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.

The goal to control short-wavelength radiation for the investigation and manipulation of ultrafast dynamics in quantum systems coevolves with the growing availability of extreme-ultraviolet (XUV) and x-ray sources from high-harmonic generation and free-electron lasers. Here, we present an XUV spatio-spectral phase modulator based on an intense XUV laser beam propagating through an optically thick resonant target, introducing dispersion profile variations around the resonance both perpendicular to and along the laser propagation direction. The resulting dipole radiation gets spectrally reshaped and becomes more divergent as compared to the original beam in the far field. As an experimental demonstration, the intense-XUV-induced double-peak off-axis structure in the far-field spectrum obtained at the Free-Electron Laser in Hamburg (FLASH) shows indications of the underlying XUV-driven Rabi dynamics and resonant pulse propagation effects. The presented work highlights a ubiquitous phenomenon occurring when an intense laser beam passes through a resonant medium.

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.

Photocurrent-induced harmonics appear in gases and solids due to tunnel ionization of electrons in strong fields and subsequent acceleration. In contrast to three-step harmonic emission, no return to the parent ions is necessary. Here we show that the same mechanism produces harmonics in metallic nanostructures in strong fields. Furthermore, we demonstrate how strong local field gradient, appearing as a consequence of the field enhancement, affects photocurrent-induced harmonics. This influence can shed light at the state of electron as it appears in the continuum, in particular, to its initial velocity.

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.

High-harmonic generation is a light up-conversion process occurring in a strong laser field, leading to coherent bursts of extreme ultrashort broadband radiation [Lewenstein et al., Phys. Rev. A 49 , 2117 (1994)]. As a new perspective, we propose that ultrafast strong-field electronic or photonic processes such as high-harmonic generation can potentially generate nonclassical states of light well before the decoherence of the system occurs [Gorlach et al., Nat. Commun. 11 , 4598 (2020); Stammer et al ., Phys. Rev. Lett. 128, 123603 (2022)]. This could address fundamental challenges in quantum technology such as scalability, decoherence, or the generation of massively entangled states [Lewenstein et al., Luca Argenti Michael Chini, 27 (2024)]. Here, we report experimental evidence of the nonclassical nature of the harmonic emission in several semiconductors excited by a femtosecond infrared laser. By investigating single- and double-beam intensity cross-correlation [Loudon, Rep. Prog. Phys. 43 , 913 (1980)], we measure characteristic nonclassical features in the single-photon statistics. We observe two-mode squeezing in the generated harmonic radiation, which depends on the laser intensity that governs the transition from super-Poissonian to Poissonian photon statistics. The measured violation of the Cauchy-Schwarz inequality realizes a direct test of multipartite entanglement in high-harmonic generation [Wasak, Phys. Rev. A 90 , 033616 (2014)]. This result is supported by the theory of multimodal detection and the Hamiltonian from which the effective squeezing modes of the harmonics can be derived [Gonoskov et al., Phys. Rev. B 109, 125110 (2024); Christ et al. New J. Phys. 13 , 033027 (2011)]. With this work, we show experimentally that high-harmonic generation is a new quantum bosonic platform that intrinsically produces nonclassical states of light with unique features such as multipartite broadband entanglement or multimode squeezing. The source operates at room temperature, using standard semiconductors and a standard commercial fiber laser, opening up new routes for the quantum industry, such as optical quantum computing, communication, and imaging.

High-harmonic generation (HHG) is a highly non-linear frequency up conversion process, mostly studied from a classical point of view. Recently, independent theoretical investigations about the quantum nature of HHG predicted several, nonclassical effects in the high-harmonic radiation [ 1, 2, 3]. In addition to the fundamental interest in understanding the physics behind HHG, a better understanding of the quantum nature of this process could potentially have a broad impact on the rapidly developing field of quantum technologies. It is in this context that present here our experimental photon statistics investigations showing the quantum nature of the HHG process.

Der Physik-Nobelpreis 2023 ehrt Pierre Agostini, Ferenc Krausz und Anne L'Huillier für ihre Pionierarbeiten zur Attosekundenphysik.

The electronic and nuclear dynamics inside molecules are essential for chemical reactions, where different pathways typically unfold on ultrafast timescales. Extreme ultraviolet (XUV) light pulses generated by free-electron lasers (FELs) allow atomic-site and electronic-state selectivity, triggering specific molecular dynamics while providing femtosecond resolution. Yet, time-resolved experiments are either blind to neutral fragments or limited by the spectral bandwidth of FEL pulses. Here, we combine a broadband XUV probe pulse from high-order harmonic generation with an FEL pump pulse to observe dissociation pathways leading to fragments in different quantum states. We temporally resolve the dissociation of a specific O2+ state into two competing channels by measuring the resonances of ionic and neutral fragments. This scheme can be applied to investigate convoluted dynamics in larger molecules relevant to diverse science fields.

Strong-field quantum electronics is emerging as a potential candidate in information processing but still coherence vs decoherence is a primary concern of the concept. Strong-field coherent processes in band gap materials have led during the last decade to the emergence of high harmonic generation in semiconductors, petahertz electronics, or strong-field quantum states. However, the coherent behavior of the sub-optical cycle-driven electrons has never been directly observed. We report here on the experimental evidence of coherent ultrashort emission of hot electrons from a nanostructured semiconductor. Our method uses sub-wavelength electric field enhancement to localize the electron emission within a nanometer-scale spot. We found similarities with the electron emission from metallic nanotips in the strong-field regime, a topic that has opened a vast domain of applications during the last decade. The electron spectra display both odd and even harmonic orders of the driving femtosecond laser frequency, a signature of the coherent nature of the electron emission and their attosecond timing. Our findings complete our knowledge of phenomena governing coherent strong-field processes in semiconductors and open perspectives for the generation of future quantum devices operating in the strong-field regime.

The use of ultrashort pulse lasers in medical treatments is increasing and is already an essential tool, particularly in the treatment of eyes, bones and skin. One of the main advantages of laser treatment is that it is fast and minimally invasive. Due to the interaction of ultrashort laser pulses with matter, X-rays can be generated during the laser ablation process. This is important not only for the safety of the patient, but also for the practitioner to ensure that the legally permissible dose is not exceeded. Although our results do not raise safety concerns for existing clinical applications, they might impact future developments at higher peak powers. In order to provide guidance to laser users in the medical field, this paper examines the X-ray emission spectra and dose of several biological materials and describes their dependence on the laser pulse energy.

Ultrashort laser-matter processing can potentially lead to hazardous X-ray emission. Starting with an overview on secondary sources, we study the dose rate and spectral emission with respect to different laser parameters available in ultrafast laboratories and for industrial laser systems. Our results indicate that annual irradiation limits are easily exceeded.

We demonstrate a method to image an object using a self-probing approach based on semiconductor high-harmonic generation. On the one hand, ptychography enables high-resolution imaging from the coherent light diffracted by an object. On the other hand, high-harmonic generation from crystals is emerging as a new source of extreme-ultraviolet ultrafast coherent light. We combine these two techniques by performing ptychography measurements with nanopatterned crystals serving as the object as well as the generation medium of the harmonics. We demonstrate that this strong field in situ approach can provide structural information about an object. With the future developments of crystal high harmonics as a compact short-wavelength light source, our demonstration can be an innovative approach for nanoscale imaging of photonic and electronic devices in research and industry.

A broad range of ultrashort laser-matter processing can potentially lead to hazardous X-ray emission. By employing a novel calibrated X-ray detector, we study the dose rate and spectral emission with respect to different laser parameters.

A large range of laser-matter applications employ ultrashort pulses and high laser intensity. Such processes can lead to unrequired X-ray generation, which represents a hazardous radiation factor even for common laboratory research-grade laser systems. We present here an analysis of the radiation dose rate and X-ray spectrum emitted during ablation of a rotating copper cylinder with respect to several laser parameters. The results show that focused sub-picosecond pulses with intensity above 1013 W/cm2 can exceed the annual irradiation limit even in one hour, requiring appropriate shielding for the safety of the researchers.

We present an experimental method capable of capturing the complete spatio-temporal dynamics of filamenting ultrashort laser pulses. By employing spatially resolved Fourier transform spectrometry in combination with the capability to terminate the filament at any length, we can follow the nonlinear dynamics in four dimensions, i.e. the transverse domain, time and filament length. Our method thus not only enables the full characterization of the filamentation process throughout its evolution, but also allows to identify and select laser pulses with desired parameters.

We use high-order harmonic spectroscopy to study ionization dynamics in a macroscopic target with tunable density, spanning over six orders of magnitude. In an in situ pump-probe experiment, the target is prepared at different densities with varying degrees of laser-induced ionization. High-order harmonic radiation is generated in the pre-ionized target, and a steepening in the decrease of the harmonic yield is observed for increasing pre-ionization, allowing not only to identify the contributing quantum paths during high-order harmonic generation but also in determining the amount of ionization within the target. The measurements allow probing of ionization dynamics in laser-induced plasma with high spatio-temporal resolution and are specifically of interest for the optimization of the harmonic generation process in high-density targets with number densities of up to 1022 cm−3.

We show the creation of unidirectional currents in nanojunctions in strong optical fields at ambient conditions, with no phase stabilization of the driver. The mechanism is rooted in the inter-cycle interference of the electronic wavepackets in the nanogap.

We present the design, integration, and operation of the novel vacuum ultraviolet (VUV) beamline installed at the free-electron laser (FEL) FLASH. The VUV source is based on high-order harmonic generation (HHG) in gas and is driven by an optical laser system synchronized with the timing structure of the FEL. Ultrashort pulses in the spectral range from 10 to 40 eV are coupled with the FEL in the beamline FL26, which features a reaction microscope (REMI) permanent endstation for time-resolved studies of ultrafast dynamics in atomic and molecular targets. The connection of the high-pressure gas HHG source to the ultra-high vacuum FEL beamline requires a compact and reliable system, able to encounter the challenging vacuum requirements and coupling conditions. First commissioning results show the successful operation of the beamline, reaching a VUV focused beam size of about 20 µm at the REMI endstation. Proof-of-principle photo-electron momentum measurements in argon indicate the source capabilities for future two-color pump-probe experiments.