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.

The deep ultraviolet (DUV) window (200-300 nm) is essential for the characterization of (bio)chemical and material systems through the UV signatures of nucleobases, amino acids, peptide bonds, many organic moieties, and wide bandgap transitions. However, extending ultrafast spectroscopy to the DUV to access the associated electronic and structural dynamics has largely remained elusive, due to the limited bandwidth and efficiency of common femtosecond DUV pulse sources. We now close this gap and demonstrate ultra-broadband femtosecond transient absorption (TA) spanning 200-800 nm, achieving unprecedented coverage of the entire DUV window. We generate supercontinuum probe pulses through soliton self-compression in a helium-filled hollow capillary fiber at a repetition rate of 20 kHz and fully suppress their high intrinsic intensity fluctuations via a correlation matrix referencing scheme. We thus achieve detector-noise-limited TA measurements with an exceptional resolution of 9 μOD in one second, and demonstrate these novel capabilities by resolving the ultrafast spin-crossover dynamics of a Fe(II) complex in the DUV. This work opens the path to unravel previously inaccessible photophysical and photochemical dynamics encoded in the DUV.

Understanding ultrafast molecular dynamics requires experimental tools capable of capturing coupled electronic and nuclear motion on their natural timescales. Here, we introduce a novel UV-XUV pump-probe beamline that achieves a temporal resolution of approximately 3 fs, which significantly surpasses the precision available in conventional time-resolved photoelectron spectroscopy (trPES) setups. This performance is enabled by the synergistic synchronization of ultrashort tunable UV pump pulses, generated via resonant dispersive-wave emission in gas-filled hollow capillary fibers, with attosecond XUV probe pulses. We demonstrate the power of this approach by applying it to the study of photoexcited acetylacetone. Our approach distinctly resolves two sequential passages through the S2/S1 conical intersection (CI), revealing dynamical features previously inaccessible to trPES. By establishing a new level of temporal precision in trPES, this beamline opens a new regime for the time-domain observation of coupled electronic-nuclear wavepacket evolution in complex molecular systems, which is pivotal for advancing ultrafast photochemistry and molecular quantum control. A novel UV-XUV pump-probe beamline is introduced, enabling time-resolved photoelectron spectroscopy with 3-femtosecond temporal resolution. This performance is achieved through synchronization of tunable ultrashort UV pump pulses with attosecond XUV probes, establishing a new regime for observing coupled electronic-nuclear dynamics in molecular systems.

Capturing chemical dynamics in real time is a central goal of ultrafast science, necessitating measurements fast enough to track atomic motion on few-femtosecond timescales with high precision. We present time-resolved hard X-ray scattering that meets these criteria by combining 7 fs full-width at half maximum (FWHM) near-infrared laser pulses with sub10 fs FWHM hard X-ray pulses from a free-electron laser. Using heavy water’s electronic response to strong-field ionization as a benchmark, we achieve a sub-8 fs FWHM instrument response function. These optical pump X-ray probe measurements enable direct observation of chemical dynamics with angstrom spatial and few-fs temporal resolution.

We demonstrate nonlinear compression of mid-infrared pulses from a Cr:ZnSe chirped-pulse amplifier using a gas-filled stretched hollow-core fiber followed by bulk-material compression. Starting from 90 fs, 2.45 µm pulses with 5.3 mJ energy, spectral broadening in the gas-filled capillary combined with optimized dispersion management enables compression to 15 fs, less than two optical cycles at 2.45 µm, with 3.3 mJ pulse energy, corresponding to a peak power of approximately 0.12 TW. The simplicity of the approach, based on a single hollow-core fiber stage and bulk dispersion compensation, makes it scalable to higher energies and establishes a robust route to mid-infrared drivers for high harmonic generation and attosecond applications.

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.

Achieving few-femtosecond resolution for a pump-probe experiment is crucial to measuring the fastest electron dynamics. As almost all molecules resonantly absorb radiation in the UV and/or visible, generating ultrashort pump pulses in this part of the optical spectrum has huge potential for probing excited state electronic dynamics while not ionising the molecule or material. However, traditional UV-Vis pump pulses cannot achieve few-fs durations and usually operate at fixed wavelengths. Almost every material and even air begin to be absorbent and highly dispersive in this region, making the generation, transport, and characterisation of ultrashort UV pulses extremely challenging. Technology to produce few-fs widely tuneable UV pulses was recently realised by the generation of resonant dispersive wave (RDW) emission in gas-filled hollow capillary fibres (HCF) [1]. RDW is a revolutionary new way to pump photochemical systems that overcome many limitations of traditional pumping regimes.

Deep ultraviolet light at 206 nm is generated with 22% pump-to-idler efficiency by four-wave mixing the third and fundamental harmonics of an ytterbium pulsed laser in an 11 µm core argon-filled antiresonant fiber.

Tunable ultrashort laser pulses across the near-ultraviolet to near-infrared with high peak power are crucial for wide-ranging applications in science and industry. Resonant dispersive-wave emission in gas-filled hollow-core fibers is a well-established technique for generating tunable ultrashort pulses from the vacuum ultraviolet to the near-infrared. However, previous demonstrations have relied on complex and expensive laser systems to provide the necessary energetic ultrashort pump pulses. Recent advances in fiber laser technology, particularly gain-managed nonlinear amplification, offer a promising alternative pump source. In this work, we combine gain-managed nonlinear amplification with resonant dispersive-wave emission to demonstrate a compact and tunable source of sub-20 fs pulses at 4.8 MHz. We achieve a tunable output spanning from 400 nm to beyond 700 nm, with energy up to 39 nJ, pulse duration down to 13 fs, and peak power exceeding 2 MW. This compact and efficient laser source opens new avenues for deploying resonant dispersive-wave-based technologies for broader scientific and industrial applications.

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.

Resonant dispersive wave (RDW) generation in hollow capillary fibers (HCFs) is a powerful technique for producing ultrashort light pulses in the deep ultraviolet range, which are important for ultrafast spectroscopy and material processing. However, the complex nonlinear dynamics governing this process and the large associated parameter space make it challenging to achieve optimal RDW pulses with the highest peak power. In this study, Bayesian optimization (BO) is coupled with the open source Luna.jl simulation framework to optimize the HCF and pump pulse paramters for less than 5 femtosecond (fs) RDW generation at a target wavelength of 200 nm. Temporally non-structured RDW were consistently identified with peak powers of up to 14 GW, exceeding experimentally published values by up to 70 %. Furthermore, a subset of the RDW optima exhibited an energy stability that is better than that of the pump pulse. Given that this approach can be generalized to other RDW wavelengths, our findings suggest that BO is a valuable tool in developing HCF systems that support RDW generation tailored to a particular experimental need.

The nature of the insulator-to-metal phase transition in vanadium dioxide (VO 2 ) is one of the longest-standing problems in condensed-matter physics. Ultrafast spectroscopy has long promised to determine whether the transition is primarily driven by the electronic or structural degree of freedom, but measurements to date have been stymied by their sensitivity to only one of these components and/or their limited temporal resolution. Here we use ultra-broadband few-femtosecond pump-probe spectroscopy to resolve the electronic and structural phase transitions in VO 2 at their fundamental time scales. Our experiments show that the system transforms into a bad-metallic phase within 10 fs after photoexcitation, but requires another 100 fs to complete the transition, during which we observe electronic oscillations and a partial re-opening of the bandgap, signalling a transient semi-metallic state. Comparisons with tensor-network simulations and density-functional theory calculations show these features result from an unexpectedly fast structural transition, in which the vanadium dimers separate and untwist with two different timescales. Our results resolve the structural and electronic nature of the light-induced phase transition in VO 2 and establish ultra-broadband few-femtosecond spectroscopy as a powerful tool for studying quantum materials out of equilibrium.

We demonstrate compression of few-cycle ultraviolet (UV) resonant dispersive waves (RDWs) generated in a cascaded hollow capillary fiber setup using a Yb laser system. Temporal characterization is performed using both tunneling ionization with a perturbation for the time-domain observation of an electric field (TIPTOE) and self-diffraction frequency-resolved optical gating (SD-FROG), which show good agreement. Through careful dispersion management, we compress the RDW pulse to 6.9 fs at a ∼390-nm central wavelength. This is the first, to our knowledge, measurement of an RDW using the TIPTOE method and demonstrates the viability of this technique to reliably characterize few-cycle UV pulses with μJ pulse energies.

We present the extreme soliton (XSOL) beamline, which generates terawatt-scale sub-femtosecond self-compressed visible-infrared pulses as well as sub-3 fs far-ultraviolet pulses with energy exceeding 170 μJ at 240 nm and wave- length tunability down to 140 nm.

We report the generation of a multi-octave supercontinuum spanning from 350 nm to 1700 nm with exceptional spectral flatness and high conversion efficiency to both visible and near-infrared regions, by pumping a methane-filled hollow-core antiresonant fiber with 1030 nm laser pulses. The dynamics exhibited signs of both modulational instability (MI) and stimulated Raman scattering (SRS). Fiber lengths ranging from 15 cm to 200 cm were investigated along with gas pressures up to 50 bar and pump pulse durations from 220 fs up to 10 ps. The best supercontinuum, in terms of spectral width and flatness, was achieved with 220 fs pulses, 25 bar filling pressure, and 60 cm propagation length. Comparison with argon-filled fiber with matched nonlinearity and dispersion showed that the Raman contribution enhances the supercontinuum generation process compared to a pure modulational instability-based process. The average power was scaled up by increasing the pulse repetition rate to 50 kHz, but further scaling was hindered by linear and nonlinear absorption, leading to fiber damage.

Optical sources exploiting resonant dispersive wave (RDW) emission are set to revolutionize ultrafast science. We demonstrate this approach by investigating excited state dynamics in morpholine using time-resolved photoelectron imaging. Excitation at 250 nm was achieved via RDW emission inside a helium-filled capillary fibre which, when combined with a short 800 nm probe, realized an instrument response of just 11 ± 2 fs. Two pathways initiate N–H bond fission: an extremely fast (<10 fs) process and a frustrated mechanism (380 fs) with hindered electronic ground state access. Photoelectron angular distributions also indicate average molecular geometry evolving on an intermediate (~100 fs) timescale. This clean distinction between population lifetimes and structural dynamics is enabled by the excellent temporal resolution inherent in RDW-based sources. Electronic structure and nonadiabatic surface hopping calculations support our data interpretation, and the synergy between experiment and theory is vital for developing a complete mechanistic picture.