the intense attosecond pulse train with Ne is also theoretically studied via a second-order time dependent perturbation theory equations-of-motion. The results of this work, combined with the feasibility of conducting XUV-pump-XUV-probe experiments, constitute a powerful tool for many potential applications. Those include attosecond pulse metrology as well as time resolved investigations of the dynamics underlying direct and sequential double ionization and their electron correlation effects.

In coherent control schemes, pathways connecting an initial and a final state can be independently controlled by manipulating the complex amplitudes of their transition matrix elements. For paths characterized by the absorption of multiple photons, these quantities depend on the magnitude and phase between the intermediate steps, and are expected to be strongly affected by the presence of resonances. We investigate the coherent control of the photoemission process in neon using a phase-controlled two-color extreme ultraviolet pulse with frequency in proximity of an excited energy state. Using helium as a reference, we show that the presence of such a resonance in neon modifies the amplitude and phase of the asymmetric emission of photoelectrons. Theoretical simulations based on perturbation theory are in fair agreement with the experimental observations.

Two-XUV-photon double ionization of Ne, induced by an intense few-pulse attosecond train with a -4 fs envelope duration is investigated experimentally and theoretically. The experiment is performed at ELI-ALPS (Extreme Light Infrastructure Attosecond Light Pulse Source) utilizing the recently constructed 10 Hz gas phase high-order harmonic generation SYLOS GHHG-COMPACT beamline. A total pulse energy up to -1 mu J generated in argon in conjunction with high-reflectivity optics in the XUV region allowed the observation of the doubly charged state of Ne induced by 40 eV central XUV-photon energies. The interaction of the intense attosecond pulse train with Ne is also theoretically studied via second-order time-dependent perturbation theory equations of motion. The results of this work, combined with the feasibility of conducting XUV-pump-XUV-probe experiments, constitute a powerful tool for many potential applications. Those include attosecond pulse metrology as well as time-resolved investigations of the dynamics underlying direct and sequential double ionization and their electron correlation effects.

Above-threshold ionization spectra from cesium are measured as a function of the carrier-envelope phase (CEP) using laser pulses centered at 3.1  μm wavelength. The directional asymmetry in the energy spectra of backscattered electrons oscillates three times, rather than once, as the CEP is changed from 0 to 2π. Using the improved strong-field approximation, we show that the unusual behavior arises from the interference of few quantum orbits. We discuss the conditions for observing the high-order CEP dependence, and draw an analogy with time-domain holography with electron wave packets.

Recent developments in extreme ultraviolet (XUV) and x-ray radiation sources have pushed pulse energies and durations to unprecedented levels that opened up the era of non-linear XUV and x-ray optics. In this quest, laser driven high order harmonic generation sources providing attosecond resolution in the XUV spectral region enabled XUV-pump-XUV-probe experiments, while Free Electron Laser research infrastructures offer unique x-ray brilliances for highly non-linear interactions and since recently, they too entered the sub-fs temporal regime. This topical review discusses the conceptual intricacies of non-linear XUV and x-ray processes, addresses experimental particularities and highlights recent applications of such processes with emphasis to laser driven XUV-attosecond source related research.

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.

Electrodynamical processes induced in complex systems like semiconductors by strong electromagnetic fields have traditionally been described using semiclassical approaches. Although these approaches allowed the investigation of ultrafast dynamics in solids culminating in multipetahertz electronics, they do not provide any access to the quantum-optical nature of the interaction, as they treat the driving field classically and unaffected by the interaction. Here, using a full quantum-optical approach, we demonstrate that the subcycle electronic response in a strongly driven semiconductor crystal is imprinted in the quantum state of the driving field resulting in nonclassical light states carrying the information of the interaction. This vital step towards strong-field ultrafast quantum electrodynamics unravels information inaccessible by conventional approaches and leads to the development of a new class of nonclassical light sources.

Attosecond Pulse Trains (APT) generated by high-harmonic generation (HHG) of high-intensity near-infrared (IR) laser pulses have proven valuable for studying the electronic dynamics of atomic and molecular species. However, the high intensities required for high-photon-energy, high-flux HHG usually limit the class of adequate laser systems to repetition rates below 10~kHz. Here, APT's generated from the 100 kHz, 160 W, 40 fs laser system (HR1) of the Extreme Light Infrastructure Attosecond Light Pulse Source (ELI-ALPS) are reconstructed using the Reconstruction of Attosecond Beating By Interference of two-photon Transitions (RABBIT) technique. These experiments constitute the first attosecond time-resolved photoelectron spectroscopy measurements performed at 100 kHz repetition rate and the first attosecond experiments performed at ELI-ALPS. These RABBIT measurements were taken with an additional IR field temporally locked to the extreme-ultraviolet APT, resulting in an atypical omega beating. We show that the phase of the 2-omega beating recorded under these conditions is strictly identical to that observed in standard RABBIT measurements within second-order perturbation theory. This work highlights an experimental simplification for future experiments based on attosecond interferometry (or RABBIT), which is particularly useful when lasers with high average powers are used.

We report the observation of multiple ionization of argon through multi-XUV-photon absorption induced by an unprecedentedly powerful laser driven high-order harmonic generation source. Comparing the measured intensity dependence of the yield of the different argon charge states with numerical calculations we can infer the different channels---direct and sequential---underlying the interaction. While such studies were feasible so far only with free electron laser (FEL) sources, this paper connects highly nonlinear XUV processes with the ultrashort time scales inherent to the harmonic pulses and highlights the advanced perspectives of emerging large scale laser research infrastructures.

We demonstrate the complete reconstruction of the electric field of visible-infrared pulses with energy as low as a few tens of nanojoules. The technique allows for the reconstruction of the instantaneous electric field vector direction and magnitude, thus giving access to the characterisation of pulses with an arbitrary time-dependent polarisation state. The technique combines extreme ultraviolet interferometry with the generation of isolated attosecond pulses.

1光子レーザー場誘起オージェ崩壊の観測とコヒーレント制御 東大院工,東北大多元研,オーストラリア国立大,ミラノ工科大,ELI-ALPS,フ ライブルク大学,European XFEL,マックス・プランク核物理,シンクロトロン・ エレットラ,ENEA C.R. Frascati,スウィンバーン工科大 ○石川顕一,D. Iablonskyi,上田潔,A. Kheifets,G. Sansone,P. Carpeggiani,M. Reduzzi,A. Comby,T. Csizmadia,S Kühn,E. Ovcharenko, T. Mazza,M. Meyer,A. Fischer,C. Callegari,O. Plekan,P. Finetti,E. Allaria,L. Giannessi,B. Diviacco,D. Gauthier,E. Roussel,E. Ferrari,K. C. Prince

Nature Photon. 11, 383–389 (2017); published online 29 May 2017; corrected after print 29 June 2017. In the version of this Article originally published, the following sentences were missing from the Acknowledgements: “Financial support by the Alexander von Humboldt Foundation (Project Tirinto) and the Italian Ministry of Research (project FIRB no.

Summary form only given. We demonstrate a tool for quantitative measurements in the linear and non-linear extreme ultraviolet (XUV) spectral region measuring spatially resolved atomic ionization products at the focus of an XUV beam [1, 2]. The ionizing radiation is a comb of the 11th-15th harmonics of a Ti:Sapphire femtosecond laser beam produced in a Xenon gas jet. The spatial ion distribution at the focus of the harmonics is recorded using an ion microscope detector [2, 3]. Spatially resolved single- and two-photon ionization products of Argon and Helium are observed. From such ion distributions single- and two-photon generalized cross sections have be extracted by a self-calibrating method. This is the first observation of spatially resolved two-XUV-photon ionization at the focus of the XUV radiation which constitutes an important step towards future single-shot temporal characterization of attosecond (asec) pulses [4].

We demonstrate a tool for quantitative measurements in the extreme ultraviolet (EUV) spectral region measuring spatially resolved atomic ionization products at the focus of an EUV beam. The ionizing radiation is a comb of the 11 th –15 th harmonics of a Ti:Sapphire femtosecond laser beam produced in a Xenon gas jet. The spatial ion distribution at the focus of the harmonics is recorded using an ion microscope. Spatially resolved single- and two-photon ionization products of Argon and Helium are observed. From such ion distributions single- and two-photon generalized cross sections can be extracted by a self-calibrating method. The observation of spatially resolved two-EUV-photon ionization constitutes an initial step towards future single-shot temporal characterization of attosecond pulses.

We combine interferometric detection of single gold nanoparticles, single molecule microscopy, and fluorescence lifetime measurement to study the modification of the fluorescence decay rate of an emitter close to a nanoparticle. In our experiment, gold particles with a diameter of 15 nm were attached to single dye molecules via double-stranded DNA of different lengths. Nanoparticle-induced lifetime modification (NPILM) has promise in serving as a nanoscopic ruler for the distance range well beyond 10 nm, which is the upper limit of fluorescence resonant energy transfer (FRET). Furthermore, the simultaneous detection of single nanoparticles and fluorescent molecules presented in this work provides new opportunities for single molecule biophysical studies.

We examine the optical near-field interaction between different types of scanning tips and single oriented fluorescent molecules. We demonstrate the influence of a tip on the excitation intensity as well as on the integrated fluorescence signal, the excited state lifetime, and the angular emission of single molecules. By using a standard model describing the radiation of an oscillating dipole close to a nanosphere or a flat interface, we interpret our observations and describe some central criteria for obtaining fluorescence enhancement or quenching.

Based on a scanning near-field optical microscope (SNOM) we present measurements on the optical interaction of a single molecule and a single gold nanoparticle. The phenomena of field enhancement, emission quenching and fluorescence enhancement are studied with outstanding control over all key parameters. We have attached a single gold nanoparticle to the extremity of a pulled glass fiber tip and approached it to within a few nanometers to a sample of highly dispersed molecules. Dielectric nanostructures can give rise to a localization of the electromagnetic field to small volumes. The resulting excitation enhancement for a fluorescent molecule depends its dipole orientation which in a crystalline matrix is defined by the guest-host system and can be chosen normal or at an angle to the substrate surface. An outstanding photostability of terrylene-molecules embedded in an ultrathin para-terphenyl film [1] allows us to study the details of the changing emission properties when the distance to the gold nanoparticle reaches down to 5-10 nm. If the incident wavelength is chosen to excite plasmon resonances in the nanoparticle then the local field may be amplified by a factor of 30 at the particle surface, thereby boosting the excitation rate of the molecule close by. The emission process of fluorescence light, however, can branch into additional radiative and non-radiative channels which are opening up in the presence of the nanoparticle. As a result, an increase of the observed fluorescence by up to 20 is observed, accompanied by a drastic reduction of the excited state lifetime from 24 to 1 ns. As expected, the effect is localized to a diffractionunlimited volume of 65x65x15 nm3 at the gold particle [2]. A thorough analysis shows deviations from a free-space model consisting of a sphere and a dipole in air. Also, changes in the spectral and spatial emission characteristics and the photostability are evidenced.

Influence of a single metallic nanoparticle on the emission properties of a single fluorophore is a investigated. A shear-force based scanning near-field optical microscope (SNOM) is used, to position a single gold nanoparticle in front of an ultrathin crystalline film, containing oriented molecules. By measuring the emission intensity and fluorescence lifetime, the modification of the optical properties of a single molecule due to its interaction with the gold nanoparticle is studied.

We have applied a new method to localize single dye molecules with nanometer resolution in three dimensions by means of the Stark effect. With this method we were able to resolve two molecules which were so close to each other that they underwent a strong dipole-dipole interaction. A quantitative spectral study of this system allowed us to determine the coupling parameters. In the second part of this paper we discuss our research on metal nanoparticles. We have developed a method to mount single nanoparticles at the very end of a fibre tip. We have used such probes as well defined scattering centers for apertureless scanning near-field optical microscopy. In this article we also discuss our efforts to examine the interaction of a single metal nanoparticle and a single molecule in a controlled manner.