Attosecond Metrology Research Articles

Multi-channel electronic and vibrational dynamics in polyatomic resonant high-order harmonic generation.

High-order harmonic generation in polyatomic molecules generally involves multiple channels of ionization. Their relative contribution can be strongly influenced by the presence of resonances, whose assignment remains a major challenge for high-harmonic spectroscopy. Here we present a multi-modal approach for the investigation of unaligned polyatomic molecules, using SF6 as an example. We combine methods from extreme-ultraviolet spectroscopy, above-threshold ionization and attosecond metrology. Fragment-resolved above-threshold ionization measurements reveal that strong-field ionization opens at least three channels. A shape resonance in one of them is found to dominate the signal in the 20–26 eV range. This resonance induces a phase jump in the harmonic emission, a switch in the polarization state and different dynamical responses to molecular vibrations. This study demonstrates a method for extending high-harmonic spectroscopy to polyatomic molecules, where complex attosecond dynamics are expected.

Direct observation of electron propagation and dielectric screening on the atomic length scale.

The propagation and transport of electrons in crystals is a fundamental process pertaining to the functioning of most electronic devices. Microscopic theories describe this phenomenon as being based on the motion of Bloch wave packets. These wave packets are superpositions of individual Bloch states with the group velocity determined by the dispersion of the electronic band structure near the central wavevector in momentum space. This concept has been verified experimentally in artificial superlattices by the observation of Bloch oscillations--periodic oscillations of electrons in real and momentum space. Here we present a direct observation of electron wave packet motion in a real-space and real-time experiment, on length and time scales shorter than the Bloch oscillation amplitude and period. We show that attosecond metrology (1 as = 10(-18) seconds) now enables quantitative insight into weakly disturbed electron wave packet propagation on the atomic length scale without being hampered by scattering effects, which inevitably occur over macroscopic propagation length scales. We use sub-femtosecond (less than 10(-15) seconds) extreme-ultraviolet light pulses to launch photoelectron wave packets inside a tungsten crystal that is covered by magnesium films of varied, well-defined thicknesses of a few ångströms. Probing the moment of arrival of the wave packets at the surface with attosecond precision reveals free-electron-like, ballistic propagation behaviour inside the magnesium adlayer--constituting the semi-classical limit of Bloch wave packet motion. Real-time access to electron transport through atomic layers and interfaces promises unprecedented insight into phenomena that may enable the scaling of electronic and photonic circuits to atomic dimensions. In addition, this experiment allows us to determine the penetration depth of electrical fields at optical frequencies at solid interfaces on the atomic scale.

Attosecond metrology: from electron capture to future signal processing

This article reviews the basic concepts underlying attosecond measurement and control techniques. Emphasis is given to exploring the fundamental speed limit of electronic signal processing that employs ultimate-speed electron metrology provided by attosecond technology.

MAKING OPTICAL WAVES, TRACING ELECTRONS IN REAL-TIME: THE ONSET OF THE ATTOSECOND REALM (Invited Review)

Tracing the dynamics of electrons inside atoms molecules or solids as they occur in real time resides at the forefront of modern science and technology. Advances in attosecond physics over the last decade and beyond are now enabling this essential experimental capability. Here we discuss some of the key developments in light sciences that made possible attosecond metrology and control of electronic processes inside matter on native time scales. These developments hold the promise for new, fundamental insights into the innerworkings of the microcosm as well as the identiflcation of innovative routes for light-based electronic and photonic devices operating at PHz rates.

Diagrammatic approach to attosecond delays in photoionization

We study laser-assisted photoionization by attosecond pulses using a time-independent formalism based on diagrammatic many-body perturbation theory. Our aim is to provide an ab inito route to the "delays" for this above-threshold ionization process, which is essential for a quantitative understanding of attosecond metrology. We present correction curves for characterization schemes of attosecond pulses, such as "streaking", that account for the delayed atomic response in ionization from neon and argon. We also verify that photoelectron delays from many-electron atoms can be measured using similar schemes if, instead, the so-called continuum--continuum delay is subtracted. Our method is general and it can be extended also to more complex systems and additional correlation effects can be introduced systematically.

Ultrafast X-ray pulse characterization at free-electron lasers

Researchers use single-cycle THz pulses from an optical laser to extend streaking techniques of attosecond metrology to measure the temporal profile and arrival time of individual FEL pulses with ∼5 fs precision.

Attosecond tracking of light absorption and refraction in fullerenes

The collective response of matter is ubiquitous and widely exploited, e.g., in plasmonic, optical, and electronic devices. Here we trace on an attosecond time scale the birth of collective excitations in a finite system and find distinct features in this regime. Combining quantum chemical computation with quantum kinetic methods, we calculate the time-dependent light absorption and refraction in fullerene that serve as indicators for the emergence of collective modes. We explain the numerically calculated transient features by an analytical model and point out the relevance for ultrafast photonic and electronic applications. A scheme is proposed to measure the predicted effects via the emergent attosecond metrology.

Frequency comb generation at terahertz frequencies by coherent phonon excitation in silicon

High-order nonlinear light-matter interactions in gases enable generation of x-ray and attosecond light pulses, metrology, and spectroscopy. Optical nonlinearities in solid-state materials are particularly interesting for combining optical and electronic functions for high-bandwidth information processing. Third-order nonlinear optical processes in silicon have been used to process optical signals with greater than 1 GHz bandwidths. Fundamental physical processes for a Si-based optical modulator in the THz bandwidth range, however, have not yet been explored. Here we demonstrate ultrafast phononic modulation of the optical index of Si by irradiation with intense few-cycle femtosecond pulses. The anisotropic reflectivity modulation by the resonant Raman susceptibility at the fundamental frequency of the longitudinal optical (LO) phonon of Si (15.6 THz) generates a frequency comb up to 7th-order. All optical >100 THz frequency comb generation is realized by harnessing the coherent atomic motion of the Si crystalline lattice at its highest mechanical frequency.

Use of photoelectron energy spectrum transfer equation for the measurement of a narrowband XUV pulse

To study the time evolution of a molecular state in an ultra-fast chemical reaction, the use of shorter pulses with higher photon energy and narrower bandwidth for both pump and probe is necessary. However, quick and precise measurement of their detailed time structures is a challenge. Over the last decade, great efforts have been made to measure an attosecond extreme ultraviolet (XUV) pulse. To date, several methods have been developed to measure the pulse duration and completely reconstruct it. The attosecond spectral phase interferometry for direct electric field reconstruction (SPIDER) and attosecond frequency-resolved optical gating (FROG) techniques are often used. However, these methods use state-of-the-art experimental set-ups and complicated data analysis procedures. To develop attosecond metrology for practical use (e.g. timing, measurement, evaluation, calibration, optimization, pumping, probing), we propose a quick and analytical method to precisely observe an attosecond XUV pulse with laser-assisted photo-ionization. The method is based on determining the laser-related phase of each streaked electron and using a transfer equation for one-step pulse reconstruction without any time-resolved measurements, iterative calculations, or data fitting procedures. Temporal errors of the pulse reconstruction are calculated from the XUV bandwidth. Because the transfer equation establishes a direct connection between the XUV pulse properties, the crucial laser parameters (peak intensity, phase, carrier envelope phase), the atomic ionization potential, and the measured photoelectron energy spectrum, we can use it to study any one of these properties from other known information and probe the dynamic processes of an ultra-fast reaction.

SubfemtosecondK-Shell Excitation with a Few-Cycle Infrared Laser Field

Subfemtosecond bursts of extreme ultraviolet radiation, facilitated by a process known as high-order harmonic generation, are a key ingredient for attosecond metrology, providing a tool to precisely initiate and probe ultrafast dynamics in the microcosms of atoms, molecules, and solids. These ultrashort pulses are always, and as a by-product of the way they are generated, accompanied by laser-induced recollisions of electrons with their parent ions. By using a few-cycle infrared (λ(0)=2.1 μm) driving laser, we were able to directly excite high-energy (∼870 eV) inner-shell electrons through laser-induced electron recollision, opening the door to time-resolved studies of core-level and concomitant multielectron dynamics.

Time-and-energy-resolved measurement of Auger cascades following Kr 3d excitation by attosecond pulses

We show that attosecond metrology has evolved from proof-of-principle experiments to a level where complex processes can be resolved in time that cannot be accessed using any other existing technique. The cascaded Auger decay following ionization and excitation of the 3d-subshell in Kr with subfemtosecond 94 eV soft x-ray pulses has been energy- and time-resolved in an x-ray pump–infrared probe experiment. This Auger cascade reveals rich multi-electron dynamics, which despite the fact that there are many experimental and theoretical data available, is not yet fully understood. We present time-resolved data showing the sequence of the temporal dynamics in the cascaded Auger decay. The decay time of several groups of lines has been measured, including the lines at the low-energy part of the spectrum, which are predominantly produced by the second-step Auger transitions. Our experimental data reveal long lifetimes (up to 70 fs) of the subvalence excited ionic (intermediate) states in the cascaded resonant Auger decay. Extensive theoretical calculations within the multiconfiguration Dirac–Fock (MCDF) approach show that the observed long lifetime may be attributed to the second-step Auger decay of the resonantly excited 3d−1np states with n = 6,7. Furthermore, our experimental data show that the electrons with a kinetic energy around 25 eV (generally assigned as M4,5N1N1 1S0 normal Auger lines) have a component corresponding to the second-step Auger decay of the ion after resonant Auger transition 3d−1np → 4s2 4p3 4dnp → 4s2 4p4 with a lifetime of 26 ± 4 fs.

Phase characterization of the reflection on an extreme UV multilayer: comparison between attosecond metrology and standing wave measurements

We characterize the phase shift induced by reflection on a multilayer mirror in the extreme UV range (80-93 eV) using two techniques: one based on high order harmonic generation and attosecond metrology (reconstruction of attosecond beating by interference of two-photon transitions), and a second based on synchrotron radiation and measurements of standing waves (total electron yield). We find an excellent agreement between the results from the two measurements and a flat group delay shift (±40 as) over the main reflectivity peak of the mirror.

Streaking at high energies with electrons and positrons

A detection scheme for characterizing high-energy γ-ray pulses down to the zeptosecond timescale is proposed. In contrast to existing attosecond metrology techniques, our method is not limited by atomic shell physics and therefore capable of breaking the MeV photon energy and attosecond time-scale barriers. It is inspired by attosecond streak imaging, but builds upon the high-energy process of electron–positron pair production in vacuum through the collision of a test pulse with an intense laser pulse. We discuss necessary conditions to render the scheme feasible in the upcoming Extreme Light Infrastructure laser facility.

Optical Detection of Attosecond Ionization Induced by a Few-Cycle Laser Field in a Transparent Dielectric Material

We observe an optical signature induced by the modulation of electron density inside a bulk transparent solid that is quasiperiodically ionized on an attosecond time scale by electric field peaks of a focused few-cycle laser pulse. The emitted optical signal resulting from the attosecond ionization dynamics is spatially, temporally and spectrally isolated from concomitant optical responses through the use of a noncollinear pump-probe technique. The method holds promise for developing an attosecond metrology for bulk solids, in which, unlike in the established attosecond metrology of gases and surfaces, direct detection of charged particles is unfeasible.

State-of-the-art attosecond metrology

Tracking and controlling electron dynamics in the interior of atoms, molecules as well as in solids is at the forefront of modern ultrafast science [1–5]. Time-resolved studies of these dynamics require attosecond temporal resolution that is provided by an ensemble of techniques consolidated under the term “attosecond metrology” [6,7]. This work reports the development and commissioning of what we refer to as next-generation attosecond beamline technology: the AS-1 attosecond beamline at the Max-Planck Institute of Quantum Optics. It consists of a phase-stabilized few-cycle laser system, for the generation of XUV radiation, and modules tailored for the spectral filtering and isolation of attosecond pulses as well as for their temporal characterization. The setup produces the shortest attosecond pulses demonstrated to date and combines them with advanced spectroscopic instrumentation (electron-, ion- and XUV-spectrometers). These pulses serve as temporally confined trigger events (attosecond streaking and tunneling spectroscopy) or probe pulses (attosecond absorption and photoelectron spectroscopy) enabling attosecond chronoscopy to be applied to a broad range of systems belonging to the microcosm.

Attosecond dispersion control by extreme ultraviolet multilayer mirrors

We report the first experimental demonstration of a-periodic multilayer mirrors controlling the frequency sweep (chirp) of isolated attosecond XUV pulses. The concept was proven with about 200-attosecond pulses in the photon energy range of 100-130 eV measured via photoelectron streaking in neon. The demonstrated attosecond dispersion control is engineerable in a wide range of XUV photon energies and bandwidths. The resultant tailor-made attosecond pulses with highly enhanced photon flux are expected to significantly advance attosecond metrology and spectroscopy and broaden their range of applications.

Versatile apparatus for attosecond metrology and spectroscopy

We present the AS-2 Attosecond Beamline at the Joint Laboratory for Attosecond Physics of the Max-Planck-Institut für Quantenoptik and Ludwig-Maximilians-Universität for time resolved pump/probe experiments with attosecond resolution. High harmonic generation and subsequent filtering of the generated extreme ultraviolet (XUV) continuum by means of metal filters and XUV multilayer mirrors serve for the generation of isolated attosecond laser pulses. After high harmonic generation, the remaining fundamental laser pulse is spatially separated from the attosecond XUV pulse, to what is to our knowledge for the first time, by means of a perforated mirror in a Mach-Zehnder interferometer. Active stabilization of this interferometer guarantees the necessary temporal resolution for tracking attosecond dynamics in real time. As a proof-of-principle, photoelectron streaking experiments are performed and experimental techniques for their realization are summarized. Finally we highlight the potential of the presented beamline system for future experiments in comparison with previously demonstrated attosecond beamlines.

Inconsistencies between two attosecond pulse metrology methods: A comparative study

The two basic approaches underlying most of the metrology of attosecond pulse trains are compared in the spectral region $~14\ensuremath{-}24$ eV, that is, the second-order intensity volume autocorrelation and the resolution of attosecond beating by interference of two photon transitions (RABITT). They give rather dissimilar pulse durations. It is concluded that for the present experimental conditions RABITT may underestimate the duration under measurement, due to variations of the driving intensity, but in conjunction with theory allows an estimation of the relative contributions of two different electron trajectories to the extreme-ultraviolet (XUV) radiation.

Optical Detection of Tunneling Ionization

We have experimentally detected optical harmonics that are generated due to a tunneling-ionization-induced modulation of the electron density. The optical signature of electron tunneling can be isolated from concomitant optical responses by using a noncollinear pump-probe setup. Whereas previously demonstrated tools for attosecond metrology of gases, plasmas, and surfaces rely on direct detection of charged particles, detection of the background-free time-resolved optical signal, which uniquely originates from electron tunneling, offers an interesting alternative that is especially suited for systems in which free electrons cannot be directly measured.

Coherent Beam Splitting by a Thin Grating

The wide use of beam splitters in quantum interferometry, attosecond metrology, modern quantum information processing, foundations of quantum mechanics, has been based on coherence of beams emerging from a beam splitter. Here we further develop the approach in which a beam splitter is regarded as a transformer of an incident wave field (photon field or matter wave field) into a field which has narrow maxima at the points along and in close vicinity of two or more particular lines. This description was derived by considering a thin grating as a model of a beam splitter for photons, atoms and molecules. It is applied to answer some questions raised in the discussions on the interpretation of wave particle duality.