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Lasers capable of generating attosecond X-ray pulses in the water window (282 to 533 eV) through high-order harmonic generation are normally based on inefficient, multi-stage optical parametric amplifiers or optical parametric chirped pulse amplifiers pumped by femtosecond or picosecond lasers. Here we report a very efficient single amplification stage laser based on traditional chirped pulse amplification capable of producing 4 mJ, near-transform limited 44 fs (μm. The ≈90 GW peak power is the highest value ever reached at this wavelength. In order to fully compress the laser pulses our system is built in a nitrogen box. Our system utilizes water cooled chromium doped zinc selenide (Cr2+:ZnSe) as the gain medium and is pumped by a commercial nanosecond holmium doped yttrium-aluminum-garnet (Ho:YAG) laser.

Accurate characterization of an attosecond pulse from streaking trace is an indispensable step in studying the ultrafast electron dynamics on the attosecond scale. Conventional attosecond pulse retrieval methods face two major challenges: the ability to incorporate a complete physics model of the streaking process, and the ability to model the uncertainty of pulse reconstruction in the presence of noise. Here we propose a pulse retrieval method based on conditional variational generative network (CVGN) that can address both demands. Instead of learning the inverse mapping from a streaking trace to a pulse profile, the CVGN models the distribution of the pulse profile conditioned on a given streaking trace measurement, and is thus capable of assessing the uncertainty of the retrieved pulses. This capability is highly desirable for low-photon level measurement, which is typical in attosecond streaking experiments in the water window X-ray range. In addition, the proposed scheme incorporates a refined physics model that considers the Coulomb-laser coupling and photoelectron angular distribution in streaking trace generation. CVGN pulse retrievals under various simulated noise levels and experimental measurement have been demonstrated. The results showed high pulse reconstruction consistency for streaking traces when peak signal-to-noise ratio (SNR) exceeds 6, which could serve as a reference for future learning-based attosecond pulse retrieval.

A single-stage chirped-pulse amplifier centered at 2.5 wm was developed to produce 3.5 mJ, 44 fs pulses at 1 kHz. The 80 GW peak power allows attosecond X-ray pulses generation in the water window.

The motion of electrons in the microcosm occurs on a time scale set by the atomic unit of time—24 attoseconds. Attosecond pulses at photon energies corresponding to the fundamental absorption edges of matter, which lie in the soft X-ray regime above 200 eV, permit the probing of electronic excitation, chemical state, and atomic structure. Here we demonstrate a soft X-ray pulse duration of 53 as and single pulse streaking reaching the carbon K-absorption edge (284 eV) by utilizing intense two-cycle driving pulses near 1.8-μm center wavelength. Such pulses permit studies of electron dynamics in live biological samples and next-generation electronic materials such as diamond. Isolated attosecond pulses are produced using high harmonic generation and sources of these pulses often suffer from low photon flux in soft X-ray regime. Here the authors demonstrate efficient generation and characterization of 53 as pulses with photon energy near the water window.

A new method for retrieving the spectral phase of isolated attosecond X-ray pulses from streaking traces is explored. The neural network method shows the potential for nearly instantaneous attosecond streaking phase retrieval, without use of the central momentum approximation. A neural network is trained with computer generated data that contain statistical noise and shown to correctly retrieve the phase of both computer generated and experimental attosecond streaking traces.

Compensating attosecond chirp (atto-chirp) of broadband high-order harmonic pulses in the water window region (282 to 533 eV) is a major challenge, due to the lack of natural materials that exhibit negative group velocity dispersion and low loss. Analysis shows that the amount of dispersion of fully ionized hydrogen plasma with suitable density-length product is sufficient to compensate the chirp of attosecond pulses with center photon energy above 300 eV. This is confirmed by numerical simulations based on the Strong Field Approximation.

Photoinduced quantum dynamics in molecules have hierarchical temporal structures with different energy scales that are associated with electron and nuclear motions. Femtosecond-to-attosecond transient absorption spectroscopy (TAS) using high-harmonic generation (HHG) with a photon energy below 300 eV has been a powerful tool to observe such electron and nuclear dynamics in a table-top manner. However, comprehensive measurements of the electronic, vibrational, and rotational molecular dynamics have not yet been achieved. Here we demonstrate HHG-based TAS at the nitrogen $K$K-edge (400 eV) for the first time to our knowledge and observe all the electronic, vibrational, and rotational degrees of freedom in a nitric oxide molecule at attosecond to sub-picosecond time scales. This method of employing core-to-valence transitions offers an all-optical approach to reveal the complete molecular dynamics in photochemical reactions with element and electronic state specificity.

Progress in high-harmonic generation has led to high-energy attosecond pulses with cutoff above the carbon $1s$ edge (283.8 eV). These pulses are essential to extend time-resolved spectroscopies to the water window in order to control electron dynamics in solvated organic species. Here we report a step towards this goal: the measurement, with subcycle time resolution, of the attosecond transient absorption spectrum of argon at the $2{p}^{\ensuremath{-}1} {L}_{2,3}$ edge ($\ensuremath{\sim}250$ eV) in the presence of a short-wave infrared control pulse. The measurements, supported by theoretical simulations, demonstrate the concurrent role of Auger decay and tunnel ionization in the driven evolution of inner-valence holes of polyelectronic atoms.

We demonstrate the generation of isolated 53-as soft X-ray pulses reaching the water window. Mid-infrared lasers centered at 2.5 to 8 μm are being developed to extend the attosecond X-rays to the keV range.

We present an experimental investigation of attosecond transient absorption spectroscopy near the L 2 , 3 -edge of argon gas by using an attosecond pulse in the 240-250 eV range.

A technique to improve double optical gating for generating isolated attosecond pulses is introduced. In this method, the ratio between the amplitudes of the counter-rotating pulses is altered to decrease the field strength prior to the linearly-polarized gate cycle. The result is a decrease in the preionization of the gas target used for generating high-order harmonics. In addition to improving phase matching and increasing the saturation intensity, this development also allows isolated attosecond pulse production with longer driving laser pulses.

Solid-state high-harmonic sources offer the possibility of compact, high-repetition-rate attosecond light emitters. However, the time structure of high harmonics must be characterized at the sub-cycle level. We use strong two-cycle laser pulses to directly control the time-dependent nonlinear current in single-crystal MgO, leading to the generation of extreme ultraviolet harmonics. We find that harmonics are delayed with respect to each other, yielding an atto-chirp, the value of which depends on the laser field strength. Our results provide the foundation for attosecond pulse metrology based on solid-state harmonics and a new approach to studying sub-cycle dynamics in solids.

We quantify the effects of the energy resolution of a magnetic bottle electron spectrometer in an attosecond streak camera on the accuracy of measuring the relative amplitudes of satellite pulses around the main attosecond pulse. Our numerical simulations show that the spectral resolution can be significantly improved by restricting the acceptance angle using a pinhole located near the source of the photoelectrons. The intensity of the pre- and post-pulses which are 1% and 10% of a main 25 as pulse can potentially be measured with less than 10% error by two practical time-of-flight spectrometer designs.

The generation of extremely short isolated attosecond pulses requires both a broad spectral bandwidth and control of the spectral phase. Rapid progress has been made in both aspects, leading to the generation of light pulses as short as 67 as in 2012, and broadband attosecond continua covering a wide range of extreme ultraviolet and soft X-ray wavelengths. Such pulses have been successfully applied in photoelectron and photoion spectroscopy and recently developed attosecond transient absorption spectroscopy to study electron dynamics in matter. In this Review, we discuss significant recent advances in the generation, characterization and applications of ultrabroadband, isolated attosecond pulses with spectral bandwidths comparable to the central frequency. These pulses can in principle be compressed to a single optical cycle. This review discusses significant recent advances in the generation, characterization and application of ultrabroadband isolated attosecond pulses with a spectral bandwidth comparable to the central frequency, which can in principle be compressed to a single optical cycle.

Double optical gating (DOG) technique was implemented with a two-cycle, 1.7 µm driving field to generate isolated attosecond pulses in the 100-250 eV spectrum range. The strong ellipticity dependency of the high harmonics from the 1.7 µm driving field makes polarization based gating method very efficient. When a second harmonic (SH) field is introduced, complete gating can be achieved with less ionization from the leading edge of the driving field, which yields supercontinua with a pulse energy of 0.3 nJ. We perform an attosecond streaking measurement to confirm the generation of isolated attosecond pulses.

It is demonstrated by numerical simulations that the attosecond chirp of high order harmonic pulses in the 530 to 1000 eV range can be partially compensated by the negative group delay dispersion of unionized molecular hydrogen gas. The transmission of X-rays through gas with the required pressure-length product for optimum chirp compensation is higher than 10% in the photon energy range that covers the K-edge of oxygen and L-edges of iron, cobalt, and nickel.

© 2016 Optical Society of America. The milestones of attosecond optics research in the last 15 years are briefly reviewed, and the latest trends in applications in gaseous and condensed matter are introduced. An outlook on future development of attosecond soft x-ray sources and their application is provided.

Isolated attosecond pulses with supercontinuum spectra can be used to study fast electron dynamics in atomic, molecular and condensed matter systems with unprecedented temporal resolution. In this chapter, we present the most recent advances in generation and characterization of the single attosecond pulses at iFAST, including the work demonstrating the world record of a single 67 as pulse. A continuous spectrum supporting 40 as pulses is observed and an MCP filter which can transmit an ultrabroadband supercontinuum covering the entire extreme ultraviolet and soft X-ray spectral range is demonstrated. In addition, a high resolution Magnetic-Bottle Energy Spectrometer is proposed to accurately characterize both the temporal profile and pulse contrast of isolated 25 as pulses.

Attosecond science promises to allow new forms of quantum control in which a broadband isolated attosecond pulse excites a molecular wave packet consisting of a coherent superposition of multiple excited electronic states. This electronic excitation triggers nuclear motion on the molecular manifold of potential energy surfaces and can result in permanent rearrangement of the constituent atoms. Here, we demonstrate attosecond transient absorption spectroscopy (ATAS) as a viable probe of the electronic and nuclear dynamics initiated in excited states of a neutral molecule by a broadband vacuum ultraviolet pulse. Owing to the high spectral and temporal resolution of ATAS, we are able to reconstruct the time evolution of a vibrational wave packet within the excited B′Σu1+ electronic state of H2 via the laser-perturbed transient absorption spectrum, This material is based on work supported by the DARPA PULSE program through a grant from AMRDEC under Award No. W31P4Q1310017; the Army Research Office under Awards No. W911NF-11-1-0297, No. WN911NF-14- 1-0383, and No. FA9550-16-1-0013; the Air Force Office of Scientific Research under Awards No. FA9550-15-1-0037 and No. FA9550-16-1-0149; the National Science Foundation under Award No. PHY-1506345, an Advanced Grant of the European Research Council XCHEM 290853; and European Grants MC-ITN CORINF, the European COST Action XLIC CM1204, the MINECO Project No. FIS2013-42002-R; and the ERA-Chemistry Project PIM2010EEC-00751

By driving high-order harmonic generation with an Optical Parametric Chirped Pulse Amplifier at 1.7 μm and implementing polarization gating, isolated attosecond pulses with photon energy extending to more than 400 eV were produced and characterized.

A 3 mJ, two-cycle (11.4 fs), 1 kHz, carrier-envelope phase (CEP)-stable laser at 1.7 μm is developed via a three-stage optical parametric chirped-pulse amplifier (OPCPA) in BiB 3 O 6 . This is an ideal driver for generating isolated attosecond pulses in the soft x-ray region.

As a compact and burgeoning alternative to synchrotron radiation and free-electron lasers, high harmonic generation has proven its superiority in static and time-resolved XUV spectroscopy for the past two decades and has recently gained many interests and successes in generating soft X-ray emissions covering the biologically important water window spectral region. Unlike synchrotron and free-electron sources, which suffer from relatively long pulse width or large time jitter, soft X-ray sources from high harmonic generation could offer attosecond time resolution and be synchronized with their driving field to investigate time-resolved near edge absorption spectroscopy, which could reveal rich structural and dynamical information of the interrogated samples. In this paper, we review recent progresses on generating and characterizing attosecond light sources in the water window region. We show our development of an energetic, two-cycle, carrier-envelope phase stable laser source at 1.7~$\mu$m and our achievement in producing a 53~attosecond soft X-ray pulse covering the carbon K-edge in the water window. Such source paves the ways for the next generation X-ray spectroscopy with unprecedented temporal resolution.

We extend attosecond transient absorption spectroscopy (ATAS) to the study of hydrogen molecules, demonstrating the potential of the technique to resolve – simultaneously and with state resolution – both the electronic and nuclear dynamics.

Quantum beats with periods of 5–10 fs are observed in various near-threshold autoionizing states of krypton atoms in an attosecond transient absorption experiment, such measurement allows reconstruction of the valence state wave packets.

We report the first demonstration of extreme ultraviolet (XUV) supercontinuum generation in the plateau region of the high-order harmonic spectrum indicative of a single attosecond pulse. It was accomplished by combining the generation of sub-10 fs laser pulses with the polarization gating of high harmonic generation. When an 8 fs pulse centred at 750 nm was split and delayed with a quartz plate and then recombined with a quarter waveplate, a laser pulse was created whose polarization varied rapidly from circular to linear and back to circular. The near-linearly polarized portion of the resultant pulse was only 1.3 fs long, which was much shorter than the laser pulse duration. Since the high-order harmonic generation process is susceptible to the ellipticity of the driving field, the pulse with a time-dependent ellipticity behaved like a half-cycle linearly polarized pulse for generating XUV radiation. By exciting argon gas with the pulse, a supercontinuum that covered 25–45 nm was produced, which corresp...

The motion of electrons in the microcosm occurs on a time scale set by the atomic unit of time—24 attoseconds. Attosecond pulses at photon energies corresponding to the fundamental absorption edges of matter, which lie in the soft X-ray regime above 200 eV, permit the probing of electronic excitation, chemical state, and atomic structure. Here we demonstrate a soft X-ray pulse duration of 53 as and single pulse streaking reaching the carbon K-absorption edge (284 eV) by utilizing intense two-cycle driving pulses near 1.8-μm center wavelength. Such pulses permit studies of electron dynamics in live biological samples and next-generation electronic materials such as diamond., Isolated attosecond pulses are produced using high harmonic generation and sources of these pulses often suffer from low photon flux in soft X-ray regime. Here the authors demonstrate efficient generation and characterization of 53 as pulses with photon energy near the water window.

Electrons in molecules rearrange on an attosecond timescale, much faster than the nuclear rearrangement associated with chemical bond breaking, which takes tens to hundreds of femtoseconds. In spite of these dramatically different timescales, it has been shown that the electron dynamics can influence the ensuing nuclear motion.1 Capturing, and ultimately controlling, this electronic motion may therefore open up the possibility of dictating the outcome of a chemical reaction before the nuclei even begin to move,2 by shaping the spectral amplitude and phase of the attosecond pulse to select a particular reaction pathway. Despite widespread interest, application of attosecond science to molecular targets has proceeded slowly. For pump-probe experiments, it would be ideal to generate intense isolated attosecond ‘pump’ pulses3 from below-threshold harmonics (i.e., with photon energies below the ionization threshold of the generation medium, which can excite ground state electrons to a higher molecular orbital). Such harmonics lie in the relatively low-photon-energy vacuum-UV (VUV) spectral region, where molecules are most electronically active. However, it is difficult to generate below-threshold harmonics, and until now the low conversion efficiency of attosecond pulse generation has severely limited the achievable photon flux (number of photons). Macroscopic buildup of high-order harmonics can be enhanced through phase-matching techniques. In these, the interaction between a driving laser and the medium in which the high-order harmonics are to be generated is experimentally tuned to maintain a fixed phase relationship between the driving laser and the generated harmonic fields. Phase matching techniques have been used effectively in high-order harmonic generation in the above-threshold region to scale the achievable photon flux, primarily in the extreme-UV and soft x-ray spectral regions.4 We have demonstrated a new mechanism for phase Figure 1. (a) Spectrally and angularly resolved resolution-enhanced structure (RES) generation. The atomic resonances in the vicinity of the 9th harmonic (3p6 ! 3p5ns and 3p5nd, 15–16eV) are indicated. The 11th harmonic (H11, 19–20eV) is comparatively featureless.

Even for simple systems, the interpretations of new attosecond measurements are complicated and provide only a glimpse of their potential. Nonetheless, the lasting impact will be the revelation of how short-time dynamics can determine the electronic properties of more complex systems.

Isolated attosecond pulses are powerful tools for studying electron dynamics in atoms, molecules and condensed matter. The shortest pulses achieved so far are 67 as. Challenges and approaches for further shortening such pulses are introduced.

Isolated attosecond pulses are used to probe laser-perturbed hydrogen molecules using attosecond absorption spectroscopy. We observe dynamic features in the delay-dependent absorption on both the electronic and nuclear timescales for the first time.

The eleven papers in this special section provide an overview of the status of research and development in the field of attosecond photonics They introduce new trend in driving laser and attosecond source advancement such as light wave synthesis, long wavelength few-cycle optical parametric amplifiers, and high harmonic generation in solids. Several important topics in theoretical and experimental studies of ionization dynamics induced by both strong infrared laser and attosecond pulses are covered.

Attosecond optics emerged in 2001. It deals with the generation, characterization and application of extreme ultraviolet pulses shorter than one optical cycle of visible light. The duration of isolated attosecond pulses has recently reached 67-as.

We report a method for calibrating an extreme ultraviolet spectrometer based on a flat-field grazing incidence spherical grating in the energy range of 20–30 eV. By measuring absorption lines corresponding to singly excited states in helium atoms and autoionizing states in argon atoms, the photon energy of the detected light was determined. The spectral resolution of the spectrometer, 60 meV, was obtained by deconvolving the Fano resonance profile of argon autoionizing states from the measured absorption line profiles.

Few-cycle laser induced dynamics in the D^1 Π_u←X^1 Σ_g and C^1 Π_u←X^1 Σ_g bands of vibrational wavepacket of hydrogen and deuterium molecules are studied by the attosecond transient absorption spectroscopy.

Generalized Double Optical Gating is a robust sub-cycle switch for generating ultra-broadband single isolated attosecond pulses. It is a promising approach to reach microjoule-level XUV pulse energy for attosecond pump - attosecond probe experiments.

Isolated attosecond pulses with low photon energies (~12-40 eV) are used to probe sub-optical-cycle dynamics in laser-perturbed atomic and molecular targets using attosecond transient absorption spectroscopy, revealing dynamic wavepacket interferences and coupled electron-nuclear dynamics.

An XUV continuum supporting 280 as isolated attosecond pulses is generated in argon with a 200 mJ, 17 fs Ti:Sapphire laser using the GDOG technique. The energy of the XUV pulse is over 100 nJ at generation location.

Photoabsorption spectroscopy with isolated attosecond pulses provides a means for all-optical time-domain measurement of ultrafast dynamics in bound and quasi-bound states of atoms and molecules, as well as near absorption edges in condensed matter systems. Due to the strong oscillator strength in atoms and molecules in the extreme ultraviolet and soft X-ray spectral regions as well as the high detection efficiency of the transmitted photon signal, attosecond transient absorption spectroscopy is an attractive alternative to traditional photoelectron spectroscopy techniques. We present measurements of the laser-perturbed photoabsorption of helium 1snp singly excited states and quasi-bound autoionizing states of argon with a few-femtosecond autoionization lifetime. In both systems, we observe laser-induced changes in the photoabsorption spectrum which evolve on the single-femtosecond timescale, faster than the laser light oscillation period.

Understanding and controlling the dynamic evolution of electrons in matter is among the most fundamental goals of attosecond science. While the most exotic behaviors can be found in complex systems, fast electron dynamics can be studied at the fundamental level in atomic systems, using moderately intense (≲103 W/cm2) lasers to control the electronic structure in proof-of-principle experiments. Here, we probe the transient changes in the absorption of an isolated attosecond extreme ultraviolet (XUV) pulse by helium atoms in the presence of a delayed, few-cycle near infrared (NIR) laser pulse, which uncovers absorption structures corresponding to laser-induced “virtual” intermediate states in the two-color two-photon (XUV+NIR) and three-photon (XUV+NIR+NIR) absorption process. These previously unobserved absorption structures are modulated on half-cycle (~1.3 fs) and quarter-cycle (~0.6 fs) timescales, resulting from quantum optical interference in the laser-driven atom.

We implement the polarization gating (PG) technique with a two-cycle, 1.7 μm driving field to generate an attosecond supercontinuum extending to the water window spectral region. The ellipticity dependence of the high harmonic yield over a photon energy range much broader than previous work is measured and compared with a semi-classical model. When PG is applied, the carrier-envelope phase (CEP) is swept to study its influence on the continuum generation. PG with one-cycle (5.7 fs) and two-cycle (11.3 fs) delay are tested, and both give continuous spectra spanning from 50 to 450 eV under certain CEP values, strongly indicating the generation of isolated attosecond pulses in the water window region.

We produce a 3 mJ, two-cycle (11.4 fs), 1 kHz, carrier-envelope phase (CEP)-stable laser source at 1.7 μm via a three-stage Ti:sapphire-pumped optical parametric chirped-pulse amplifier in BiB3O6. We achieve a pump-to-signal conversion efficiency of 18% in the last stage, which is, to the best of our knowledge, the highest yet achieved for near-octave bandwidth amplification. A f-to-2f measurement shows a CEP instability of 165 mrad over 1 h. This is an ideal light source for generating isolated attosecond pulses in the soft x-ray region.

Single isolated attosecond pulse is a new powerful tool for studying dynamics of correlated electron motion in pump-probe experiments [1]. Such pulses, as short as 67 attoseconds, can now be generated with the Double Optical Gating (DOG) method. The broadband XUV supercontinuum spectrum of the isolated attosecond pulse is particularly suitable for probing the time variation of the electronic states in transient absorption measurements.

Recent advances in attosecond science have relied upon the nearly instantaneous response of free electrons to an external field. However, it is still not clear whether bound electrons are able to rearrange on sublaser cycle time scales. Here, we probe the optical Stark shifts induced by a few-cycle near infrared laser field in helium bound states using isolated attosecond pulses in a transient absorption scheme and uncover a subcycle laser-induced energy level shift of the laser-dressed $1s3p$ state.

High-order-harmonic-generation yield is remarkably sensitive to driving laser ellipticity, which is interesting from a fundamental point of view as well as for applications. The most well-known example is the generation of isolated attosecond pulses via polarization gating. We develop an intuitive semiclassical model that makes use of the recently measured initial transverse momentum of tunneling ionization. The model is able to predict the dependence of the high-order-harmonic yield on driving laser ellipticity and is in good agreement with experimental results and predictions from a numerically solved time-dependent Schr\"odinger equation.