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Nonlinear optical properties of organic semiconductors (OSCs) have been extensively investigated in the perturbative regime, while strong light induced high-order processes in solid-state OSCs are less studied. Here, below-threshold harmonic generation is examined, both experimentally and theoretically, in two solid-state thin film OSCs, that is, tetraphenylporphyrin and zinc tetraphenylporphyrin. Results show that the π-π* excitations of the porphyrin ring system generate the harmonic emission. The contribution of the Brunel harmonic to the 5th harmonic emission is uncovered, where the resonant 5-photon transition (S0 → S2 transition) is found to lead to an early onset of non-perturbative behavior. A similar resonance effect is expected in Brunel harmonic generation in other organic materials.

Strong-field photoemission from nanostructures and the associated temporally modulated currents play a key role in the development of ultrafast vacuum optoelectronics. Optical light fields could push their operation bandwidth into the petahertz domain. A critical aspect of their functionality in the context of applications is the impact of charge interaction effects. Here, we investigated the photoemission and photocurrents from nanometric tungsten needle tips exposed to carrier-envelope phase (CEP)-controlled few-cycle laser fields. We report a characteristic rapid increase in the intensity-rescaled cutoff energies of emitted electrons beyond a certain intensity value. By comparison with simulations, we identify this feature as the onset of charge-interaction dominated photoemission dynamics. Our results are anticipated to be relevant also for the strong-field photoemission from other nanostructures, including photoemission from plasmonic nanobowtie antennas used in CEP-detection and for PHz-scale devices.

Molecular adsorbate reactions on nanoparticles play a fundamental role in areas such as nano-photocatalysis, atmospheric, and astrochemistry. They can be induced, enhanced, and controlled by field localization and enhancement on the nanoparticle surface. In particular, the ability to perform highly controlled near-field-mediated reactions is key to deepening our understanding of surface photoactivity on nanosystems. Here, using reaction nanoscopy, we experimentally demonstrate all-optical nanoscopic control of surface reaction yields by tailoring the near fields on nanoparticles with waveform-controlled linear and bicircular two-color laser pulses, respectively. We observe site-selective proton emission from the dissociative ionization of adsorbate molecules on Si O 2 nanoparticles as a function of the polarization and relative phase of the two-color pulses. The angularly resolved close-to-uniform mapping between the surface reaction yields and the measured ion momentum enables the observation and spatial control of molecular reactions on the nanoparticle surface with nanoscopic resolution. The experimental results are modeled and reproduced qualitatively by classical trajectory Monte Carlo simulations. Our work paves the way toward reliable all-optical control of photocatalytic chemical reactions on nanoscale surfaces.

Ultrafast light-induced molecular reactions on aerosolized nanoparticles may elucidate early steps in the photoactivity of nanoparticles with potential impact in fields ranging from chemistry and m...

Abstract We investigate the strong-field ion emission from the surface of isolated silica nanoparticles aerosolized from an alcoholic solution, and demonstrate the applicability of the recently reported near-field imaging at 720 nm [Rupp et al., Nat. Comm., 10(1):4655, 2019] to longer wavelength (2 $$\mu $$ μ m) and polarizations with arbitrary ellipticity. Based on the experimental observations, we discuss the validity of a previously introduced semi-classical model, which is based on near-field driven charge generation by a Monte-Carlo approach and classical propagation. We furthermore clarify the role of the solvent in the surface composition of the nanoparticles in the interaction region. We find that upon injection of the nanoparticles into the vacuum, the alcoholic solvent evaporates on millisecond time scales, and that the generated ions originate predominantly from covalent bonds with the silica surface rather than from physisorbed solvent molecules. These findings have important implications for the development of future theoretical models of the strong-field ion emission from silica nanoparticles, and the application of near-field imaging and reaction dynamics of functional groups on isolated nanoparticles. Graphical abstract

Droplets provide unique opportunities for the investigation of laser-induced surface chemistry. Chemical reactions on the surface of charged droplets are ubiquitous in nature and can provide critical insight into more efficient processes for industrial chemical production. Here, we demonstrate the application of the reaction nanoscopy technique to strong-field ionized nanodroplets of propanediol (PDO). The technique’s sensitivity to the near-field around the droplet allows for the in-situ characterization of the average droplet size and charge. The use of ultrashort laser pulses enables control of the amount of surface charge by the laser intensity. Moreover, we demonstrate the surface chemical sensitivity of reaction nanoscopy by comparing droplets of the isomers 1,2-PDO and 1,3-PDO in their ion emission and fragmentation channels. Referencing the ion yields to gas-phase data, we find an enhanced production of methyl cations from droplets of the 1,2-PDO isomer. Density functional theory simulations support that this enhancement is due to the alignment of 1,2-PDO molecules on the surface. The results pave the way towards spatio-temporal observations of charge dynamics and surface reactions on droplets.

Ion microscopy is an established technique for laser focus diagnostics and the accurate, intensity-resolved measurement of laser ionization processes. In the present feasibility study, we discuss a new ion microscope design, which improves its resolution across a large range of magnifications and simplifies its operation. Instead of the common two einzel lens configuration, which is usually optimized for a fixed magnification, we propose a generic design consisting of an array of equally spaced ring electrodes, whose individually adjustable voltages are controlled by an evolutionary algorithm. In this way, we can realize aberration minimized magnifications between 25 and 100. Moreover, the algorithm can adjust the voltage settings under changing experimental conditions and facilitates autofocusing for user-defined magnification.

Regarded as the most important ion in interstellar chemistry, the trihydrogen cation, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\rm{H}}}_{{{3}}}^{+}$$\end{document}H3+, plays a vital role in the formation of water and many complex organic molecules believed to be responsible for life in our universe. Apart from traditional plasma discharges, recent laboratory studies have focused on forming the trihydrogen cation from large organic molecules during their interactions with intense radiation and charged particles. In contrast, we present results on forming \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\rm{H}}}_{{{3}}}^{+}$$\end{document}H3+ from bimolecular reactions that involve only an inorganic molecule, namely water, without the presence of any organic molecules to facilitate its formation. This generation of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\rm{H}}}_{{{3}}}^{+}$$\end{document}H3+ is enabled by “engineering” a suitable reaction environment comprising water-covered silica nanoparticles exposed to intense, femtosecond laser pulses. Similar, naturally-occurring, environments might exist in astrophysical settings where hydrated nanometer-sized dust particles are impacted by cosmic rays of charged particles or solar wind ions. Our results are a clear manifestation of how aerosolized nanoparticles in intense femtosecond laser fields can serve as a catalysts that enable exotic molecular entities to be produced via non-traditional routes., The H3+ ion plays a key role in interstellar chemistry and can be formed from organic compounds upon interaction with charged particles or radiation. Here the authors demonstrate that H3+ can also be formed from water adsorbed on silica nanoparticles exposed to intense laser pulses, conditions that mimic the impact of charged particles on dust in astrophysical settings.

Measuring the field of visible light with high spatial resolution has been challenging, as many established methods only detect a focus-averaged signal. Here, we introduce a near-field method for optical field sampling that overcomes that limitation by employing the localization of the enhanced near-field of a nanometric needle tip. A probe field perturbs the photoemission from the tip, which is induced by a pump pulse, generating a field-dependent current modulation that can easily be captured with our electronic detection scheme. The approach provides reliable characterization of near-petahertz fields. We show that not only the spiral wavefront of visible femtosecond light pulses carrying orbital angular momentum (OAM) can be resolved but also the field evolution with time in the focal plane. Additionally, our method is polarization sensitive, which makes it applicable to vectorial field reconstruction.

We present a novel approach to single-shot characterization of the spectral phase of broadband laser pulses. Our method is inexpensive, insensitive to alignment and combines the simplicity and robustness of the dispersion scan technique, that does not require spatio-temporal pulse overlap, with the advantages of single-shot pulse characterization methods such as single-shot frequency-resolved optical gating at a real-time reconstruction rate of several Hz.

Solid-state devices capable to react on the sub-cycle evolution of optical fields can pave the way towards petahertz electronics or provide new diagnostic devices for few-cycle lasers [1] . Such devices might rely on an effect of a transient metallization of wide-bandgap materials demonstrated in dielectrics [2] and semiconductors [3] . The observation of optical current control so far has been limited to millijoule-class laser systems which hinders the development towards the miniaturization and mass availability of the potential devices as these high-pulse-energy systems are bulky and have low repetition rate. Here, we report on the transient metallization and CEP-driven current control induced in a compact setup at 80 MHz repetition rate in dielectric SiO 2 , HfO 2 and semiconducting GaN with pJ-class pulses for the first time to our knowledge.

Future PHz electronic devices may be able to perform operations on few-femtosecond time-scales. Such devices are based on the ability to control currents induced by intense few-cycle laser pulses. Investigations of this control scheme have been based on complex, amplified laser systems, typically delivering mJ or sub-mJ-level laser pulses, limiting the achievable clock rate to the kHz regime. Here, we demonstrate transient metallization and lightwave-driven current control with 300-pJ laser pulses at 80 MHz repetition rate in dielectric media (HfO2 and fused silica), and the wide-bandgap semiconductor GaN. We determine the field strength dependence of optically induced currents in these media. Supported by a theoretical model, we show scaling behaviors that will be instrumental in the construction of PHz electronic devices.

We demonstrate a novel method to measure the temporal electric field evolution of ultrashort laser pulses. Our technique is based on the detection of transient currents in air plasma. These directional currents result from subcycle ionization of air with a short pump pulse and the steering of the released electrons with the pulse to be sampled. We assess the validity of our approach by comparing it with different state-of-the-art laser-pulse characterization techniques. Notably, our method works in ambient air and facilitates a direct measurement of the field waveform, which can be viewed in real time on an oscilloscope in a similar way as a radio frequency signal.

We investigate the interaction of Xe with isolated attosecond XUV pulses. Specifically, we calculate the ion yields and determine the pathways leading to the formation of ionic charged states up to Xe$^{5+}$. To do so, in our formulation we account for single-photon absorption, sequential multi-photon absorption, direct two-photon absorption, single and double Auger decays, and shake-off. We compare our results for the ion yields and for ion yield ratios with recent experimental results obtained for 93 eV and 115 eV attosecond XUV pulses. In particular, we investigate the role that a sequence of two single-photon ionization processes plays in the formation of Xe$^{4+}$. We find that each one of these two processes ionizes a core electron and thus leads to the formation of a double core-hole state. Remarkably, we find that the formation of Xe$^{5+}$ involves a direct two-photon absorption process and the absorption of a total of three photons., Comment: 8 pages, 4 figures

The ability to measure and control the carrier–envelope phase (CEP) of few-cycle laser pulses is of paramount importance for both frequency metrology and attosecond science. Here, we present a phase meter relying on CEP-dependent photocurrents induced by circularly polarized few-cycle pulses focused between electrodes in ambient air. The new device facilitates compact, single-shot CEP measurements under ambient conditions and promises CEP tagging at repetition rates orders of magnitude higher than most conventional CEP detection schemes, as well as straightforward implementation at longer wavelengths.

We present laser-waveform-dependent relativistic high harmonics from plasma surfaces, and use spectral interferometry to understand its generation process. The attotrain structure as well as the field-driven plasma surface motion during the process are revealed.

At intensities below-the-recollision threshold, we show that re-collision-induced excitation with one electron escaping fast after re-collision and the other electron escaping with a time delay via a Coulomb slingshot motion is one of the most important mechanisms of non-sequential double ionization, for strongly-driven He at 400 nm. Slingshot-NSDI is a general mechanism present for a wide range of low intensities and pulse durations. Anti-correlated two-electron escape is its striking hallmark. This mechanism offers an alternative explanation of anti-correlated two-electron escape obtained in previous studies., Comment: 6 pages, 3 figures

The interpretation of experimental data from novel mid-infrared few-cycle laser sources requires an understanding of ionization mechanisms and knowledge about related ion yields. Experimental studies have indicated sequential double ionization as the dominant process above 1014 W cm−2. These results contradict a recent prediction that in this spectral region, non-sequential processes dominate the double ionization of xenon up to intensities of about 1015 W cm−2. In either case, the ratio of doubly to singly charged xenon yield reported in previous studies has been limited to a few percent, indicating a regime well below the onset of saturation of the double ionization process. We present an experimental study of double ionization of xenon and krypton atoms exposed to intense near four-cycle pulses at 3.2 μm. Our experiments rely on the ion microscopy technique, which facilitates the detection of ions originating from a restricted region within the interaction volume, thereby reducing the impact of focal averaging. Our measurements suggest that at intensities of close to 1.2 × 1014 W cm−2, double ionization of xenon and krypton is already significantly saturated. In particular, we find a doubly to singly charged yield ratio of about 75 percent for xenon and 25 percent for krypton. We compare our results with the predictions of different models accounting for the effects of volume averaging and focal geometry. We find that in the deeply saturated regime of our experiment, the Perelomov–Popov–Terentyev theory significantly underestimates the observed double ionization yield.

The migration of hydrogen atoms resulting in the isomerization of hydrocarbons is an important process which can occur on ultrafast timescales. Here, we visualize the light-induced hydrogen migration of acetylene to vinylidene in an ionic state using two synchronized 4 fs intense laser pulses. The first pulse induces hydrogen migration, and the second is used for monitoring transient structural changes via Coulomb explosion imaging. Varying the time delay between the pulses reveals the migration dynamics with a time constant of 54 ± 4 fs as observed in the H+ + H+ + CC+ channel. Due to the high temporal resolution, vibrational wave-packet motions along the CC- and CH-bonds are observed. Even though a maximum in isomerization yield for kinetic energy releases above 16 eV is measured, we find no indication for a backwards isomerization — in contrast to previous measurements. Here, we propose an alternative explanation for the maximum in isomerization yield, namely the surpassing of the transition state to the vinylidene configuration within the excited dication state.

The interaction of ultra-intense laser pulses with matter opened the way to generate the shortest light pulses available nowadays in the attosecond regime. Ionized solid surfaces, also called plasma mirrors, are promising tools to enhance the potential of attosecond sources in terms of photon energy, photon number and duration especially at relativistic laser intensities. Although the production of isolated attosecond pulses and the understanding of the underlying interactions represent a fundamental step towards the realization of such sources, these are challenging and have not yet been demonstrated. Here, we present laser-waveform-dependent high-order harmonic radiation in the extreme ultraviolet spectral range supporting well-isolated attosecond pulses, and utilize spectral interferometry to understand its relativistic generation mechanism. This unique interpretation of the measured spectra provides access to unrevealed temporal and spatial properties such as spectral phase difference between attosecond pulses and field-driven plasma surface motion during the process., High-order harmonic generation is explored in gases, solids and plasmas with moderate to high intensity lasers. Here the authors show spectral interferometry of HHG from relativistic plasma and its potential as a source of intense isolated attosecond pulses.

The study of core electron dynamics through nonlinear spectroscopy requires intense isolated attosecond extreme ultraviolet or even X-ray pulses. A robust way to produce these pulses is high-harmonic generation (HHG) in a gas medium. However, the energy upscaling of the process depends on a very demanding next-generation laser technology that provides multi-terawatt (TW) laser pulses with few-optical-cycle duration and controlled electric field. Here, we revisit the HHG process driven by 16-TW sub-two-cycle laser pulses to reach high intensity in the 100-eV spectral region and beyond. We show that the combination of above barrier-suppression intensity with a long generation medium significantly enhances the isolation of attosecond pulses compared to lower intensities and/or shorter media and this way reduces the pulse duration as well as field-stability requirements on the laser driver. This novel regime facilitates the real-time observation of electron dynamics at the attosecond timescale in atoms, molecules, and solids.

We investigate the carrier-envelope phase and intensity dependence of the longitudinal momentum distribution of photoelectrons resulting from above-threshold ionization of argon by few-cycle laser pulses. The intensity of the pulses with a center wavelength of 750\,nm is varied in a range between $0.7 \times 10^{14}$ and $\unit[5.5 \times 10^{14}]{W/cm^2}$. Our measurements reveal a prominent maximum in the carrier-envelope phase-dependent asymmetry at photoelectron energies of 2\,$U_\mathrm{P}$ ($U_\mathrm{P}$ being the ponderomotive potential), that is persistent over the entire intensity range. Further local maxima are observed at 0.3 and 0.8\,$U_\mathrm{P}$. The experimental results are in good agreement with theoretical results obtained by solving the three-dimensional time-dependent Schr\"{o}dinger equation (3D TDSE). We show that for few-cycle pulses, the carrier-envelope phase-dependent asymmetry amplitude provides a reliable measure for the peak intensity on target. Moreover, the measured asymmetry amplitude exhibits an intensity-dependent interference structure at low photoelectron energy, which could be used to benchmark model potentials for complex atoms.

We demonstrate multiphoton ionization of inner-shell electrons in Xenon with 100-eV attosecond pulses. This was achieved with a novel XUV source based on high-harmonic generation in the gas phase driven with multi-TW few-cycle laser pulses.

Nonlinear light-matter interactions in the extreme ultraviolet (XUV) are a prerequisite to perform XUV-pump/XUV-probe spectroscopy of core electrons. Such interactions are now routinely investigate ...

Non-sequential double ionization (NSDI) is a process in which two electrons are ripped off an atom or molecule by a strong laser field in a correlated manner. Although NSDI has been the subject of numerous experimental and theoretical studies over the past three decades, the exact mechanisms responsible for the observed energy and momentum sharing between the electrons generated in the process, are not yet fully understood. The main reason lies in the fact that the theoretical description of the complex correlated many body dynamics that govern NSDI is exceedingly difficult. A particularly challenging task for theory is the modeling of NSDI dynamics over time scales exceeding one period of the laser field. As a result, most calculations are restricted to a single laser cycle. On the experimental side, in contrast, kinematically complete experiments on single-cycle NSDI has long been prohibitively challenging. Therefore, the comparison between theory and experiments has been limited, so far, to a very qualitative level. We review recent results obtained from first kinematically complete NSDI experiments in the single cycle regime. We illustrate the insight gained from these experiments, and discuss how the new experimental data may facilitate verification of theoretical models on a quantitative level.

Intense, multi-color laser fields permit the control of the ionization of atoms and the steering of electron dynamics. Here, we present the efficient collinear creation of the second and third harmonic of a 790 nm femtosecond laser followed by a versatile field synthesizer for the three color fields' composition. Using the device, we investigate the strong-field ionization of neon by fields composed of the fundamental, and the second or third harmonic. The three-color device offers sufficient flexibility for the coherent control of strong-field processes and for time-resolved pump-probe studies.

A three-dimensional semiclassical model is used to study double ionization of Ar when driven by a near-infrared and near-single-cycle laser pulse for intensities ranging from 0.85$\times$10$^{14}$ W/cm$^{2}$ to 5$\times$10$^{14}$ W/cm$^{2}$. Asymmetry parameters, distributions of the sum of the two electron momentum components along the direction of the polarization of the laser field and correlated momenta are computed as a function of intensity and of the carrier envelope phase. A very good agreement is found with recently obtained results in kinematically complete experiments employing near-single-cycle laser pulses. Moreover, the contribution of the direct and delayed pathways of double ionization is investigated for the above observables. Finally, an experimentally obtained anti-correlation momentum pattern at higher intensities is reproduced with the three-dimensional semiclassical model and shown to be due to a transition from strong to soft recollisions with increasing intensity., Comment: 11 pages, 10 figures

Femtosecond pulses with tailored laser fields are indispensable for studying the control of ultrafast chemical transformations [1] and for uncovering the realms of strong field physics in atoms, molecules, solids [2], and nanoscale targets [3]. For such studies pulses with well-controlled electric field, synthesized from hollow-core fiber compressors, have shown their application potential [4]. More commonly used femtosecond laser systems, however, deliver pulses that span less than an octave in spectral bandwidth, making the synthesis of tailored fields with such pulses highly desirable. Here, we present results from an efficient multi-harmonic generator and synthesizer (see Fig. 1(a)), converting femtosecond pulses delivered from an amplified Ti:sapphire system into a three-color beam with desired field waveform. Recently, the careful 3D-modelling of the propagation of tightly focused, broadband light fields from femtosecond oscillators in nonlinear and birefringent media allowed for efficient third harmonic generation with an efficiency of 30% [5]. Here, we followed the same approach, with a design goal for an efficient conversion of 25 fs pulses into three femtosecond beams centered around 260 nm, 390 nm and 780 nm. Together with the harmonic generator our device permits versatile multi-harmonic field synthesis, where first all three beams are separated, and then combined into a single three-color field. The beam separation and recombination allows for independent modification of each of the beams, where not only temporal delays (with interferometric stability) but also the polarization state of each of the beams can be altered.

Nonlinear photoionization with energetic FEL pulses has opened up new horizons for the investigation of inner-shell electron dynamics in atomic and molecular systems [1]. So far, however, the limited temporal resolution (typically a few tens of femtoseconds) achievable with FELs has hampered the time resolution of these dynamics.

In this work we explored strong field-induced decay of doubly excited transient Coulomb complex Ar ∗∗ → Ar 2+ + 2e. We measured the correlated two-electron emission as a function of carrier envelop phase (CEP) of 6 fs pulses in the non-sequential double ionization (NSDI) of argon. Classical model calculations suggest that the intermediate doubly excited Coulomb complex loses memory of its formation dynamics. We estimated the ionization time difference between the two electrons from NSDI of argon and it is 200 ± 100 as (N Camus et al, Phys. Rev. Lett. 108, 073003 (2012)).

We have investigated nuclear dynamics in bound and dissociating acetylene molecular ions in a time-resolved reaction microscopy experiment with a pair of few-cycle pulses. Different ionization processes were observed for acetylene. These time-dependent ionization processes are simulated using semi-classical on-the-fly dynamics revealing the underling mechanisms.

Selected features of nonsequential double ionization have been qualitatively reproduced by a multitude of different (quantum and classical) approaches. In general, however, the typical uncertainty of laser pulse parameters and the restricted number of observables measured in individual experiments leave room for adjusting theoretical results to match the experimental data. While this has been hampering the assessment of different theoretical approaches leading to conflicting interpretations, comprehensive experimental data that would allow such an ultimate and quantitative assessment have been missing so far. To remedy this situation we have performed a kinematically complete measurement of single-cycle multiple ionization of argon over a one order of magnitude range of intensity. The momenta of electrons and ions resulting from the ionization of the target gas are measured in coincidence, while each ionization event is tagged with the carrier-envelope phase and intensity of the 4-fs laser pulse driving the process. The acquired highly differential experimental data provide a benchmark for a rigorous test of the many competing theoretical models used to describe nonsequential double ionization.

Proton migration is a ubiquitous process in chemical reactions related to biology, combustion, and catalysis. Thus, the ability to control the movement of nuclei with tailored light, within a hydrocarbon molecule holds promise for far-reaching applications. Here, we demonstrate the steering of hydrogen migration in simple hydrocarbons, namely acetylene and allene, using waveform-controlled, few-cycle laser pulses. The rearrangement dynamics are monitored using coincident 3D momentum imaging spectroscopy, and described with a quantum-dynamical model. Our observations reveal that the underlying control mechanism is due to the manipulation of the phases in a vibrational wavepacket by the intense off-resonant laser field., Comment: 5 pages, 4 figures

We report on the generation of high-order harmonics in gas media using a sub-5 fs multi-ten-terawatt laser system. The application of this source towards XUV-pump/XUV-probe experiments with photon energies around 100 eV is discussed.

We have investigated nuclear dynamics in bound and dissociating acetylene molecular ions in a time-resolved reaction microscopy experiment with a pair of few-cycle pulses. Vibrating bound acetylene cations or dissociating dications are produced by the first pulse. The second pulse probes the nuclear dynamics by ionization to higher charge states and Coulomb explosion of the molecule. For the bound cations, we observed vibrations in acetylene (HCCH) and its isomer vinylidene (CCHH) along the CC-bond with a periodicity of around 26 fs. For dissociating dication molecules, a clear indication of enhanced ionization is found to occur along the CH- and CC-bonds after 10 fs to 40 fs. The time-dependent ionization processes are simulated using semi-classical on-the-fly dynamics revealing the underling mechanisms.

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 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. 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 report techniques developed to utilize three-dimensional momentum information as feedback in adaptive femtosecond control of molecular dynamics. Velocity map imaging is used to obtain the three-dimensional momentum map of the dissociating ions following interaction with a shaped intense ultrafast laser pulse. In order to recover robust feedback information, however, the two-dimensional momentum projection from the detector must be inverted to reconstruct the full three-dimensional momentum of the photofragments. These methods are typically slow or require manual inputs and are therefore accomplished offline after the images have been obtained. Using an algorithm based upon an “onion-peeling” (also known as “back projection”) method, we are able to invert 1040 × 1054 pixel images in under 1 s. This rapid inversion allows the full photofragment momentum to be used as feedback in a closed-loop adaptive control scheme, in which a genetic algorithm tailors an ultrafast laser pulse to optimize a specific outcome. Examples of three-dimensional velocity map image based control applied to strong-field dissociation of CO and O2 are presented.

Subfemtosecond control of the breaking and making of chemical bonds in polyatomic molecules is poised to open new pathways for the laser-driven synthesis of chemical products. The break-up of the C-H bond in hydrocarbons is an ubiquitous process during laser-induced dissociation. While the yield of the deprotonation of hydrocarbons has been successfully manipulated in recent studies, full control of the reaction would also require a directional control (that is, which C-H bond is broken). Here, we demonstrate steering of deprotonation from symmetric acetylene molecules on subfemtosecond timescales before the break-up of the molecular dication. On the basis of quantum mechanical calculations, the experimental results are interpreted in terms of a novel subfemtosecond control mechanism involving non-resonant excitation and superposition of vibrational degrees of freedom. This mechanism permits control over the directionality of chemical reactions via vibrational excitation on timescales defined by the subcycle evolution of the laser waveform.

We study triple-ionization-induced, spatially asymmetric dissociation of ${\mathrm{N}}_{2}$ using angular streaking in an elliptically polarized laser pulse in conjunction with few-cycle pump-probe experiments. The kinetic-energy-release dependent directional asymmetry in the ion sum-momentum distribution reflects the internuclear distance dependence of the fragmentation mechanism. Our results show that for 5--35-fs near-infrared laser pulses with intensities reaching 10${}^{15}$ W/cm${}^{2}$, charge exchange between nuclei plays a minor role in the triple ionization of N${}_{2}$. We demonstrate that angular streaking provides a powerful tool for probing multielectron effects in strong-field dissociative ionization of small molecules.

Intense isolated attosecond pulses with >100 eV photon energy are produced via high-order-harmonic generation using 80mJ sub-5 fs laser pulses. The generation of these attosecond pulses and their application to non-linear XUV-XUV pum-probe experiments is discussed.

The tunnelling of an electron through a suppressed atomic potential followed by its motion in the continuum, is the fundamental mechanism underlying strong-field laser-atom/molecule interactions. Due to its quantum nature, the interaction is governed by the phase of the released electron wave-packets. Thus, detailed mapping of the electron wave-packet interferences provides essential insight into the physics underlying the interaction. A process which is providing access to the intricacies of the interaction is the generation of high order harmonics of the laser frequency. The phase-amplitude distribution of the emitted extreme-ultraviolet (EUV), carries the complete information about the harmonic generation process and vice-versa. Thus, the visualization of the EUV-spatial-amplitude-distribution, as it results from interfering electron wave-packet contributions, is of crucial importance. Restrictions to this accomplishment are due to the spatially integrating measurement approaches applied so far, that average out the phase effects in the generation process. In this work, we demonstrate a method which overcomes this obstacle. An EUV-spatial-amplitude-distribution-image is induced from the imprint on the measured spatial distribution of ions, produced through EUV-photon ionization of atoms. Interference extremes in the image carry phase information about the interfering electron wave-packets. The present approach provides detailed insight on the strong-field laser-atom interaction mechanism, while establishing the era of phase selective interaction studies. Furthermore, it paves the way for substantial enhancement of the spectral and temporal precision of measurements by in-situ controlling the phase distribution of the emitted radiation and/or spatially selecting the EUV-radiation-atom interaction products., Comment: 13 pages main text with 4 figures, 7 pages supporting material with 3 figures

We present a comparative study of nonsequential double ionization (NSDI) of N${}_{2}$ and Ar exposed to near-single-cycle laser pulses. The NSDI process is investigated using carrier-envelope-phase-tagged electron-ion coincidence spectroscopy. The measured NSDI spectra of N${}_{2}$ and Ar exhibit a striking resemblance. In particular, the correlated two-electron momentum distribution arising from NSDI of N${}_{2}$ also displays a cross-shape very similar to that reported for Ar [Bergues et al., Nat. Commun. 3, 813 (2012)]. We interpret our results in terms of recollision-excitation with subcycle depletion and discuss how this mechanism accounts for the observed similarities and differences in the ionization behavior of the two species.

Shaping ultrafast laser pulses using adaptive feedback can manipulate dynamics in molecular systems, but extracting information from the optimized pulse remains difficult. Experimental time constraints often limit feedback to a single observable, complicating efforts to decipher the underlying mechanisms and parameterize the search process. Here we show, using two strong-field examples, that by rapidly inverting velocity map images of ions to recover the three-dimensional photofragment momentum distribution and incorporating that feedback into the control loop, the specificity of the control objective is markedly increased. First, the complex angular distribution of fragment ions from the nω+C2D4→C2D3++D interaction is manipulated. Second, isomerization of acetylene (nω+C2H2→C2H2(2+)→CH2++C+) is controlled via a barrier-suppression mechanism, a result that is validated by model calculations. Collectively, these experiments comprise a significant advance towards the fundamental goal of actively guiding population to a specified quantum state of a molecule.

We explore the dissociative ionization of CO with carrier-envelope-phase (CEP) tagged few-cycle laser pulses. We observe the CEP dependence of the directional emission of C${}^{p+}$ and O${}^{q+}$ fragments from transient CO${}^{(p+q)+}$ ions, where $p+q\ensuremath{\le}3$ and $q\ensuremath{\le}1$. At ${I}_{0}=3.5\ifmmode\times\else\texttimes\fi{}{10}^{14}$ W/cm${}^{2}$, a ${180}^{\ensuremath{\circ}}$ phase difference between the C${}^{+}$ and O${}^{+}$ fragments from the ($p=1$, $q=0$) and ($p=0$, $q=1$) channels reflects the orientation dependence of the CO ionization. At ${I}_{0}=1.2\ifmmode\times\else\texttimes\fi{}{10}^{15}$ W/cm${}^{2}$, we find a $\ensuremath{\sim}$35${}^{\ensuremath{\circ}}$ phase shift between the C${}^{2+}$ fragments from the ($p=2$, $q=0$) and ($p=2,\phantom{\rule{4pt}{0ex}}q=1$) channels, in contrast to the ${180}^{\ensuremath{\circ}}$ shift previously observed between the C${}^{2+}$ fragment channels at ${I}_{0}=6\ifmmode\times\else\texttimes\fi{}{10}^{14}$ W/cm${}^{2}$ [Phys. Rev. Lett. 106, 073004 (2011)].