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.

Light fields with a central wavelength of 2 µm are very well suited for strong-field-driven charge carrier control: Their photon energy lies far below the band gap of many materials, while their oscillation period remains significantly shorter than the coherence time of charge carrier oscillations. The resulting potential for field-driven charge carrier control is contingent on the reproducibility of the field structure of such ultrashort laser pulses. Here, we present a compact 200-kHz laser system that delivers ultrashort pulses with a duration of less than 20 fs in the spectral range around 2 µm and with a pulse energy of 25 µJ. The electric field structure of the 2-µm pulses is characterized in detail. In particular, the carrier-envelope phase (CEP) is measured over a wide range of timescales, from microseconds to hours. Passive stabilization due to difference frequency generation results in a root mean square value of carrier-envelope phase noise of less than 70 mrad over all measured time scales. The applicability of the pulses is demonstrated by measuring CEP-dependent high-order harmonic spectra with energies of up to 160 eV.

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 experimentally demonstrate the generation of sub-two-cycle optical pulses using a cascaded post-compression scheme based on two bulk multi-pass cells. Starting from 220 fs, 100 μJ pulses around 1030 nm, sequential spectral broadening and pulse compression reduce the pulse duration first to 50 fs and ultimately to 5.5 fs (1.6 optical cycles) at a central wavelength of 1058 nm. The ultra-broadband spectrum is enabled by a dispersion-controlled cavity. Comprehensive spectral, temporal, and spatial characterization confirms excellent pulse quality. Despite operating the second multi-pass cell at peak powers several hundred times above the critical power for self-focusing in fused silica, no significant spatio-spectral or spatio-temporal distortions are observed, enabling direct use of the compressed 40 μJ pulses in further experiments. Numerical simulations of the nonlinear spectral broadening show excellent agreement with the experimental results, supporting the underlying physical picture. These findings establish bulk multi-pass cells as an efficient, compact, and robust platform for generating few-cycle pulses with excellent beam quality, offering considerable potential for strong-field and ultrafast applications, including extreme-ultraviolet generation and isolated attosecond pulse production.

Ultrafast lasers with simultaneously high average and peak power have become indispensable for driving a multitude of applications, including high-harmonic generation, strong-field physics, and particle source applications. Both parametric amplifiers and post-compressed Ytterbium lasers have emerged as prime platforms to meet these demands. While multi-pass cell (MPC) based post-compression offers broadband output with high beam quality, it provides limited wavelength tunability and suffers from temporal contrast degradation. Conversely, optical parametric amplifiers (OPAs) provide spectral tunability and high temporal contrast but they are limited by low pump-to-signal conversion efficiency and spatial beam inhomogeneities. Here, we introduce the Optical Parametric Multi-Pass Cell Amplifier (OPMPC), a hybrid architecture that overcomes the limitations of both schemes. Our approach utilizes two non-collinearly intersecting MPCs providing broadband parametric amplification of the seed pulses and complete idler removal after each pass through the crystal, thereby suppressing back-conversion. We experimentally demonstrate a record pump-to-signal power conversion efficiency of 43

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.

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.

We report pulse energy scaling enabled by the use of Laguerre–Gaussian single-vortex (LG0,l) beams for spectral broadening in a sub-40 cm long Herriott-type bulk multi-pass cell. Beams with orders l = 1–3 are generated using a spatial light modulator, facilitating rapid and precise reconfiguration of the experimental conditions. 180 fs pulses with 610 μJ pulse energy are post-compressed to 44 fs using an LG0,3 beam, boosting the peak power of an ytterbium laser system from 2.5 GW to 9.1 GW. The spatial homogeneity of the output LG0,l beams is quantified, and the topological charge is spectrally-resolved and is shown to be conserved after compression by employing a custom spatiotemporal coupling measurement setup.

>Combining high peak power and high average power has long been a key challenge of ultrafast laser technology,crucial for applications such as laser-plasma acceleration and strong-field physics. A promising solution lies in post-compressed ytterbium lasers, but scaling these to high pulse energies presents a major bottleneck. Post-compression techniques, particularly Herriott-type multi-pass cells (MPCs), have enabled large peak power boosts at high average powers but their pulse energy acceptance reaches practical limits defined by setup size and coating damage threshold. In this work, we address this challenge and demonstrate, to our knowledge, a novel type of compact, energy-scalable MPC (CMPC). By employing a novel MPC configuration and folding the beam path,the CMPC introduces a new degree of freedom for downsizing the setup length, enabling compact setups even for large pulse energies. We experimentally and numerically verify the CMPC approach, demonstrating post-compression of 8 m J pulses from 1 ps down to 51 fs in atmospheric air using a cell roughly 45 cm in length at low fluence values. Additionally, we discuss the potential for energy scaling up to 200 m J with a setup size reaching2.5 m. Our work presents a new approach to high-energy post-compression, with up-scaling potential far beyond the demonstrated parameters. This opens new routes for achieving the high peak and average powers necessary for demanding applications of ultrafast lasers.

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.

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.

Diffraction gratings are among the most fundamental optical elements. They are essential in ultrafast optics for manipulating the spectral phase of coherent ultrashort light pulses in grating stretchers, compressors, and 4f pulse shapers. In this work, we have exploited an effect associated with diffraction from a single grating, which can produce large amounts of spatially-dependent group delay dispersion. Leveraging this effect, we propose a novel single-shot dispersion-scan technique, enabling the characterization of longer pulse durations than previously possible, with arbitrary chirp. We demonstrate this method by measuring 170 fs pulses from a commercial ytterbium-based laser system, both before and after post-compression in a multi-pass cell.

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.

Isolated attosecond pulse (IAP) generation usually involves the use of short-medium gas cells operated at high pressures. In contrast, long-medium schemes at low pressures are commonly perceived as inherently unsuitable for IAP generation due to the nonlinear phenomena that challenge favourable phase-matching conditions. Here we provide clear experimental evidence on the generation of isolated extreme-ultraviolet attosecond pulses in a semi-infinite gas cell, demonstrating the use of extended-medium geometries for effective production of IAPs. To gain a deeper understanding we develop a simulation method for high-order harmonic generation (HHG), which combines nonlinear propagation with macroscopic HHG solving the 3D time-dependent Schrödinger equation at the single-atom level. Our simulations reveal that the nonlinear spatio-temporal reshaping of the driving field, observed in the experiment as a bright plasma channel, acts as a self-regulating mechanism boosting the phase-matching conditions for the generation of IAPs.

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.