The propagation of femtosecond pulses in guided structures is a matter of both fundamental and practical interest in nonlinear optics. In particular, hollow-core waveguides (HCWs) filled with a gas medium are fabricated and used as devices for the generation of attosecond pulses from high-order harmonics. In this process, the configuration of the laser field (intensity and phase) inside the waveguide is of crucial importance for enhancing the (well-known, low) efficiency of high-order harmonic generation (HHG). Here, we present numerical calculations which demonstrate the main features of the propagation process in fabricated HCWs. We consider a variety of experimental parameters like gas pressure, waveguide size, laser wavelength, and pulse energy and duration. In particular, the beam profile at the fiber input is found to be a sensitive parameter which influences the whole evolution of the laser field along the propagation. Our model is based on a split-step method modified to account for propagation in ionized media and is validated against experimental and theoretical data from the literature. Our results contribute to the description of the main features of beam propagation in HCWs and provide guiding directions for designing efficient configurations for HHG.

We present a compact high-harmonic generation approach – termed as strongly overdriven regime – providing continuum radiation ranging from 18 to 140 eV albeit using long driving laser pulses, acting as a flexible and intense extreme-ultraviolet source.

High-harmonic generation (HHG) normally requires a careful adjustment of the driving laser intensity (typically $10^{14} - 10^{15}$ W/cm$^2$) and gas medium parameters to enable good phase matching conditions. In contrast with conventional wisdom, we present experimental results indicating phase-matched HHG in all rare gases, using a high-density medium and a driver laser intensity of around $10^{16}$ W/cm$^2$. The experimental results are corroborated by theoretical simulations, which indicate that ionization-induced self-phase modulation and plasma defocusing self-regulate the driver laser intensity to a level that is appropriate for good phase matching. A ten-fold broadening of the NIR spectrum is observed, which results in the generation of continuous spectra from $18-140$ eV in spite of using 50-fs-long driving pulses. The presented scheme represents a simple and versatile concept for the generation of XUV and soft X-ray continua, which are ideally suited for transient absorption and reflection spectroscopy.

We report on the reconstruction of ultrashort laser pulses from computer-simulated and experimental second harmonic generation-frequency resolved optical gating (SHG-FROG) spectrograms. In order to retrieve the spectral amplitude and phase we use a convolutional neural network trained on simulated SHG-FROG spectrograms and the corresponding spectral-domain fields employed as labels for the network, which is a complex field encompassing the full information about the amplitude and phase. Our results show excellent retrieval capabilities of the neural network in case of the simulated pulses. Although trained only on computer generated data, the method shows promising results regarding experimentally measured pulses.

Filamentation is a nonlinear optical phenomenon that involves the interplay between Kerr lens self-focusing and plasma defocusing, holding promise for practical applications in open-air environments over long distances. This process significantly alters the characteristics of laser pulses, making the control of laser properties a pivotal issue. In this study, we investigated the effects of laser energy and chirp on the spectral modulation of femtosecond laser pulses caused by filamentation in air. Negatively and positively chirped pulses exhibit distinct characteristics in the spectra measured after the filamentation. Negatively chirped pulses have broader spectral broadening and a stronger dependence on initial laser energy than positively chirped pulses, making them suitable for generating few-cycle pulses and applications requiring white light. In contrast, positively chirped pulses can be advantageous for applications requiring long-distance transmission of high-energy laser pulse. Our findings reveal the fundamental characteristics of femtosecond laser filamentation in air, thereby expanding the scope of possible atmospheric applications.

High-intensity laser pulses covering the ultraviolet to terahertz spectral regions are nowadays routinely generated in a large number of laboratories. In contrast, intense extreme-ultraviolet (XUV) pulses have only been demonstrated using a small number of sources including free-electron laser facilities [1-3] and long high-harmonic generation (HHG) beamlines [4-9]. Here we demonstrate a concept for a compact intense XUV source based on HHG that is focused to an intensity of $2 \times 10^{14}$ W/cm$^2$, with a potential increase up to $10^{17}$ W/cm$^2$ in the future. Our approach uses tight focusing of the near-infrared (NIR) driving laser and minimizes the XUV virtual source size by generating harmonics several Rayleigh lengths away from the NIR focus. Accordingly, the XUV pulses can be refocused to a small beam waist radius of 600 nm, enabling the absorption of up to four XUV photons by a single Ar atom in a setup that fits on a modest (2 m) laser table. Our concept represents a straightforward approach for the generation of intense XUV pulses in many laboratories, providing novel opportunities for XUV strong-field and nonlinear optics experiments, for XUV-pump XUV-probe spectroscopy and for the coherent diffractive imaging of nanoscale structures.

The front photopyroelectric configuration (FPPE), making use of air as a coupling fluid between the sample and sensor, was applied to measure the thermal effusivity of some solid thermoelectric materials. The investigated samples were ZnO, CuCrO 2 , Cu 4 Sn 7 S 16 , TiS 3 and two samples of high manganese silicide (HMS) thermoelectric materials. Most of these materials are porous and consequently, the classical PPE method, making use of standard coupling fluids between sensor and sample, cannot be used due to the fact that the coupling fluid penetrates inside the sample and leads to incorrect results. With this work we extend (to thermoelectric solids) the area of application of a method, recently proposed by Salazar et al. (Measurement 121: 96, 2018). Experimentally, the thermal effusivity is obtained from a multi-parametric fit of the phase of the FPPE signal as a function of the modulation frequency (with sample’s thermal effusivity, thickness of the sensor-sample air gap and heat losses by convection and radiation, as fitting parameters). It was demonstrated that, in some particular cases, the three parameters are independent and consequently, the solution of the fit is unique. Where possible, the obtained results have been compared with data from the literature and good agreement was found.

We demonstrate that two different single attosecond pulses (SAP) can be obtained from naturally separated spectral domains formed during high-order harmonic generation and propagation in a gas medium. We propose a feasible experimental configuration in which one can obtain an SAP in a lower energy domain (<300 eV), or another SAP in a higher energy domain (>300 eV). Without filtering, a double attosecond pulse emission with fixed temporal separation is obtained. The gap between the two spectral domains is close to the onset of the water window.

The ongoing development of intense high-harmonic generation (HHG) sources has recently enabled highly non-linear ionization of atoms by the absorption of at least 10 extreme-ultraviolet (XUV) photons within a single atom (Senfftleben et al, arXiv:1911.01375). Here we investigate how the generation of these very intense HHG pulses in our 18-m-long beamline is aided by the reshaping of the fundamental, few-cycle, near-infrared (NIR) driving laser within a 30-cm-long HHG Xe medium. Using an incident NIR intensity that is higher than what is required for phase-matched HHG, signatures of reshaping are found by measuring the NIR blueshift and the fluorescence from the HHG medium along the propagation axis. These results are well reproduced by numerical calculations that show temporal compression of the NIR pulses in the HHG medium. The simulations predict that after refocusing an XUV beam waist radius of 320 nm and a clean attosecond pulse train can be obtained in the focal plane, with an estimated XUV peak intensity of 9 × 1015 W cm−2. Our results show that XUV intensities that were previously only available at large-scale facilities can now be obtained using moderately powerful table-top light sources.

We perform a multi-dimensional parameter scan in the generation of high-order harmonics, with the main purpose to find the macroscopic conditions that optimize the harmonic yield in a specific spectral domain, around 40 eV for this particular case. The scanned parameters are the laser pulse energy, gas pressure, interaction cell position relative to focus and the cell length, while the fixed parameters are chosen to model a loose focusing configuration which is used in many existing laboratories. We performed the simulations with a 3D non-adiabatic model complemented by a detailed analysis of the phase matching mechanisms involved in an efficient harmonic generation. Based on the results we identify a range of parameter combinations that lead to a high yield in the specified spectral domain. The method and results presented here can be the framework for the design and construction of high flux high-order harmonic generation beamlines.

We demonstrate that by filtering high harmonics it is possible to obtain two different single attosecond pulses (SAP) resulting from naturally separated spectral domains formed during propagation in the macroscopic medium. We propose a feasible experimental configuration in which one can obtain a SAP in a lower energy domain (<300 eV), or another SAP in a higher energy domain (>300 eV). Without filtering, a double attosecond pulse emission with fixed temporal separation is obtained. The gap between the two spectral domains is close to the onset of the water window.

In this work, we numerically study a self-guiding process in which ionization plays a dominant role and analyze its effect on high-order harmonic generation (HHG) in gases. Although this type of self-guiding-termed "plasma-core-induced self-guiding" in previous works-limits the achievable cutoff by regulating the intensity of the laser beam, it provides favorable conditions for phase matching, which is indispensable for high-flux-gas highharmonic sources. To underline the role of self-guiding in efficient HHG, we investigate the time-dependent phase-matching conditions in the guided beam and show how the spatiotemporally constant fundamental intensity contributes to the constructive buildup of the harmonic field in a broad photon energy range up to the provided cutoff. (c) 2019 Optical Society of America

We numerically study the physical mechanism of the chirp compensation of the attosecond XUV pulses obtained by high-order harmonic generation. We show by detailed analysis that the macroscopic aspects of fundamental pulse propagation are essential in the formation and temporal-spectral-spatial properties of the attosecond pulses. Partial chirp compensation is already achievable due to the self-phase-modulation of the fundamental pulse. Further, by propagating the attosecond pulses in preformed plasma, we can fully compensate the inherent positive chirp of the attosecond pulses which were built up from the short electron trajectories in the harmonic generation process.

Along with the review of the technological frame that will be available at the Extreme Light Infrastructure Attosecond Light Pulse Source (ELI-ALPS) we present considerations applicable to large-scale attosecond sources driven by high-power laser pulses.

In this work we present the results obtained with an artificial neural network (ANN) which we trained to predict the expected output of high-order harmonic generation (HHG) process, while exploring a multi-dimensional parameter space. We argue on the utility and efficiency of the ANN model and demonstrate its ability to predict the outcome of HHG simulations. In this case study we present the results for a loose focusing HHG beamline, where the changing parameters are: the laser pulse energy, gas pressure, gas cell position relative to focus and medium length. The physical quantity which we predict here using ANN is directly related to the total harmonic yield in a specified spectral domain (20–40 eV). We discuss the versatility and adaptability of the presented method.

The study of core electron dynamics through nonlinear spectroscopy requires intense isolated attosecond extremeultraviolet or even X-ray pulses. A robust way to produce these pulses is high-harmoni ...

We study high-order harmonics generated by an intense multicycle 800 nm laser pulse in the presence of a weaker 1300 nm mid-infrared (MIR) pulse. Additionally to the previously observed effects of yield enhancement, cutoff extension, and continuum generation, we report here a spectral shift of harmonic peaks controlled by the delay of the MIR pulse. We explain this effect considering the half-cycle to half-cycle behavior of the two-color field. (C) 2018 Optical Society of America

Nonlinear light-matter interactions, such as filamentation or high-order harmonic generation, are at the heart of nonlinear optics. Scaling of such effects is crucial to benefit optimally from novel laser developments. We introduce and discuss a general scaling model for nonlinear light-matter interactions in gases.