Zoltán Filus

High-resolution, momentum-resolved photoelectron emission microscopy is employed to investigate ejected electrons extending all the way from the directly emitted primary photoelectrons to the slow secondary electrons resulting from the ensuing collision cascade. Excitation with unpolarised radiation from a helium light source enables the valence band to be mapped across the entire first surface Brillouin zone. The measured quasi-particle bands show very good quantitative agreement with state-of-the-art density functional theory supplemented by many-body corrections within the GW approximation, establishing a reliable reference for the initial-state electronic structure. Extreme-ultraviolet light of controlled polarisation from a high-harmonic generation source is used to access high-lying conduction-band states. The polarisation-dependent momentum distributions of directly photoemitted electrons reveal the roles of initial-state symmetry and final conduction-band states. In the low-energy secondary-electron regime we map out, for the first time, the momentum-space distribution of the quasi-bound in-plane resonance in the conduction band which acts as a doorway state for secondary-electron emission through the X-peak. Six-fold-symmetric intensity patterns at characteristic kinetic energies above vacuum, independent of photon energy and light polarisation, indicate cascading secondary-electron emission through preferential conduction-band channels. These nearly non-dispersive features arise from hybridised, sp2-derived σ∗ conduction-band states. By disentangling contributions from the valence bands, conduction bands mediating direct photoemission, and low-energy resonances populated by the slowing-down electron cascade, our results provide unprecedented experimental access to the valence-band electronic structure and multiple emission channels of bulk graphite, relevant also to other layered quantum materials.

Dynamic interference is an elusive strong-field effect where photoelectrons from intense laser pulses interfere in time, forming rich kinetic energy patterns. Here, we present the first experimental demonstration of isolated dynamic interference using a novel two-color scheme: chirped laser-assisted dynamic interference. Isolation was achieved with a crossed-polarization setup combining an extreme ultraviolet harmonic field and an infrared pulse with tailored spectrotemporal properties. Beyond prior works, our approach enables precise control over interfering trajectories, yielding holographic interference patterns and advancing our understanding of strong-field phenomena.

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.

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 beamline of the Extreme Light Infrastructure—Attosecond Light Pulse Source facility. Versatile spectral and temporal shaping of the XUV pulses is 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 × 1010 photons/s at 39.7 eV selected with 700 meV full width at half maximum 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 µm). 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 a ptychographic algorithm on experimentally recorded XUV-infrared 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 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 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.

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.