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.

High-order harmonic generation has become a standard technique for producing attosecond extreme ultraviolet (XUV) pulses in the laboratory, yet the high flux necessary for nonlinear XUV photoionization remains accessible to only a few research groups. Here, we introduce the SYLOS Compact high-harmonic beamline at the Extreme Light Infrastructure Attosecond Light Pulse Source, specifically designed to provide the flux required for nonlinear optics in the XUV. We present a detailed characterization of the beam line demonstrating its capability to generate and utilize both intense attosecond pulse trains and isolated attosecond pulses. We further showcase the 2-XUV–photon double ionization of neon (Ne) and argon (Ar), achieved in a user campaign. The results underscore the beamline’s capability to support cutting-edge attosecond experiments and investigations of ultrafast electron dynamics on the attosecond scale.

We report on the development of a carrier-envelope phase (CEP)-stable 1030 nm fiber-based laser system producing 6.2 fs pulses achieved via the multi-pass cell (MPC) post-compression technique with 402 W average power at 100 kHz repetition rate. This system employs an upgraded three-stage MPC compression scheme exhibiting excellent beam quality properties for this intensity region. Active stabilization locks the CEP noise below 430 mrad root mean square. This work represents the first demonstration of a coherently combined fiber laser system simultaneously achieving such exceptional average power, CEP stability and sub-two-cycle pulse durations. Similar to all other light sources of the Extreme Light Infrastructure Attosecond Light Pulse Source, this newly developed system is accessible to the international research community in peer-reviewed open user calls of the Extreme Light Infrastructure European Research Infrastructure Consortium.

High-harmonic generation (HHG) in gaseous targets is the most widespread method to produce coherent extreme-ultraviolet (XUV) pulses with sub-femtosecond duration. However, this process has intrinsically low efficiency, and substantial research and development is devoted worldwide to increase the achievable photon flux through this highly nonlinear light–matter interaction process. In this work, we show the strong interplay of phase matching and absorption in gas-pressure gradients, substantially affecting macroscopic HHG efficiency. Through detailed experimental analysis and supporting numerical studies, we highlight their significance, particularly at the boundaries of the interaction volume. The concluded results have implications in the massively expanding application possibilities of HHG sources requiring high photon flux, for example in the semiconductor industry, in nanoscale imaging of biological and industrial samples, or in nonlinear optics in the XUV regime.

We demonstrate enhanced spectral broadening of femtosecond pulses (330 fs, 1030 nm, 5-200 kHz) in a hollow-core fiber (HCF) using CO2/helium gas mixtures. While increasing pure CO2 pressure broadens the spectrum and induces a Raman redshift, it causes substantial optical loss. Introducing a helium buffer enables strong broadening at reduced CO2 pressures, significantly decreasing loss. Propagating pulses through a 1-m-long, 300-μm-core HCF, we show that helium enhances spectral width, extends the Raman shift, and improves transmitted power by facilitating spatial cooling. This binary molecular/noble gas approach provides a robust, low-loss route for spectral engineering in HCFs.

The ELI ERIC facility offers international users ultrashort laser sources via the ALPS facility focusing on few-cycle laser drivers for attosecond pulses, particle beams and ultrahigh intensity interactions. The HR1 system supports attosecond high harmonic generation (HHG) and time-resolved spectroscopy at 100 kHz. However, its high repetition rate, while aiding statistical analysis, poses thermal challenges and limits certain applications requiring lower repetition rates. To address this, the HR Alignment laser system was developed for the HHG beamline at the ELI-ALPS facility. This new system delivers sub-6 fs, 1 mJ pulses with a tunable repetition rate (from 10 Hz to 10 kHz) and carrier-envelope phase (CEP) stabilization. It utilizes an ytterbium-doped potassium gadolinium tungstate front-end, multi-pass cell compression, and chirped mirrors. Characterization confirms excellent energy and CEP stability (below 300 mrad), beam quality and temporal contrast, matching the HR1 laser's performance. This compact, stable system provides high-flux attosecond generation for reaction microscopy enhancing ultrafast research in the ELI-ALPS facility.

Semiconductor crystals driven by strong mid-infrared pulses offer advantages for studying many-body physics and ultrafast optoelectronics via high-harmonic generation. While the process has been used to study solids in the presence strong mid-infrared fields, its potential as an attosecond light source is largely underexplored. We demonstrate that high-harmonics emitted from zinc-oxide crystals produce attosecond pulses, measured through spectroscopy of alkali metals. Using a cross-correlation approach, we photoionize Cesium atoms with vacuum-ultraviolet high-harmonics in the presence of a mid-infrared laser field. We observe oscillations in the photoelectron yield, originating from the instantaneous polarization of atoms by the laser field. The phase of these oscillations encodes the attosecond synchronization of the high-harmonics and is used for attosecond pulse metrology. This source opens new spectral windows for attosecond spectroscopy, enabling studies of bound-state dynamics in natural systems with low ionization energies, while facilitating the generation of non-classical entangled light states in the visible-VUV.

Attosecond time-resolution experiments using noncollinear interferometers require precise and active control of the optical delay to prevent instabilities - including both slow drifts and rapid vibrations - that can obscure the time evolution of the physical system under investigation. In this work, we present the design and results of stability measurements for a double interferometer setup for extreme ultraviolet-infrared pump-probe spectroscopy. The attosecond pump-probe setup is driven by a high-average-power, high-repetition-rate laser system and offers sub-optical-cycle (+/-81 as) stability with a fast feedback rate over extended periods (up to several days). Due to the noncollinear arrangement, the setup enables independent control of both amplitude and phase in the two arms even across significantly different spectral regions. As a proof of concept, we demonstrate attosecond beating in angle-resolved photoemission during two-photon, two-color photoionization, highlighting the broad potential of the system for kinematically and dynamically complete studies of atomic-scale light-matter interactions.

We report on the development of a carrier-envelope phase (CEP) stable 1030 nm fiber-based laser system producing 6.2 fs pulses achieved via the multi-pass cell (MPC) post-compression technique with 402 W average power at 100 kHz repetition rate. This system employs an upgraded three-stage MPC compression scheme exhibiting excellent beam quality properties for this intensity region. Active stabilization locks the CEP noise below 430 mrad RMS. This work represents the first demonstration of a coherently combined fiber laser system simultaneously achieving such exceptional average power, CEP stability, and sub-two-cycle pulse durations. Similarly to all other light sources of the Extreme Light Infrastructure Attosecond Light Pulse Source (ELI ALPS), this newly developed system is accessible to the international research community in peer-reviewed open user calls of Extreme Light Infrastructure European Research Infrastructure Consortium (ELI ERIC).

Strongly laser-driven semiconductor crystals offer substantial advantages for the study of many-body physics and ultrafast optoelectronics via the high harmonic generation process. While this phenomenon has been employed to investigate the dynamics of solids in the presence of strong laser fields, its potential to be utilized as an attosecond light source has remained unexploited. Here, we demonstrate that the high harmonics generated through the interaction of mid–infrared pulses with a ZnO crystal leads to the production of attosecond pulses, that can be used to trace the ultrafast ionization dynamics of alkali metals. In a cross–correlation approach, we photoionize Cesium atoms with the vacuum-ultraviolet (VUV) high-harmonics in the presence of a mid-infrared laser field. We observe strong oscillations of the photoelectron yield originating from the instantaneous polarization of the atoms by the laser field. The phase of the oscillations encodes the attosecond synchronization of the ionizing high-harmonics and is used for attosecond pulse metrology. This light source opens a new spectral window for attosecond spectroscopy, paving the way for studies of systems with low ionization potentials including neutral atoms, molecules and solids. Additionally, our results highlight the significance of the source for generating non–classical massively entangled light states in the visible–VUV spectral region.

We present a compact high-harmonic generation approach – termed as strongly overdriven regime – providing continuum radiation ranging from 18 to 140 eV albeit using long driving laser pulses, acting as a flexible and intense extreme-ultraviolet source.

We investigate high rep-rate operation and spatio-spectral challenges in the petawatt laser of ELI ALPS. User readiness, next steps and potential issues of development towards the final 2 PW 17 fs output are also discussed.

In recent years laser driven attosecond sources based on loose geometry high order harmonic generation reach focused intensities as high as to induce multi-photon multiple ionization or even strong-field effects in the extreme ultraviolet spectral range. In this chapter, we review four such sources developed through collaborative efforts between FORTH, ELI-ALPS and the University of Lund together with recent results obtained in the above mentioned topics utilizing these sources.

We compare multiple temporal pulse characterization techniques in three different pulse duration regimes from 15 fs to sub-5 fs, as there are no available standards yet for measuring such ultrashort pulses. To accomplish this, a versatile post-compression platform was developed, where the 100 fs near infrared pulses were post-compressed to the sub-two-cycle regime in a hybrid, three-stage configuration. After each stage, the duration of the compressed pulse was measured with the d-scan, TIPTOE and SRSI techniques and the retrieved temporal intensity profiles, spectrum and spectral phases were compared. Spectral homogeneity was also measured with an imaging spectrometer to understand the input coupling conditions of the temporal measurements. Our findings suggest that the different devices give similar results in terms of temporal intensity profile, however they are extremely sensitive to alignment and to beam quality, especially in the case of the shortest pulses. We address specific steps of measurement procedures, which paves the way towards the standardization of pulse characterization in the near future.

Monitoring the carrier-envelope phase (CEP) is of paramount importance for experiments involving few cycle intense laser fields. Common measurement techniques include f-2f interferometry or stereo-ATI setups. These approaches are adequate, but are challenging to implement on demand, at different locations as additional metrology tools, in intense few cycle laser-matter interaction experiments, such as those prevalent in sophisticated user beamlines. In addition there are inherent difficulties for CEP measured at non-conventional laser wavelengths (like e.g. mid infrared) and measurements above 10 kHz laser repetition rates, on single shot basis. Here we demonstrate both by simulations and by experiments a machine learning (ML) driven method for CEP estimation in the mid infrared, which is readily generalizable for any laser wavelength and possibly up to MHz repetition rates. The concept relies on the observation of the spectrum of high harmonic generation (HHG) in bulk material and the use of ML techniques to estimate the CEP of the laser. Once the ML model is trained, the method provides a way for cheap and compact real-time CEP tagging. This technique can complement the otherwise sophisticated monitoring of CEP, and is able to capture the complex correlation between the CEP and the observable HHG spectra.

We compare multiple temporal pulse characterization techniques in three different pulse duration regimes from 15 fs to sub-5 fs, as there are no available standards yet for measuring such ultrashort pulses. To accomplish this, a versatile post-compression platform was developed, where the 100 fs near infrared pulses were post-compressed to the sub-two-cycle regime in a hybrid, three-stage configuration. After each stage, the duration of the compressed pulse was measured with the d-scan, TIPTOE and SRSI techniques and the retrieved temporal intensity profiles, spectrum and spectral phases were compared. Spectral homogeneity was also measured with an imaging spectrometer to understand the input coupling conditions of the temporal measurements. Our findings suggest that the different devices give similar results in terms of temporal intensity profile, however they are extremely sensitive to alignment and to beam quality, especially in the case of the shortest pulses. We address specific steps of measurement procedures, which paves the way towards the standardization of pulse characterization in the near future.

We measured the duration of few-cycle pulses with different characterization techniques (d-scan, SRSI and TIPTOE) at four post-compression stages. We compared the retrieved temporal shapes, spectra and phases to see their reliability and limitations.

Advancements in light engineering have led to the creation of pulsed laser sources capable of delivering high-repetition-rate, high-power few-cycle laser pulses across a wide spectral range, enabling exploration of many fascinating nonlinear processes occurring in all states of matter. High-harmonic generation, one such process, which converts the low-frequency photons of the driver laser field into soft x-rays, has revolutionized atomic, molecular, and optical physics, leading to progress in attosecond science and ultrafast optoelectronics. The Extreme Light Infrastructure, Attosecond Light Pulse Source (ELI ALPS) facility pioneers state-of-the-art tools for research in these areas. This paper outlines the design rationale, capabilities, and applications of plasma- and gas-based high-repetition-rate (1 kHz to 100 kHz) attosecond extreme ultraviolet (XUV) beamlines developed at ELI ALPS, highlighting their potential for advancing various research fields.