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.

At laser wavelengths longer than the prominent 1 μm range of high-power ytterbium-doped lasers, nonlinear phase shifts produced in nonlinear media for spectral broadening and subsequent pulse compression decrease drastically. Consequently, at the 2 μm wavelength range, the threshold of the applicable peak power for pulse compression in gas-filled multipass cells increases. The common approach of choosing a Herriott multipass cell configuration close to the concentric resonator does not necessarily lead to the highest total nonlinear phase shift, due to a restriction of the total number of reflections on the cell mirrors of a given size. Therefore, an analytical approach is presented here to maximize the nonlinear phase shift for a given set of mirrors, considering lossless and dispersionless propagation. Furthermore, to achieve pulse compression with gas-filled multipass cells for relatively low peak powers at wavelengths around 2 μm, we developed a high-pressure gas cell and demonstrated experimentally pulse compression in the negative- and positive dispersion regimes, with achieved pulse durations of around 40 fs and 55 fs respectively.

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.

Incoherent Diffraction Imaging - IDI - is a diffraction-based imaging technique that has been recently proposed to exploit the partial coherence of incoherently scattered light to retrieve structural information from the scattering centers. Similar to the stellar intensity interferometry of Hanbury Brown and Twiss, the signal builds up on the second-order spatial correlations of the emitted light. The complex spatial distribution of the target is thereby encoded in the spatial intensity fluctuations of the scattered light. The first experimental realisations of this imaging technique have been realised using the fluorescence excited by an ultra-short X-ray pulse at Free Electron Laser (FEL) facilities. Here, we propose an alternative set-up based on a table-top Pseudo-Thermal Light Source. This set-up allows us to explore IDI under a wide range of physically relevant conditions as well as to benchmark numerical and analytical models currently used to determine the imaging capabilities of this technique.

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.

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.

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.

We describe a beamline where few-femtosecond ultraviolet (UV) pulses are generated and synchronized to few-cycle near-infrared (NIR) and extreme ultraviolet (XUV) attosecond pulses. The UV light is obtained via third-harmonic generation in argon or neon gas when focusing a phase-stabilized NIR driving field inside a glass cell that was designed to support high pressures for enhanced conversion efficiency. A recirculation system allows reducing the large gas consumption required for the nonlinear process. Isolated attosecond pulses are generated using the polarization gating technique, and the photon spectrometer employed to characterize the XUV radiation consists of a new design based on the combination of a spherical varied-line-space grating and a cylindrical mirror. This design allows for compactness while providing a long entrance arm for integrating different experimental chambers. The entire interferometer is built under vacuum to prevent both absorption of the XUV light and dispersion of the UV pulses, and it is actively stabilized to ensure an attosecond delay stability during experiments. This table-top source has been realized with the aim of investigating UV-induced electron dynamics in neutral states of bio-relevant molecules, but it also offers the possibility to implement a manifold of novel time-resolved experiments based on photo-ionization/excitation of gaseous and liquid targets by ultraviolet radiation. UV pump-XUV probe measurements in ethyl-iodide showcase the capabilities of the attosecond beamline.

Ultraviolet (UV) light that penetrates our atmosphere initiates various photochemical and photobiological processes. However, the absence of extremely short UV pulses has so far hindered our ability to fully capture the mechanisms at the very early stages of such processes. This is important because the concerted motion of electrons and nuclei in the first few femtoseconds often determines molecular reactivity. Here we investigate the dissociative dynamics of iodomethane following UV photoexcitation, utilizing mass spectrometry with a 5 fs time resolution. The short duration of the UV pump pulse (4.2 fs) allows the ultrafast dynamics to be investigated in the absence of any external field, from well before any significant vibrational displacement occurs until dissociation has taken place. The experimental results combined with semi-classical trajectory calculations provide the identification of the main dissociation channels and indirectly reveal the signature of a conical intersection in the time-dependent yield of the iodine ion. Furthermore, we demonstrate that the UV-induced breakage of the C-I bond can be prevented when the molecule is ionized by the probe pulse within 5 fs after the UV excitation, showcasing an ultrafast stabilization scheme against dissociation. The authors investigate the ultrafast dissociative dynamics of iodomethane induced by sudden ultraviolet photoexcitation. They showcase how subsequent ionization within a 5-femtosecond window can stabilize the molecule and prevent fragmentation.

This work presents experimental results on High-order Harmonic Generation in a sub-mm glass cell using long driving pulses. The results show a remarkable extension of the cutoff explicable in the framework of the nonadiabatic regime.

Chiral molecules, used in applications such as enantioselective photocatalysis1, circularly polarized light detection2 and emission3 and molecular switches4,5, exist in two geometrical configurations that are non-superimposable mirror images of each other. These so-called (R) and (S) enantiomers exhibit different physical and chemical properties when interacting with other chiral entities. Attosecond technology might enable influence over such interactions, given that it can probe and even direct electron motion within molecules on the intrinsic electronic timescale6 and thereby control reactivity7-9. Electron currents in photoexcited chiral molecules have indeed been predicted to enable enantiosensitive molecular orientation10, but electron-driven chiral dynamics in neutral molecules have not yet been demonstrated owing to the lack of ultrashort, non-ionizing and perturbative light pulses. Here we use time-resolved photoelectron circular dichroism (TR-PECD)11-15 with an unprecedented temporal resolution of 2.9 fs to map the coherent electronic motion initiated by ultraviolet (UV) excitation of neutral chiral molecules. We find that electronic beatings between Rydberg states lead to periodic modulations of the chiroptical response on the few-femtosecond timescale, showing a sign inversion in less than 10 fs. Calculations validate this and also confirm that the combination of the photoinduced chiral current with a circularly polarized probe pulse realizes an enantioselective filter of molecular orientations following photoionization. We anticipate that our approach will enable further investigations of ultrafast electron dynamics in chiral systems and reveal a route towards enantiosensitive charge-directed reactivity.

The response of metal nanostructures to optical excitation leads to localized surface plasmon (LSP) generation with nanoscale field confinement driving applications in, for example, quantum optics and nanophotonics. Field sampling in the terahertz domain has had a tremendous impact on the ability to trace such collective excitations. Here, we extend such capabilities and introduce direct sampling of LSPs in a more relevant petahertz domain. The method allows to measure the LSP field in arbitrary nanostructures with subcycle precision. We demonstrate the technique for colloidal nanoparticles and compare the results to finite-difference time-domain calculations, which show that the build-up and dephasing of the plasmonic excitation can be resolved. Furthermore, we observe a reshaping of the spectral phase of the few-cycle pulse, and we demonstrate ad-hoc pulse shaping by tailoring the plasmonic sample. The methodology can be extended to single nanosystems and applied in exploring subcycle, attosecond phenomena.