Attosecond light sources based on high-order harmonic generation (HHG) constitute to date the only table-top solution for producing coherent broadband radiation covering the spectral range from the extreme ultraviolet to the soft x-rays. The so-called emission cutoff can be extended towards higher photon energies by increasing the driving wavelength at the expense of conversion efficiency. An alternative route is to overdrive the process by using higher laser intensities, with the challenging requirement of interacting with higher plasma densities over short propagation distances. Here, we address this challenge by exploiting free-space propagation in a differentially pumped glass chip designed for optimal gas confinement over sub-mm lengths. By driving HHG with multicycle pulses at either 800 nm or 1500 nm, we demonstrate a cutoff extension by a factor of two compared to conventional phase matching approaches and surpassing the present record using multicycle fields. Our three-dimensional propagation simulations, in excellent agreement with the experiment, confirm that gas confinement is crucial since efficient phase matching of cutoff harmonics occurs only for short propagation lengths. Additionally, we show that the high photon energy component is not only temporally confined to the leading edge of the driving pulse, but also spatially confined in the near-field to an off-axis contribution due to reshaping of the driving field along propagation inside the medium. Our findings contribute to the fundamental understanding of HHG across different regimes.

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.

Modulating electron-phonon coupling offers a route to control structural displacements and tune material functionality. Valence-to-conduction band transitions, however, provide limited leverage over the driving force. Here, we demonstrate coherent lattice dynamics in trigonal tellurium using free-electron laser pulses tuned to the Te N4,5-edge. Over a broad fluence range, the oscillation amplitude obeys the displacive excitation of coherent phonons framework, extended to core resonance with twice the driving efficiency of a visible pump. Ab initio calculations decompose the force into competing multiband contributions, inaccessible to optical excitation, whose balance shifts as carriers relax. Tunable extreme-ultraviolet and X-ray pulses thus open a regime in which the displacive response is set by band-dependent coupling to the lattice, not by the number and temperature of the photocarriers alone.

Second-harmonic generation (SHG) is a widely used nonlinear optical process that has been optimized for high frequency conversion efficiency, broad bandwidths, and optimum spatial and temporal quality of the generated light. However, limitations of the generation efficiency arise due to temporal dispersion and pulse walk-off effects introduced by the nonlinear crystal. To circumvent these limitations, quasi-waveguide schemes, in particular multi-pass cells (MPCs), can be employed, providing flexible phase-tuning capabilities. In this work, we demonstrate for the first time broadband and efficient SHG in an MPC. SHG efficiencies of 72

Q-switched lasers are compact, cost-effective, and highly pulse energy-scalable sources for nanosecond-scale laser pulses. The technology has been developed for many decades and is widely used in scientific, industrial and medical applications. However, their inherently narrow bandwidth imposes a lower limit on pulse duration-typically in the few-hundred-picosecond range-limiting the applicability of Q-switched technology in fields that require ultrafast laser pulses in the few-picosecond or femtosecond regime. In contrast, mode-locked lasers can produce broad-band, ultrafast ( <1 ps) pulses, but are complex, expensive, and typically require a large footprint. To bridge the parameter gap between these two laser platforms-in terms of pulse duration and achievable peak power-we here propose a Herriott-type multi-pass cell (MPC) based post-compression scheme for shortening the pulse durations of Q-switched lasers down to the ultrafast, picosecond regime. We experimentally demonstrate post-compression of 0.5 ns, 1 mJ pulses from a Q-switched laser to 24 ps using a compact glass-rod MPC for spectral broadening. We verify this result numerically and show that compression down to a few picoseconds is possible using the nanosecond MPC (nMPC). Through spectral filtering approaches, the nMPC suppresses detrimental nonlinear processes such as stimulated Raman scattering, which have set severe limitations for fiber-based post-compression of Q-switched lasers until today. Our results pave the way to cost-efficient and compact ultrafast laser platforms based on Q-switched laser technology.

Water’s polarity and hydrogen-bond network give rise to its unique chemical and biochemical behaviour. Its vibrational motions, occurring on a few-femtosecond timescale, govern ultrafast energy transfer within the hydrogen-bond network. However, direct real-time observation of these motions has remained elusive due to the extreme temporal resolution required. Here, we investigate the ground-state vibrational dynamics of liquid water initiated by a sub-5 fs near-infrared (NIR) pump pulse via Impulsive Stimulated Raman Scattering (ISRS). Using few-fs ultraviolet (UV) probe pulses transmitted through a 5 µm-thick liquid jet, we monitor the coherent vibrational wave packet dominated by the OH stretch mode, exhibiting a 10 fs oscillation period and a 25 fs damping time. These results reveal the rapid dephasing of the OH stretch mode preceding its relaxation through coupling to the bending vibrations, highlighting the importance of intermolecular couplings of liquid water in the high frequency vibrational dynamics. Water vibrational motion, which occurs on the few-femtosecond timescale and underpins energy transfer within the hydrogen bonding network, has remained challenging to observe in real time due to constraints in time resolution. Here, the authors investigate the ground state vibrational dynamics of liquid water using a sub-5 fs near-infrared pump pulse and few-fs ultraviolet probe pulses, observing rapid dephasing of the OH stretch mode that precedes its relaxation via coupling to the bend modes.

The photophysics of nucleobases has been the subject of both theoretical and experimental studies over the past decades due to the challenges posed by resolving the steps of their radiationless relaxation dynamics, which cannot be described in the framework of the Born-Oppenheimer approximation (BOA). In this context, the ultrafast dynamics of 2-thiouracil has been investigated with a time-resolved NEXAFS study at the Free Electron Laser FLASH. Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS) can be used to observe electronic transitions in ultrafast molecular relaxation. We performed time-resolved UV-pump/X-ray probe absorption measurements at the sulfur 2s (L1) and 2p (L2/3) edges. We are able to identify absorption features corresponding to the S2 (ππ*) and S1 (nπ*) electronic states. We observe a delay of 102 ± 11 fs in the population of the nπ* state with respect to the initial optical excitation and interpret the delay as the time scale for the S2 → S1 internal conversion. We furthermore identify oscillations in the absorption signal that match a similar observation in our previous X-ray photoelectron spectroscopy study on the same molecule.

In the context of ultrafast spectroscopy, the availability of few-femtosecond UV pulses is key to disclose the role of electron dynamics in photo-activated biochemically relevant processes. Here, we present an optical setup for the generation of UV pulses spectrally tunable between 270 and 350 nm with transform limited durations of 3.0 and 2.9 fs, respectively, and repetition rates up to 50 kHz, using resonant dispersive wave (RDW) emission in an argon-filled hollow-core fiber. The RDW emission is driven by 1030 nm sub-20 fs pulses produced by post-compressing an Yb-based laser with a dispersion-engineered multi-pass cell (MPC). The combination of an MPC with a capillary constitutes a compact source to deliver few-femtosecond UV pulses at high-repetition rates, which is ideal for statistically demanding experiments in molecular physics.

Ultrafast molecular phenomena, such as inter- and intramolecular energy transfer, play a pivotal role in determining the final functionality of photoactive biochemically-relevant systems. Photosynthesis is a notable example [1]. Understanding how the natural environment influences the light-induced ultrafast dynamics requires to study the molecules in solution [2]. Our goal is to experimentally examine molecules embedded in water clusters on the femtosecond timescale through time resolved XUV photoelectron spectroscopy [3]. However, the diluted nature of the target samples, leading to low statistics, calls for XUV sources operating in the multi-kHz regime.

Few-femtosecond ultraviolet (UV) light pulses are required for the real-time monitoring of electron dynamics occurring in electronically-excited neutral molecules [1]. We report the design and implementation of a compact, integrated microfluidic chip for broadband UV generation. The chip features a differentially pumped glass cell (Fig. 1a) that enhances gas confinement and improves the conversion efficiency of third harmonic generation driven by few-cycle temporally-reshaped near-infrared (NIR) pulses [2]. The resulting UV pulses (typical spectra are shown in Fig. 1b and 1c) were subsequently combined with few-femtosecond near-infrared (NIR) probe pulses to study the ultrafast excited-state dynamics of acetone. Previous studies have shown that resonant excitation to the Rydberg states of acetone via a 2-UV photon transition is followed by ultrafast relaxation via non-adiabatic crossings between the electronic states [3]. However, these studies have lacked the temporal resolution to fully resolve the dynamics occurring on a sub-10 fs timescale. Here, we take advantage of the unprecedented resolution offered by the UV source described above to monitor the ultrafast dynamics of (excited) acetone. Data were acquired using a double-sided spectrometer capable of simultaneous mass spectrometry and electron velocity map imaging.

Extreme light confinement in plasmonic nanosystems enables novel applications in photonics, sensor technology, energy harvesting, biology, and quantum information processing. Fullerenes represent an extreme case for nanoplasmonics: They are subnanometer carbon-based molecules showing high-energy and ultrabroad plasmon resonances; however, the fundamental mechanisms driving the plasmonic response and the corresponding collective electron dynamics are still elusive. Here, we uncover the dominant role of electron correlations in the dynamics of the giant plasmon resonance (GPR) of the subnanometer system C 60 by using attosecond photoemission chronoscopy. We find a characteristic photoemission delay of up to about 300 attoseconds that is purely induced by coherent large-scale electron correlations in the plasmonic potential. These results provide insights into the nature of the plasmon resonances in subnanometer systems and open perspectives for advancing nanoplasmonic applications.

Free-electron lasers (FELs) are the world's most brilliant light sources with rapidly evolving technological capabilities in terms of ultrabright and ultrashort pulses over a large range of accessible photon energies. Their revolutionary and innovative developments have opened new fields of science regarding nonlinear light-matter interaction, the investigation of ultrafast processes from specific observer sites, and approaches to imaging matter with atomic resolution. A core aspect of FEL science is the study of isolated and prototypical systems in the gas phase with the possibility of addressing well-defined electronic transitions or particular atomic sites in molecules. Notably for polarization-controlled short-wavelength FELs, the gas phase offers new avenues for investigations of nonlinear and ultrafast phenomena in spin orientated systems, for decoding the function of the chiral building blocks of life as well as steering reactions and particle emission dynamics in otherwise inaccessible ways. This roadmap comprises descriptions of technological capabilities of facilities worldwide, innovative diagnostics and instrumentation, as well as recent scientific highlights, novel methodology and mathematical modeling. The experimental and theoretical landscape of using polarization controllable FELs for dichroic light-matter interaction in the gas phase will be discussed and comprehensively outlined to stimulate and strengthen global collaborative efforts of all disciplines.

Electron scattering cross sections are essential for the quantitative description of electron-induced processes in nanoconfined aqueous systems. Here we report on energy- and size-dependent cross sections for water clusters with diameters between '1.0 and 2.5 nm and electron energies below '32 eV. The cross sections of the clusters are up to a factor of 2 larger than those of water, suggesting a reduced dielectric screening in nanoconfinement. The cross sections show no significant cluster size dependence. A slight increase of cross sections with increasing electron energy is attributed to the increasing contribution from electronic scattering channels. As the simplest form of nanoconfined water, nanoscale water clusters represent an important model system.

The interaction between nuclear and electronic states plays a fundamental role in many nuclear processes. While such nucleus-electron couplings have been extensively studied using nuclear physics methodologies, recent advancements in light sources have introduced novel schemes for nuclear photoexcitation. This breakthrough enables a new class of experiments that could revolutionize our understanding and control of nuclear states. In this work, we review the research on electron processes associated with nuclear transitions, and the light sources that have been used for nuclear photoexcitation. Furthermore, we explore the potential for establishing time-resolved spectroscopy of electron-nucleus couplings, drawing connections to state-of-the-art light sources and time-resolved studies in atomic and molecular physics. By addressing both challenges and opportunities, we aim to provide a roadmap for future research in this interdisciplinary field. This emerging platform, linking nuclear science, atomic physics, and photonics, holds great promise for driving groundbreaking scientific and technological advancements.

Ultraviolet (UV) radiation plays a pivotal role in initiating photochemical and photobiological processes. However, capturing the initial stages of molecular dynamics that occur immediately after UV light absorption presents a significant challenge. This difficulty arises from the limited availability of UV-pumped time-resolved setups that can achieve the necessary few-femtosecond resolution. [1]. In this work, we present the first combined experimental and theoretical study of the ultrafast dissociation dynamics of the benchmark molecule iodomethane (CH3I) performed with few-fs UV pulses. In CH3I, a single UV-photon transition leads to a rapid cleavage of the C-I bond [2]. Furthermore, the presence of a conical intersection (CI) near the Franck-Condon (FC) region makes iodomethane a popular test case for investigating the effects of UV-induced non-adiabatic crossings at ultrafast timescales.

We report on a new versatile transportable endstation for controlled molecule (eCOMO) experiments providing a combination of molecular beam purification by electrostatic deflection and simultaneous ion and electron detection using velocity-map imaging (VMI). The b-type electrostatic deflector provides spatial dispersion of species based on their effective-dipole-moment-to-mass ratio. This enables selective investigation of molecular rotational quantum states, conformers, and molecular clusters. Furthermore, the double-sided VMI spectrometer equipped with two high-temporal-resolution event-driven Timepix3 cameras provides detection of all generated ions independently of their mass-over-charge ratio and electrons. To demonstrate the potential of this novel apparatus, we present experimental results from our investigation of carbonyl sulfide (OCS) after ionization. In particular, we provide the characterization of the molecular beam, electrostatic deflector, and electron- and ion-VMI spectrometer. The eCOMO endstation delivers a platform for ultrafast dynamics studies using a wide range of light sources from table-top lasers to free-electron-laser and synchrotron-radiation facilities. This makes it suitable for research activities spanning from atomic, molecular, and cluster physics, over energy science and chemistry, to structural biology.

The two most notable methods for producing short laser pulses are mode-locked and Q-switched lasers. While mode-locked lasers generate pulses in the pico- (ps) to femtosecond (fs) range, Q-switched lasers typically produce pulses at the nanosecond (ns) or 100 ps level [1]. Thus, mode-locking has been the superior technology when ultrashort pulses or high peak powers are required. On the other hand, Q-switched lasers can have great advantages owing to their simplicity, compactness, low costs, and the large pulse energies that can be generated [2]. To bridge the parameter gap between mode-locked and Q-switched lasers, pulse post-compression appears attractive. One possible method is self-phase-modulation (SPM) in optical fibers [3]. However, stimulated-Raman-scattering limits the possible peak powers to far below 1 kW and thus pulse energies to lower than μJ level for ns pulses [3]. Another SPM-based method are Multi-pass cells (MPCs), which have become a standard technology for post-compression of mode-locked lasers [4]. However, MPCs have not yet been used to compress Q-switched laser pulses due to their inherently low peak powers. Here, we demonstrate a new MPC type, facilitating post-compression of pulses with order-of-magnitude lower peak power than typical bulk-MPCs [4]. We use a fused silica block as the nonlinear medium and compress 1 mJ pulses from the nanosecond down to the picosecond regime.

A microfluidic chip integrating a gas cell and two lateral differential pumping stages is used to generate ultraviolet supercontinua via third-harmonic generation in neon or argon. Spectra spanning from 200 to 325 nm are obtained with pulse energies up to ~0.8 μJ, corresponding to 0.2% conversion efficiency.