We report on a multimode laser-diode (LD) pumped 4.5-GHz Kerr-lens mode-locked laser, which can directly deliver 360 fs pulses with an average power of over 3.7 W.

High-order harmonics were generated from mono- and polycrystaline molybdenum disulfide (MoS2) monolayers with an infrared femtosecond pulse. We control the Orbital Angular Momentum (OAM) and spatial polarization distribution of the generation beam by using a liquid crystal Q-plate. We then measure the OAM and the full polarization map of the emitted harmonics. We observe that monocrystaline MoS2 behaves as a polarization converter, while polycrystaline MoS2 may be used as a phase mask.

Owing to their high mobility and immunity to topological deflection, skyrmions in antiferromagnetic (AFM) systems are gaining attention as a potential solution for next-generation magnetic data storage. Synthetic antiferromagnets (SAFs) offer a promising avenue to tune the properties of the individual magnetic layers, facilitating the conditions necessary for skyrmions to be used in practical devices. Despite recent advancements achieving fast skyrmion mobility, the nucleation of small and rigid circular skyrmions without an external field remains challenging in SAFs. Theoretical predictions suggest that optical vortex (OAM) beams can stabilize skyrmionic spin textures by transferring their spin and orbital angular momentum to the magnetic material. Here, this intriguing proposal is delved into and the creation of sub-50 nm compact skyrmions in SAFs using OAM beams is successfully demonstrated, eliminating the need for external magnetic fields. Additionally, the results underscore the importance of beam energy and the number of pulses, as both factors play critical roles in the stabilization of these AFM skyrmionic textures. This breakthrough is significant as it paves the way for stabilizing true zero-field skyrmions in AFM systems, where magnetization is minimally affected by external magnetic fields. This work will open a potential avenue for stabilizing small, compact skyrmions in antiferroic systems, facilitating their implementation in logic and memory devices. Skyrmions in antiferromagnets are attracting interest for next-gen magnetic data storage due to their high mobility and immunity to topological deflection. Despite advancements in skyrmion mobility, nucleating small, rigid skyrmions without an external field remains challenging in SAFs. Here it is demonstrated that optical vortex beams can stabilize skyrmionic textures, creating sub-50 nm skyrmions in SAFs without external fields. image

We demonstrate on a high-power 1-GHz Kerr-lens mode-locked (KLM) ytterbium (Yb):CYA laser delivering 149-fs pulses with an average power of 11.1 W. The corresponding single-pulse energy and peak power are 10.3 nJ and 60.8 kW, respectively. The mode-locking operation can be consistently sustained with root mean square (RMS) values of power fluctuations of only 0.52% for 100 min. To the best of our knowledge, this is the highest average power ever reported from a GHz femtosecond mode-locked oscillator.

We present the experimental realization of spectrally tunable, ultrashort, quasi-monochromatic extreme ultraviolet (XUV) pulses generated at 100 kHz repetition rate in a user-oriented gas high harmonic generation (GHHG) beamline of the Extreme Light Infrastructure - Attosecond Light Pulse Source (ELI ALPS) facility. Versatile spectral and temporal shaping of the XUV pulses are accomplished with a double-grating, time-delay compensated monochromator accommodating the two composing stages in a novel, asymmetrical geometry. This configuration supports the achievement of high monochromatic XUV flux (2.8+/-0.9*1e10 photons/s at 39.7 eV selected with 700 meV FWHM bandwidth) combined with ultrashort pulse duration (4.0+/-0.2 fs using 12.1+/-0.6 fs driving pulses) and small spot size (sub-100 um). Focusability, spectral bandwidth, and overall photon flux of the produced radiation were investigated covering a wide range of instrumental configurations. Moreover, complete temporal (intensity and phase) characterization of the few-femtosecond monochromatic XUV pulses - a goal that is difficult to achieve by conventional reconstruction techniques - has been realized using ptychographic algorithm on experimentally recorded XUV-IR pump-probe traces. The presented results contribute to in-situ, time-resolved experiments accessing direct information on the electronic structure dynamics of novel target materials.

We report the first demonstration of time-resolved X-ray absorption spectroscopy to track previously undetected photoinduced dynamics of a paradigmatic crystalline conjugated polymer: poly(3-hexylthiophene) (P3HT) commonly used in solar cell devices. The pi to pi* transition, the first step of solar energy conversion, is pumped with a 15 fs optical pulse and the dynamics are probed by an attosecond soft X-ray pulse at the carbon K-edge. We observe direct spectroscopic signatures of the initially hot excitonic state, which is delocalized over multiple polymer chains, undergoing a rapid evolution on a sub 50 fs timescale which can be directly associated with cooling and localization to form the lowest excitonic state on a single polymer chain. This sensitivity of time-resolved X-ray spectroscopy to the primary electron dynamics occurring directly after excitation paves the way for new insights in a wide range of organic optoelectronic materials.

High-repetition rate attosecond pulse sources are indispensable tools for time-resolved studies of electron dynamics, such as coincidence spectroscopy and experiments with high demands on statistics or signal-to-noise ratio, especially in the case of solid and big molecule samples in chemistry and biology. Although with the high-repetition rate lasers, such attosecond pulses in a pump-probe configuration are possible to achieve, until now, only a few such light sources have been demonstrated. Here, by shaping the driving laser to an annular beam, a 100 kHz attosecond pulse train (APT) is reported with the highest energy so far (51 pJ/shot) on target (269 pJ at generation) among the high-repetition rate systems (>10 kHz) in which the attosecond pulses were temporally characterized. The on-target pulse energy is maximized by reducing the losses from the reflections and filtering of the high harmonics, and an unprecedented 19% transmission rate from the generation point to the target position is achieved. At the same time, the probe beam is also annular and low loss of this beam is reached by using another holey mirror to combine with the APT. The advantages of using an annular beam to generate attosecond pulses with a high-average power laser are demonstrated experimentally and theoretically. The effect of nonlinear propagation in the generation medium on the annular-beam generation concept is also analyzed in detail.

We present current status and recent experiments carried out with the attosecond beamlines of Extreme Light Infrastructure Attosecond Light Pulse Source driven by the 100 kHz, high-repetition-rate (HR-1) laser system.

We present the design and implementation of a new, modular gas target suitable for high-order harmonic generation using high average power lasers. To ensure thermal stability in this high heat load environment, we implement an appropriate liquid cooling system. The system can be used in multiple-cell configurations, allowing us to control the cell length and aperture size. The cell design was optimized with heat and flow simulations for thermal characteristics, vacuum compatibility, and generation medium properties. Finally, the cell system was experimentally validated by conducting high-order harmonic generation measurements using the 100 kHz high average power HR-1 laser system at the Extreme Light Infrastructure Attosecond Light Pulse Source (ELI ALPS) facility. Such a robust, versatile, and stackable gas cell arrangement can easily be adapted to different experimental geometries in both table-top laboratory systems and user-oriented facilities, such as ELI ALPS.

The occupied and unoccupied electronic states of MoS2 monolayer isolated flake were studied using laboratory based photoemission electron microscope (PEEM) nanoESCA equipped with He-I photon source. PEEM real-space imaging allowed selecting the high quality flake. Altogether, the data will allow accurately recovering the band structures of MoS2. The band structures will be used in future pump-probe experiments to explore the dynamics of electrons in the conduction band and photo-induced multitopological states using trefoil polarization.

Attosecond pulses were widely utilized to investigate ultra-fast phenomena in the natural time scale of electron motion - attosecond (10 −18 s) - since its first demonstrations 20 years ago [1] , [2] . However, the repetition rate of most attosecond sources were limited to several kHz, which hindered many applications such as the photoelectron emission from solids where the space charge effect is needed to be alleviated, and the statistic measurements where less events every laser shot are preferred. Hence, to achieve high signal-to-noise ratio, the only method is to increase the repetition rate. In addition, the substantially shortened time for data acquisition is beneficial for most experiments. Thanks to the high-repetition-rate (HR) laser, HR attosecond pulses are possible now.

In the literature it is well known that the produced high-order harmonics inherit certain properties from the driving laser especially in terms of spectral characteristics [1] , therefore introducing modulation in the generating laser spectrum we can have significant impact on the high harmonic generation process [2] , [3] .

We propose a technique to control the spectral characteristics of high-harmonic sources. The method is easily implementable in any attosecond-pulse generation beamline, regardless of the driving laser, thereby satisfying different experimental needs.

π -conjugated semiconducting polymers have the potential to provide low cost, flexible and thin optoelectronic devices such as light-emitting diodes, field effect transistors and solar cells. The ultrafast dynamics of tightly bound, Frenkel excitons in these materials are crucial for the performance of these devices [1] , [2] . In this work we demonstrate the first application of soft X-ray transient absorption spectroscopy to study exciton dynamics in an organic semiconducting polymer, poly(3-hexylthiophene) (P3HT). The samples are pumped with a ∼ 15 fs optical pulse resonant with the π → π * transition at 2.25 eV, relevant to solar energy conversion. The probe is a soft X-ray supercontinuum extending to 350 eV with attosecond pulse duration generated via high harmonic generation [3] . Figure 1a shows the transient absorption spectrum of P3HT in the vicinity of the carbon K edge. At all positive time delays a blue shift in the absorption edge is observed giving rise to the strong increase in absorption at 286.25 eV. At short time delays this is accompanied by a weaker positive differential absorption feature 1.2 eV below the edge. The low energy feature is highly transient and decays with a lifetime of 16 8 fs. With the support of TDDFT simulations ( Fig. 1b ), we associate the spectral blue shift with the localised ± singlet exciton and the low energy transient feature as a direct spectroscopic signature of delocalisation of the exciton between polymer chains. Our results suggest that the photoexcited H-aggregate type state undergoes rapid localisation on a sub 50 fs timescale consistent with theoretical predictions for this polymer [4] .

We report the generation of 50 pJ attosecond pulse trains at 100-kHz using a high average-power annular laser beam, which is the highest one until now among systems of repetition rate higher than 10 kHz.

We generate high-order harmonics in a gaseous medium with tunable photon energy using time-domain interferometry of double pulses in a noncollinear generation geometry. The method is based on the fact that the generated harmonics inherit certain spectral properties of the driving laser. The two temporally delayed laser pulses, identical in all parameters, are produced by a custom-made split-and-delay unit utilizing wave-front splitting without a significant energy loss. The arrangement is easy to implement in any attosecond pulse generation beamline, and is suitable for the production of an extreme ultraviolet source with simply and quickly variable central photon energy, useful for a broad range of applications.

We investigate the electron quantum path interference (QPI) effects during high harmonic generation in atomic gas medium driven by ultrashort chirped laser pulses. To achieve that, we identify and vary the different experimentally relevant control parameters of such a driving laser pulse influencing the high harmonic spectra. Specifically, the impact of the pulse duration (from the few-cycle to the multi-cycle domain), peak intensity and instantaneous frequency is studied in a self-consistent manner. Simulations involving macroscopic propagation effects are also considered. The study aims to reveal the microscopic background behind a variety of interference patterns capturing important information both about the fundamental laser field and the generation process itself. The results provide guidance towards experiments with chirp control as a tool to unravel, explain and utilize the rich and complex interplay between QPIs including the tuning of the periodicity of the intensity dependent oscillation of the harmonic signal, and the curvature of spectrally resolved Maker fringes.

We generate attosecond pulse train (APT) in argon driven by the high repetition rate (HR) laser of the extreme light infrastructure-attosecond light pulse source (ELI-ALPS), providing 100 kHz, 80 W, 1030 nm, 40 fs pulses from a fiber chirped-pulse amplification (fiber-CPA) laser system. Under the current operating conditions of the high harmonic generation beamline (HR-GHHG), we observed the average pulse duration to be 395 as measured using the technique of reconstruction of attosecond beating by interference of two-photon transitions. The beamline uses an annular-shape laser beam so that the main part of the driving laser co-propagating with the APT can be eliminated by reflection on a holey mirror. An additional 100 nm aluminum foil is used to filter out the remaining laser and the low order harmonics, allowing 2 pJ APT with a bandwidth from 25 eV to 50 eV to be transported to the target position where the APT interacts with matter. The implementation of the HR-GHHG beamline in ELI-ALPS delivering attosecond pulse trains at 100 kHz paves the way for time-resolved experiments in the infrastructure, especially those that involve rare events and coincidence analysis, both of which need high statistics.

This work theoretically investigates high-order harmonic generation in rare-gas atoms driven by two temporally delayed ultrashort laser pulses. Apart from their temporal delay, the two pulses are identical. Using a single-atom model of the laser-matter interaction it is shown that the photon energy of the generated harmonics is controllable within the range of one eV-a bandwidth comparable to the photon energy of the fundamental field-by varying the time delay between the generating laser pulses. It is also demonstrated that high-order harmonics generated by double pulses have advantageous characteristics, which mimick certain properties of an extreme ultraviolet monochromator. With the proposed method, a simpler setup at a much lower cost and comparatively higher spectral yield can be implemented in contrast to other approaches.

The reconstruction of ultrashort optical pulses with a complex intensity substructure is demonstrated using the Self-Referenced Spectral Interferometry (SRSI) pulse characterization technique with a modified phase retrieval algorithm. A correction spectral phase term is extracted by the manipulation of the temporal interferogram, allowing the treatment of scenarios with complicated pulse shapes, where the original algorithm fails. The improved SRSI algorithm is verified through the application on two temporally well-separated pulses having the same polarization direction and spectral shape, generated by duplicating 37 fs-long amplified pulses of a Ti:Sa based laser system. The spectral phase of highly chirped double pulses with equal or different amplitude ratios is numerically retrieved. The collinear and achromatic experimental arrangement results in a compact and easy-to-align system.