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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.

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.

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.

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.

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 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.

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.

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)].

Since its discovery in 1982 by L'Huillier and coworkers [1], non-sequential double ionization (NSDI) of atoms in strong laser fields has been the subject of numerous experimental and theoretical studies. Yet, the mechanisms that govern this process are not fully understood. Advances in ultra-fast laser technology have permitted the generation of light pulses with durations close to a single optical cycle [2], which can be used to study quantum dynamics with attosecond time resolution. Using a reaction microscope (REMI) in combination with the recently developed single-shot carrier-envelope phase (CEP) measurement technique [3], we investigate the sub-cycle dynamics of the NSDI process in argon and neon atoms exposed to near single-cycle laser pulses.

Strong-field closed-loop control schemes using shaped ultrafast laser pulses have been used to selectively fragment a variety of molecules in recent years. The resulting pulses are often complex and resist an easy mechanistic interpretation. We report on the use of velocity map imaging to study the dissociative ionization of CO molecules by optimally-shaped ultrafast laser pulses. Using this technique, a mechanism is identified for the optimized CO${}^{+}$ $\ensuremath{\rightarrow}$ C $+$ ${\mathrm{O}}^{+}$ dissociation, and some of the observed control over the CO${}^{+}$ dissociation branching ratio is ascribed to an angular discrimination effect. Furthermore, we demonstrate that the acquisition of two-dimensional velocity map images is rapid enough to incorporate directly into the adaptive control loop.

Single-shot carrier-envelope-phase (CEP) tagging is combined with a reaction mircoscope (REMI) to investigate CEP-dependent processes in atoms. Excellent experimental stability and data acquisition longevity are achieved. Using this approach, we study the CEP effects for nonsequential double ionization of argon in 4-fs laser fields at 750 nm and an intensity of 1.6x10{sup 14} W/cm{sup 2}. The Ar{sup 2+} ionization yield shows a pronounced CEP dependence which compares well with recent theoretical predictions employing quantitative rescattering theory [S. Micheau et al., Phys. Rev. A 79, 013417 (2009)]. Furthermore, we find strong CEP influences on the Ar{sup 2+} momentum spectra along the laser polarization axis.

We study the photodetachment of ${\mathrm{H}}^{\ensuremath{-}}$, ${\mathrm{F}}^{\ensuremath{-}}$, and ${\mathrm{Br}}^{\ensuremath{-}}$ in a short laser pulse of 800 nm wavelength and $6\ifmmode\times\else\texttimes\fi{}{10}^{14}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$ peak intensity. Photoelectron spectra, recorded with the use of an imaging technique, reveal a substantial contribution from the sequential process of double detachment of halogen negative ions. The saturation effect is shown to play a crucial role in this process. The role of the alignment of atoms produced by photodetachment is discussed.

We investigate the photodetachment process in the negative fluorine ion in a strong linearly polarized laser field. Angle-resolved momentum distributions of photoelectrons are measured with the use of an imaging technique for a wide range of laser frequencies and peak intensities. The nonmonotonic structure recorded in photoelectron spectra is interpreted in terms of the quantum interference effect predicted by a Keldysh-like theory. In particular, the dependence of the interference term on the laser parameters is used to explain the origin of the observed spectral features. Our results unambiguously show that the length gauge is the proper one to use in the frame of the strong-field approximation.

The sub-cycle dynamics in non-sequential double ionization of argon are studied by combining a reaction microscope with a single-shot carrier-envelope phase measurement technique.

A new method to measure the carrier-envelope phase (CEP) of a strong few-cycle laser pulse is introduced. The new concept relies on above threshold ionization (ATI) in circularly polarized laser fields and allows for CEP tagging every single laser pulse generated in few-cycle laser systems at multi-kHz repetition rates. The implementation of the new method is described and discussed quantitatively. It is shown that the introduced circular-polarization phase-meter may offer a series of advantages over existing CEP-tagging schemes based on ATI in linearly polarized laser fields.

Strong few-cycle light fields with stable electric field waveforms allow controlling electrons on time scales down to the attosecond domain. We have studied the dissociative ionization of randomly oriented DCl in 5 fs light fields at 720 nm in the tunneling regime. Momentum distributions of D(+) and Cl(+) fragments were recorded via velocity-map imaging. A waveform-dependent anti-correlated directional emission of D(+) and Cl(+) fragments is observed. Comparison of our results with calculations indicates that tailoring of the light field via the carrier envelope phase permits the control over the orientation of DCl(+) and in turn the directional emission of charged fragments upon the breakup of the molecular ion.

In strong laser fields, sub-femtosecond control of chemical reactions with the carrier-envelope phase (CEP) becomes feasible. We have studied the control of reaction dynamics of acetylene and allene in intense few-cycle laser pulses at 750 nm, where ionic fragments are recorded with a reaction microscope. We find that by varying the CEP and intensity of the laser pulses it is possible to steer the motion of protons in the molecular dications, enabling control over deprotonation and isomerization reactions. The experimental results are compared to predictions from a quantum dynamical model, where the control is based on the manipulation of the phases of a vibrational wave packet by the laser waveform. The measured intensity dependence in the CEP-controlled deprotonation of acetylene is well captured by the model. In the case of the isomerization of acetylene, however, we find differences in the intensity dependence between experiment and theory. For the isomerization of allene, an inversion of the CEP-dependent asymmetry is observed when the intensity is varied, which we discuss in light of the quantum dynamical model. The inversion of the asymmetry is found to be consistent with a transition from non-sequential to sequential double ionization., Comment: 15 pages, 8 figures

The observation and manipulation of electron dynamics in matter call for attosecond light pulses, routinely available from high-order harmonic generation driven by few-femtosecond lasers. However, the energy limitation of these lasers supports only weak sources and correspondingly linear attosecond studies. Here we report on an optical parametric synthesizer designed for nonlinear attosecond optics and relativistic laser-plasma physics. This synthesizer uniquely combines ultra-relativistic focused intensities of about 1020 W/cm2 with a pulse duration of sub-two carrier-wave cycles. The coherent combination of two sequentially amplified and complementary spectral ranges yields sub-5-fs pulses with multi-TW peak power. The application of this source allows the generation of a broad spectral continuum at 100-eV photon energy in gases as well as high-order harmonics in relativistic plasmas. Unprecedented spatio-temporal confinement of light now permits the investigation of electric-field-driven electron phenomena in the relativistic regime and ultimately the rise of next-generation intense isolated attosecond sources.