Attosecond pulses produced by high-order harmonic generation in gases driven by intense laser fields have become a cornerstone technique for probing ultrafast electronic motion in matter. These applications require a good knowledge of the temporal and spectral properties of the emitted radiation. In this work, we generate a train of 2 to 3 attosecond pulses that we characterize using 2-color laser-assisted photoionization. An unexpected spectral behavior, with more pulses at high energies than at low energies, is observed when the carrier-to-envelope phase of the laser field is changed by 90°. High-order harmonic generation simulations indicate that the time-dependent phase matching of the harmonics contributes in a nontrivial way to the structure of the pulse train. Two-color laser-assisted photoionization enables us to unravel the dynamical influence of subcycle phase matching on the spectral properties of the attosecond pulse train, going beyond the predictions of the response of a single atom to a strong laser field.

Time-resolved imaging of plasmonic near fields is well-established at visible wavelengths but remains largely unexplored in the short-wavelength infrared (SWIR) range. Here, we use time-resolved photoemission electron microscopy (TR-PEEM) to directly visualize ultrafast plasmon dynamics in silver nanowires driven by a few-cycle SWIR pulses. We observe strong electric-field enhancement localized at the nanowire ends, oscillating synchronously with the SWIR optical cycle. Both the local intensity and the temporal response vary within and between individual nanowires. Time-dependent linear modeling reveals that subwavelength variations in nanowire length significantly affect the plasmon dynamics, while the precise end morphology influences both dynamics and field enhancement. Electromagnetic simulations predict additional substrate-induced plasmon modes beyond the nanowire's fundamental longitudinal resonances. Because the imaged photoemission signal depends highly nonlinearly on the local electric field at the wire-vacuum interface, it is dominated by emission from the nanowire ends, suppressing weaker substrate-induced modes. The photoemission yield shows a lower-than-expected power-law dependence with incident field intensity, suggesting the potential onset of strong-field effects beyond simple perturbative multiphoton emission. These results establish silver nanowires as efficient local SWIR field concentrators and demonstrate that their tunable ultrafast plasmonic responses can be imaged and filtered using photoelectrons, offering promising avenues for nanoscale photonic applications and ultrafast control of electron emission.

Twenty-five years have passed since the first experimental demonstration of attosecond pulses, marking the advent of our ability to resolve and control electron motion in real time. What began as a technological breakthrough - generating the shortest flashes ever produced - has evolved into a powerful approach for probing and steering electronic dynamics in atoms, molecules, and solids. This roadmap, authored by leading experts in the field, surveys the recent rapid progress in the generation and characterization of attosecond pulses, emerging attosecond measurement and control techniques, and their expanding range of applications. It reviews current and future developments in attosecond light sources, including novel laser technologies, waveform synthesizers, new schemes for high-order harmonic generation, attosecond pulse generation at free-electron lasers, and structured light. Advances in attosecond measurement methodologies are also discussed, encompassing all-attosecond pump-probe spectroscopy, attosecond four-wave mixing, attosecond microscopy, spectroscopy with light transients, and attosecond interferometry. Furthermore, the roadmap addresses applications of attosecond spectroscopy to reveal electron dynamics in molecules and condensed matter systems from both theoretical and experimental perspectives, and highlights emerging directions at the interface with quantum optics and quantum entanglement. Overall, this work aims to serve as a comprehensive resource for navigating the evolving landscape of attosecond science.

We present a general theoretical framework for multiphoton processes driven by quantum light fields, establishing a direct link between photon statistics and photoelectron observables. Our results show that the autocorrelation and cross-correlation functions, which quantify the underlying photon statistics, are directly mapped onto the resulting photoelectron spectra. Although our framework is broadly applicable, we demonstrate specifically in the example of reconstruction of attosecond beating by interference of two-photon transitions (RABBIT) the influence of the light statistical properties. In this approach, the amplitude, contrast and phase of the oscillations of the sideband signal as a function of pump-probe delay reveal the quantum nature of light. We analyze these observables across several quantum configurations, including correlated infrared and harmonic modes, as well as the uncorrelated case with non-classical harmonic statistics, thereby establishing a general framework for quantum-light RABBIT spectroscopy. We compare the analytical theory with numerical simulations for the case of classical harmonics and an infrared field in a squeezed coherent state, obtaining excellent agreement. Our results reveal how the interplay between classical and quantum correlations dictates the coherence of the photoemission process, providing a new window into the quantum-optical foundations of attosecond science.

High-order harmonic generation enables the up-conversion of intense infrared or visible femtosecond laser pulses into extreme-ultraviolet attosecond pulses. However, the highly nonlinear nature of the process results in low conversion efficiency, which can be a limitation for applications requiring substantial pulse energy, such as nonlinear attosecond time-resolved spectroscopy or single-shot diffractive imaging. Refocusing of the attosecond pulses is also essential to achieve higher intensities but difficult in practice due to strong chromatic aberrations. In this work, we address both the generation and the refocusing of attosecond pulses by sculpting the driving beam into a ring-shaped intensity profile with no spatial phase variations, referred to as a hollow Gaussian beam. Our experimental and theoretical results reveal that hollow Gaussian beams efficiently redistribute the driving-laser energy in a ring-shaped area at focus, where the harmonics are generated with low divergence. In addition, unlike in standard Gaussian-driven schemes, this divergence decreases with increasing harmonic order. Although generated as a ring, our numerical simulations show that the attosecond pulses produced in such an extended area can be refocused with greatly reduced chromatic spread therefore enabling higher intensities—up to 3 times compared to those generated with Gaussian driving beams with the same Rayleigh length. This approach opens pathways for compact and powerful attosecond light sources driven by structured light beams.

The field of attosecond physics has expanded significantly in recent years, yet experimental facilities supporting attosecond pump–attosecond probe spectroscopy remain rare. Here, we present a newly constructed beamline for the generation and application of energetic, isolated extreme ultraviolet (XUV) and soft x-ray attosecond pulses via upscaling of high-harmonic generation (HHG) in a gas medium. The fundamental properties of the HHG radiation—energy, beam profile, spectrum, and divergence—are characterized and optimized. The source delivers up to 55 nJ of pulse energy within the Zr window (65–150 eV) with high stability (∼5%–10%) and a divergence of 0.1 mrad. Numerical simulations identify optimal operating conditions consistent with experimental results. Temporal super-resolution of the driving laser is applied, resulting in a broadened spectral continuum. Furthermore, the beamline includes a split-and-delay stage before focusing the HHG radiation to a <6μm spot for pump–probe experiments using two distinct focusing optics. Spatially resolved ion microscopy is employed to trace the generated ions at the focus. The presented beamline is designed for nonlinear XUV studies with isolated attosecond pulses.

We perform an experimental two-color high-order harmonic generation study in argon with the fundamental of an ytterbium ultrashort pulse laser and its second harmonic. The intensity of the second harmonic and its phase relative to the fundamental are varied while keeping the total intensity constant. We extract the optimum values for the relative phase and ratio of the two colors which lead to a maximum yield enhancement for each harmonic order in the extreme ultraviolet spectrum. Within the three-step model, the yield maximum can be associated with a flat electron return time versus return energy distribution. An analysis of different distributions allows to predict the required relative two-color phase and ratio for a given harmonic order, total laser intensity, fundamental wavelength, and ionization potential.

Ultrashort laser pulses are essential in advanced research, medicine and industry, but the precise characterization remains challenging. The absolute phase of ultrashort light pulses, the so-called carrier-envelope phase (CEP), enables the control of light-matter interaction in the few cycle regime [1]. Its precise measurement is therefore essential, in particular at single-shot and high repetition rate. With the new class of industrial grade lasers based on Ytterbium- and Thulium-doped materials, repetition rates of hundreds of kHz to MHz are readily available together with CEP-stable and few-cycle versions. We have demonstrated a novel, all-optical method to measure the CEP of ultrashort laser pulses, in single-shot for every laser shot, at a repetition rate of 200 kHz [2]. Here, we extend the parameter space of our technique to 586 kHz, which is the fastest single-shot CEP detection ever performed, to the best of our knowledge.

A photoelectron, emitted due to the absorption of light quanta as described by the photoelectric effect, is often characterized experimentally by a classical quantity, its momentum. However, since the photoelectron is a quantum object, its rigorous characterization requires the reconstruction of the complete quantum state, the photoelectron's density matrix. Here, we use quantum state tomography to fully characterize photoelectrons emitted from helium and argon atoms upon absorption of ultrashort, extreme ultraviolet light pulses. While in helium we measure a pure photoelectronic state, in argon, spin-orbit interaction induces entanglement between the ion and the photoelectron, leading to a reduced purity of the photoelectron state. Our work shows how state tomography gives new insights into the fundamental quantum aspects of light-induced electronic processes in matter, bridging the fields of photoelectron spectroscopy and quantum information, and offering new spectroscopic possibilities for quantum technology.

High-order harmonic generation (HHG) enables the up-conversion of intense infrared or visible femtosecond laser pulses into extreme-ultraviolet attosecond pulses. However, the highly nonlinear nature of the process results in low conversion efficiency, which can be a limitation for applications requiring substantial pulse energy, such as nonlinear attosecond time-resolved spectroscopy or single-shot diffractive imaging. Refocusing of the attosecond pulses is also essential to achieve a high intensity, but difficult in practice due to strong chromatic aberrations. In this work, we address both the generation and the refocusing of attosecond pulses by sculpting the driving beam into a ring-shaped intensity profile with no spatial phase variations, referred to as a Hollow Gaussian beam (HGB). Our experimental and theoretical results reveal that HGBs efficiently redistribute the driving laser energy in the focus, where the harmonics are generated on a ring with low divergence, which furthermore decreases with increasing order. Although generated as a ring, the attosecond pulses can be refocused with greatly reduced chromatic spread, therefore reaching higher intensity. This approach enhances the intensity of refocused attosecond pulses and enables significantly higher energy to be delivered in the driving beam without altering the focusing conditions. These combined advantages open pathways for compact, powerful, tabletop, laser-driven attosecond light sources.

Intense ultrashort-pulsed laser systems based on Ti:sapphire chirped pulse amplifiers (CPA) operating at 800 nm have been a workhorse of ultrafast science for several decades. More recently, driven by the progress in laser technology, sources with central wavelengths further in the infrared have emerged as powerful tools, in particular to generate soft X-ray (SXR) radiation via high-harmonic generation (HHG). Optical parametric chirped pulse amplification (OPCPA) is a prominent technique to generate few-cycle pulses in the infrared, allowing broad amplification bandwidth at high repetition rates. However, achieving high-flux coherent SXR emission remains challenging due to the unfavorable scaling of HHG conversion efficiency with longer wavelengths. Therefore, the development of high average power infrared drivers capable of efficient SXR generation, along with straightforward and accurate pulse characterization, remains an active area of research.

High-order harmonic generation (HHG) in gases is inherently inefficient, prompting ongoing efforts to enhance the output of coherent table-top XUV sources. In this study, we investigate the influence of the laser pulse duration on HHG using a compact post-compression method featuring a bulk multi-pass cell. This setup enables tunable Fourier-limited pulse durations as short as 42 fs. We examine the HHG yield as a function of the laser intensity for pulse durations ranging from 42 fs to 180 fs while maintaining identical focusing conditions and generating medium. Our findings reveal that, for a given intensity, there exists an optimum pulse duration – not necessarily the shortest – that maximizes conversion efficiency. This optimum pulse duration increases as the intensity decreases. The experimental results are corroborated by numerical simulations, which show the dependence of HHG yield on the duration and peak intensity of the driving laser and underscore the importance of the interplay between light-matter interaction and phase-matching in the nonlinear medium. Our conclusion allows us to understand why HHG could be demonstrated in 1988 with pulses as long as 40 ps pulses and intensities of only a few 10^13 W/cm^2.

The dispersion-scan technique (d-scan) is a well-established technique for the measurement of ultrashort laser pulses. While a single-shot version of d-scan is relatively easy to implement for sub-10 fs laser pulses, this becomes increasingly difficult as the pulse duration increases due to the need for a larger dispersion range, which scales quadratically with the Fourier transform limit (FTL). Here, we present a solution using a grism (a combination of prism and grating) to measure tens of femtosecond pulses without modifying the principle of the single-shot d-scan. Our grism, made of a zinc sulfide prism and a transmission grating, achieves a dispersion range window of 3200 fs(2). We demonstrate this new method, to our knowledge, by characterizing pulses with an FTL of 25 fs and different spectral phases, comparing the results with a conventional d-scan.

Coherent extreme ultraviolet (XUV) radiation can be generated in a tabletop configuration via high-order harmonic generation (HHG) in gases. While HHG is inherently inefficient, increasing the conversion efficiency (CE) is crucial for many applications, such as coincidence spectroscopy [1] and coherent imaging [2], which require high XUV throughput. Previous calculations suggest that one way to improve the CE is to use shorter laser pulses [3]. To investigate this experimentally, it is essential to vary the Fourier-transform-limited (FTL) pulse duration independently of other generation parameters, such as focusing geometry.

The photoelectric effect explained by Einstein is often regarded as a one-electron phenomenon, whereas the interaction of the escaping electron with other electrons, referred to as electron correlation, plays an important role in multielectron systems. In this Letter, we study the attosecond photoionization of the outer s subshell of argon in its substantial minimum cross-section region formed by electron correlation, which was theoretically predicted in 1972 and experimentally confirmed using synchrotron radiation. Combining high-spectral resolution attosecond interferometry experiments and novel theoretical calculations allows us to identify the most essential electron correlations affecting the time of photoemission, solving the long-standing inconsistency between measurements and theories, and demonstrating the contribution of coherent couplings with shakeup channels. The measurement of time delays gives unprecedented insight into the photoionization process, unraveling details of the atomic potential experienced by the escaping electron and capturing its dynamics.

There is a growing interest in reconstructing the density matrix of photoelectron wavepackets, in particular in complex systems where decoherence can be introduced either by a partial measurement of the system or through coupling with a stochastic environment. To this end, several methods to reconstruct the density matrix, quantum state tomography protocols, have been developed and tested on photoelectrons ejected from noble gases following absorption of extreme ultraviolet (XUV) photons from attosecond pulses. It remains a challenge to obtain model-free, single scan protocols that can reconstruct the density matrix with high fidelities. Current methods require extensive measurements or involve complex fitting of the signal. Efficient single-scan reconstructions would be of great help to increase the number of systems that can be studied. We propose a new and more efficient protocol that is able to reconstruct the continuous variable density matrix of a photoelectron in a single time delay scan. It is based on measuring the coherences of a photoelectron created by absorption of an XUV pulse using a broadband infrared (IR) probe that is scanned in time and a narrowband IR reference that is temporally fixed to the XUV pulse. We illustrate its performance for a Fano resonance in He as well as mixed states in Ar arising from spin-orbit splitting. We show that the protocol results in excellent fidelities and near-perfect estimation of the purity.

We investigate the spatial characteristics of high-order harmonic radiation generated in argon, and observe cross-like patterns in the far field. An analytical model describing harmonics from an astigmatic driving beam reveals that these patterns result from the order and generation position dependent divergence of harmonics. Even small amounts of driving field astigmatism may result in cross-like patterns, coming from the superposition of individual harmonics with spatial profiles elongated in different directions. By correcting the aberrations using a deformable mirror, we show that fine-tuning the driving wavefront is essential for optimal spatial quality of the harmonics.

We present a novel, interferometric, two-color, high-order harmonic generation setup based on a turn-key Ytterbium-doped femtosecond laser source and its second harmonic. Each interferometer arm contains a spatial light modulator with individual capabilities to manipulate the spatial beam profiles and to stabilize the relative delay between the fundamental and the second harmonic. In addition, separate control of the relative power and focusing geometries of the two color beams is implemented to conveniently perform automated scans of multiple parameters. A live diagnostics system gives continuous information during ongoing measurements.