Broadband attosecond pulse shaping

Intense isolated attosecond pulses from two-color few-cycle laser driven relativistic surface plasma

Ultrafast plasma dynamics play a pivotal role in the relativistic high harmonic generation, a phenomenon that can give rise to intense light fields of attosecond duration. Controlling such plasma dynamics holds key to optimize the relevant sub-cycle processes in the high-intensity regime. Here, we demonstrate that the optimal coherent combination of two intense ultrashort pulses centered at two-colors (fundamental frequency, [Formula: see text] and second harmonic, [Formula: see text]) can lead to an optimal shape in relativistic intensity driver field that yields such an extraordinarily sensitive control. Conducting a series of two-dimensional (2D) relativistic particle-in-cell (PIC) simulations carried out for currently achievable laser parameters and realistic experimental conditions, we demonstrate that an appropriate combination of [Formula: see text] along with a precise delay control can lead to more than three times enhancement in the resulting high harmonic flux. Finally, the two-color multi-cycle field synthesized with appropriate delay and polarization can all-optically suppress several attosecond bursts while favourably allowing one burst to occur, leading to the generation of intense isolated attosecond pulses without the need of any sophisticated gating techniques.

Selecting two-photon sequential ionization pathways in H2 through harmonic filtering

Recent experiments in gas-phase molecules have shown the versatility of using attosecond pulse trains combined with IR femtosecond pulses to track and control excitation and ionization yields on the attosecond timescale. The interplay between electron and nuclear motions drives the light-induced transitions favoring specific reaction paths, so that the time delay between the pulses can be used as the tracking parameter or as a control knob to manipulate the molecular dynamics. Here, we present ab initio simulations on the hydrogen molecule to demonstrate that by filtering the high harmonics in an attosecond pulse train one can quench or enhance specific quantum paths thus dictating the outcome of the reaction. It is then possible to discriminate the dominant sequential processes in two-photon ionization, as for example molecular excitation followed by ionization or the other way around. More interestingly, frequency filters can be employed to steer the one- and two-photon yields to favor electron emission in a specific direction.

Accuracy and precision of the RABBIT technique

One of the most ubiquitous techniques within attosecond science is the so-called reconstruction of attosecond beating by interference of two-photon transitions (RABBIT). Originally proposed for the characterization of attosecond pulses, it has been successfully applied to the accurate determination of time delays in photoemission. Here, we examine in detail, using numerical simulations, the effect of the spatial and temporal properties of the light fields and of the experimental procedure on the accuracy of the method. This allows us to identify the necessary conditions to achieve the best temporal precision in RABBIT measurements. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

Aperiodic CrSc multilayer mirrors for attosecond water window pulses

Extending single attosecond pulse technology from currently sub-200 eV to the so called 'water window' spectral range may enable for the first time the unique investigation of ultrafast electronic processes within the core states of bio-molecules as proteins or other organic materials. Aperiodic multilayer mirrors serve as key components to shape these attosecond pulses with a high degree of freedom and enable tailored short pulse pump-probe experiments. Here, we report on chirped CrSc multilayer mirrors, fabricated by ion beam deposition with sub-angstrom precision, designed for attosecond pulse shaping in the 'water window' spectral range.

Formation of a single attosecond pulse via interaction of resonant radiation with a strongly perturbed atomic transition

We propose a technique to form a single few-cycle attosecond pulse from vacuum ultraviolet or extreme ultraviolet radiation via resonant interaction with hydrogenlike atoms, irradiated by a high-intensity far-off-resonant laser field. The laser field strongly perturbs excited atomic energy levels via the Stark effect and ionizes atoms from the excited states. We show that an isolated attosecond pulse can be formed using either a short incident femtosecond pulse of the resonant radiation or a steep front edge of the laser field. We propose an experimental realization of a single subfemtosecond pulse formation at 121.6 nm in atomic hydrogen and a single sub-100 as pulse formation at 13.5 nm in Li(2+) plasma.

Compression methods for XUV attosecond pulses

Attosecond extreme-ultraviolet (XUV) pulses generated in gases via high-order harmonic generation typically carry an intrinsic positive chirp. Compression of such pulses has been demonstrated using metallic transmission filters, a method with very limited tunability. We compare here the compression achievable with a diffraction grating based method with that of metallic filters using simulated high harmonic waveforms in the transmission window of metal films.

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.

Few-cycle attosecond pulses via periodic resonance interaction with hydrogenlike atoms

We show that it is possible to produce nearly bandwidth-limited few-cycle attosecond pulses based on periodic resonance interaction of a quasi-monochromatic radiation with the bound states of hydrogenlike atoms. A periodic resonance is provided by a far-off-resonant laser field with intensity much below the atomic ionization threshold via periodic tunnel ionization from the excited states and adiabatic Stark splitting of the excited energy levels. Without external synchronization of the spectral components, it is possible to produce 135 as pulses at 13.5 nm in Li²⁺-plasma controlled by radiation of a mode-locked Nd:YAG laser, as well as 1.25 fs pulses at 122 nm in atomic hydrogen controlled by radiation of a CO₂ laser.

Control of the attosecond synchronization of XUV radiation with phase-optimized mirrors

We report on the advanced amplitude and phase control of attosecond radiation allowed by specifically-designed multilayer XUV mirrors. We first demonstrate that such mirrors can compensate for the intrinsic chirp of the attosecond emission over a large bandwidth of more than 20 eV. We then show that their combination with metallic foils introduces a third-order dispersion that is adjustable through the mirror's incidence angle. This results in a controllable beating allowing the radiation to be shaped from a single to a series of sub-100 as pulses.

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.

Attosecond electron spectroscopy using a novel interferometric pump-probe technique

We present an interferometric pump-probe technique for the characterization of attosecond electron wave packets (WPs) that uses a free WP as a reference to measure a bound WP. We demonstrate our method by exciting helium atoms using an attosecond pulse (AP) with a bandwidth centered near the ionization threshold, thus creating both a bound and a free WP simultaneously. After a variable delay, the bound WP is ionized by a few-cycle infrared laser precisely synchronized to the original AP. By measuring the delay-dependent photoelectron spectrum we obtain an interferogram that contains both quantum beats as well as multipath interference. Analysis of the interferogram allows us to determine the bound WP components with a spectral resolution much better than the inverse of the AP duration.

Separation of high order harmonics with fluoride windows

The ensemble of lower orders produced in high order harmonic generation can be efficiently temporally separated by propagation in a fluoride window while still preserving their femtosecond pulse duration. We present calculations for MgF2, CaF2, and LiF windows for the third, fifth, and seventh harmonics of 800 nm. We use this simple and inexpensive technique in a pump/probe experiment to resolve femtosecond dynamics in the ethylene molecule.

Attosecond synchronization of high-order harmonics from midinfrared drivers

The group delay dispersion, also known as the attochirp, of high-order harmonics generated in gases has been identified as the main intrinsic limitation to the duration of Fourier-synthesized attosecond pulses. Theory implies that the attochirp, which is inversely proportional to the laser wavelength, can be decreased at longer wavelength. Here we report the first measurement of the wavelength dependence of the attochirp using an all-optical, in situ method [N. Dudovich, Nature Phys. 2, 781 (2006)10.1038/nphys434]. We show that a 2 microm driving wavelength reduces the attochirp with respect to 0.8 microm at comparable intensities.

Coherent electron scattering captured by an attosecond quantum stroboscope

We demonstrate a quantum stroboscope based on a sequence of identical attosecond pulses that are used to release electrons into a strong infrared (IR) laser field exactly once per laser cycle. The resulting electron momentum distributions are recorded as a function of time delay between the IR laser and the attosecond pulse train using a velocity map imaging spectrometer. Because our train of attosecond pulses creates a train of identical electron wave packets, a single ionization event can be studied stroboscopically. This technique has enabled us to image the coherent electron scattering that takes place when the IR field is sufficiently strong to reverse the initial direction of the electron motion causing it to rescatter from its parent ion.