Search

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). 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.

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., 6 pages, 4 figures

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., JPhys Photonics, 2 (3), ISSN:2515-7647

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.

The problem of propagation of an ultraintense pulse in a gas medium is solved by using a 3D nonadiabatic model extended to the case of multiple ionisation of atoms in the interaction region. A complex space – time coupling of the pulse during its propagation and strong spatial and temporal changes in the field distribution are shown.

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. (Less)

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 as plasma-core induced self-guiding in previous works --- limits the achievable cut-off by regulating the intensity of the laser beam, it provides favorable conditions for phase matching, which is indispensable for high-flux gas high-harmonic 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 spatio-temporally constant fundamental intensity contributes to the constructive build-up of the harmonic field in a broad photon-energy range up to the provided cut-off., 14 pages, 7 figures

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&ndash, 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 extreme ultraviolet or even X-ray pulses. A robust way to produce these pulses is high-harmonic generation (HHG) in a gas medium. However, the energy upscaling of the process depends on a very demanding next-generation laser technology that provides multi-terawatt (TW) laser pulses with few-optical-cycle duration and controlled electric field. Here, we revisit the HHG process driven by 16-TW sub-two-cycle laser pulses to reach high intensity in the 100-eV spectral region and beyond. We show that the combination of above barrier-suppression intensity with a long generation medium significantly enhances the isolation of attosecond pulses compared to lower intensities and/or shorter media and this way reduces the pulse duration as well as field-stability requirements on the laser driver. This novel regime facilitates the real-time observation of electron dynamics at the attosecond timescale in atoms, molecules, and solids.

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.

In this paper, we numerically investigate the possibility to obtain a lighthouse emission for the attosecond pulses produced by high-order harmonics of a strong mid-infrared fundamental pulse without any optical element inserted in the path of the generating beam. The parameters of the driving pulse, focusing geometry, gas medium and detection configuration are currently experimentally feasible. Here, we study in detail the specific propagation conditions of the laser beam, and describe the exact mechanism of the sensitive space-time variation of the medium's refractive index that lead to the dynamic wavefront rotation. This basic requirement for the lighthouse phenomenon is transmitted to the harmonic bursts, which are emitted with different divergence in successive optical half-cycles, thus can be detected in the far field at increasing distances from the optical axis. In this configuration, spectral filtering of the harmonics is not necessary, therefore the total harmonic pulse power might be used in further pump-probe experiments.

The availability of tunable mid-infrared laser systems capable to deliver high intensity few cycle pulses offers the perspective of generating ultrashort radiation in the extreme UV range through the high-order harmonic generation (HHG) of the fundamental. In particular, such systems make possible the superposition of multiple colors of incommensurable frequencies that gives rise to time gating of HHG, thereby realizing the necessary condition leading to isolated attosecond pulse (IAP) generation. In this paper, the HHG resulting from the superposition of two incommensurable frequencies is analyzed with the aim to find out the conditions that the relative intensity and frequency of the two fields have to fulfill in order to generate isolated attosecond pulses. It is observed that the two fields can be superimposed in parallel and perpendicular polarization, both giving rise to IAP under appropriate conditions.

We present a method to trace quantum trajectories of electrons in a high-order harmonic generation process. The method relies on the saddle-point technique and, unlike previously developed analytical techniques, it enables an arbitrary variation in time of the driving fields. The trajectory phases yield information about phase-matching conditions at the optical cycle level, especially useful in investigating attosecond pulse generation. Specially prepared laser pulses, as well as propagation effects can be included in these phase-matching calculations. The method can handle both one- and two-dimensional electron trajectories produced by corresponding driving fields. We show the potentials of the method by calculating trajectories and phase-matching conditions for a polarization gate configuration proposed recently [Altucci, C.; Esposito, R.; Tosa, V.; Velotta, R. Opt. Lett. 2008, 33, 2943–2945.] to obtain isolated attosecond pulse from multi-cycle laser pulses.

We study two-color high-order harmonic generation in Neon with 790 nm and 1300 nm driving laser fields and observe an extreme-ultraviolet continuum that extends to photon energies of 160 eV. Using a 6-mm-long, high pressure gas cell, we optimize the HHG yield at high photon energies and investigate the effect of ionization and propagation under phase-matching conditions that allow us to control the temporal structure of the XUV emission. Numerical simulations that include the 3D propagation of the two-color laser pulse show that a bright isolated attosecond pulse with exceptionally high photon energies can be generated in our experimental conditions due to an efficient hybrid optical and phase-matching gating mechanism.

A general scaling formalism for nonlinear light-matter interactions in gases is presented and experimentally verified. The formalism enables to conveniently extrapolate nonlinear phenomena, such as filamentation or high-order harmonic generation, to new laser parameters.

Scaling attosecond sources to higher pulse energy and/or repetition rate can benefit many applications. We present a scaling framework for nonlinear light-matter interactions, applicable to attosecond pulse generation and other nonlinear phenomena as e.g. filamentation.

Extrapolating nonlinear phenomena, such as filamentation, to new parameters as e.g. to higher pulse energy is often challenging. We here present a general scaling model for nonlinear light-matter interactions in gases and proof it experimentally.

The critical behavior of a one-dimensional Burridge–Knopoff type spring-block model, aimed to describe magnetization phenomena, is studied by Monte Carlo type computer simulations. Disorder is introduced through randomly distributed pinning centers and the magnetization process is modeled through a relaxation dynamics. The distribution of avalanche sizes (jumps in magnetization) is studied for different disorder values. The results indicate that the model exhibits a disorder-driven phase transition. Estimates for some critical exponents and scaling laws are given.

Nonlinear optical methods are becoming ubiquitous in many areas of modern photonics. They are, however, often limited to a certain range of input parameters, such as pulse energy and average power, since restrictions arise from, for example, parasitic nonlinear effects, damage problems and geometrical considerations. Here, we show that many nonlinear optics phenomena in gaseous media are scale-invariant if spatial coordinates, gas density and laser pulse energy are scaled appropriately. We develop a general scaling model for (3+1)-dimensional wave equations, demonstrating the invariant scaling of nonlinear pulse propagation in gases. Our model is numerically applied to high-order harmonic generation and filamentation as well as experimentally verified using the example of pulse post-compression via filamentation. Our results provide a simple recipe for up-or downscaling of nonlinear processes in gases with numerous applications in many areas of science., This manuscript has 9 pages including 3 figures and two tables

Studies of the quasi-phase-matched harmonics generated in spatially modulated silver plasma are presented. We analyze the influence of the driving and heating pulse energies on the enhancement and spectral tuning of the quasi-phase-matched harmonics and show the tuning of those enhanced groups of harmonics by tilting the multislit mask placed in front of the ablated targets. Calculations performed in the laser-produced plasmas demonstrate the joint influence of the electrons that appeared during target ablation and tunnel ionization on the tuning of the quasi-phase-matching conditions in the plasmas.

A simple mechanical spring-block model is introduced for studying magnetization phenomena and in particularly the Barkhausen noise. The model captures and reproduces the accepted microscopic picture of domain wall movement and pinning. Computer simulations suggest that this model is able to reproduce the main characteristics of hysteresis loops and Barkhausen jumps. In the thermodynamic limit the statistics of the obtained Barkhausen jumps follows several scaling laws, in qualitative agreement with the experimental results. The simplicity of the model and the invoked mechanical analogies makes it attractive for computer simulations and pedagogical purposes., Comment: Revtex, 8 pages, 6 figures

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.

We numerically investigate the use of strong THz radiation in assisting single attosecond pulse generation by few-cycle, 800 nm laser pulses. We optimize focusing conditions to generate short and powerful single attosecond pulses of high-energy photons by keeping the parameters of the THz field within the limits achieved experimentally. We show that using optimal focusing geometry isolated attosecond pulses shorter than 100 as can be obtained even in the absence of further gating or XUV compression techniques, using an 8 fs generating pulse. Furthermore, quantum path control of short- and long-trajectory components is demonstrated by varying the delay between the THz and IR pulses.

High-order harmonic generation in the presence of a chirped THz pulse is investigated numerically with a complete 3D non-adiabatic model. The assisting THz pulse illuminates the HHG gas cell laterally inducing quasi-phase-matching. We demonstrate that it is possible to compensate the phase mismatch during propagation and extend the macroscopic cutoff of a propagated strong IR pulse to the single-dipole cutoff. We obtain two orders of magnitude increase in the harmonic efficiency of cutoff harmonics (�170 eV) using a THz pulse of constant wavelength, and a further factor of 3 enhancement when a chirped THz pulse is used. High-order harmonic generation (HHG) in a noble gas medium is currently the widely used method to produce coherent radiation in the XUV and soft X-ray regime. The elementary laser-atom interaction leading to HHG is understood via the three-step model [1], but the complete description of the HHG process requires considering the elementary laser-atom interaction together with the macroscopic aspects of laser and harmonic field propagation in the ionized gaseous medium. The temporal, spectral and spatial distortions of the fundamental pulse during propagation result in a varying intensity and phase which strongly influence the produced harmonic radiation on both the single-atom and the macroscopic level [2]. The ultimate goal in an experiment is to obtain intense, coherent XUV or soft X-ray radiation which in turn are useful in the designed applications like molecular imaging or pump-probe experiments [3]. In Fig. 1 we show a typical case of phase mismatch (PMM) of the harmonic radiation along the HHG cell

We numerically investigate high-order harmonic generation where a properly tailored THz pulse assists the process of regular HHG in a static cell. The strong IR pulse (800 nm, 20 fs, 0.16-0.3 mJ pulse energy) propagates along the cell while the assisting THz pulse illuminates the cell laterally. We demonstrate that the phase mismatch can be compensated during propagation and the macroscopic cutoff can be extended up to the single-dipole cutoff. The amplified spectral domain can be controlled by adjusting the IR pulse energy and the THz pulse parameters. We obtain two orders of magnitude increase in the harmonic efficiency in the cutoff region (150-200 eV), in a quasi-phasematching regime.

The production of isolated attosecond pulses is nowadays based on the generation of high harmonics driven by ultrafast and intense laser sources in gases. The generation of such pulses requires the confinement of the harmonic emission within a single event, thus obtaining a continuous XUV spectrum. Here we report on the generation of continuous XUV spectra produced by an ultrabroadband optical parametric amplifier operating in the mid-IR and generating multi-cycle pulses, thus avoiding the use of few-cycle Ti:sapphire lasers and, thanks to the favorable cutoff scaling law for mid-IR drivers, paving the way to the production of isolated attosecond pulses well above the 100-eV photon energy.

We present a new quasi-phase-matching method for high-order harmonic generation where a weak THz pulse assists the process, and we demonstrate by numerical simulations more than two orders of magnitude increase in the harmonic yield at photon energies ∼170 eV.

High-order harmonic generation by few-cycle 800 nm laser pulses in neon gas in the presence of a strong terahertz (THz) field is investigated numerically with propagation effects taken into account. Our calculations show that the combination of THz fields with up to 12 fs laser pulses can be an effective gating technique to generate single attosecond pulses. We show that in the presence of the strong THz field only a single attosecond burst can be phase matched, whereas radiation emitted during other half cycles disappears during propagation. The cutoff is extended and a wide supercontinuum appears in the near-field spectra, extending the available spectral width for isolated attosecond pulse generation from 23 to 93 eV. We demonstrate that phase-matching effects are responsible for the generation of isolated attosecond pulses, even in conditions when single-atom response yields an attosecond pulse train.

Macroscopic effects for high-order harmonic generation in the presence of strong terahertz fields are analyzed. The effect of terahertz fields on high-order harmonic generation was first investigated in [1]. Here we further investigate the process using a complete three dimensional non-adiabatic macroscopic model [2], and show that propagation effects are very important when dealing with terahertz fields, and, under certain circumstances even help the generation of single attosecond pulses (SAP).

We report on our experimental and theoretical investigations on the generation of high-order harmonics driven by 1500 nm few-cycle laser pulses in xenon. In contrast to the common belief, we found experimental evidence suggesting that harmonic generation driven by mid-infrared laser pulses can be realized with high efficiency; in particular, an enhancement of very high harmonic orders can be achieved under suitable conditions of the laser–medium interaction. The experimental results were simulated by a 3D non-adiabatic model. The theoretical outcomes confirm the experimental findings and provide a physical explanation for the counter-intuitive results. In particular, a time-dependent phase-matching analysis threw light on the generation mechanisms at a timescale of half optical cycle of the fundamental pulse.

The study of ultrafast phenomena in atoms, molecules and solid-state matter is nowadays accessing a new realm, thanks to the production of isolated attosecond pulses (IAPs) of XUV radiation by high-order harmonic generation (HHG) [1]. The classical route to the generation of continuous XUV spectra leading to IAPs exploits shaped few-cycle laser pulses, like in polarization gating [2] and two-color gating [3] techniques. In this work we will show that such techniques can be implemented with an ultrabroadband optical parametric amplifier operating in the mid-IR and generating multi-cycle pulses, thus avoiding the use of a few-cycle Ti:Sapphire laser source and, thanks to the favorable cutoff scaling law for mid-IR drivers [3], paving the way to the production of IAPs well above the 100-eV photon energy.

High-order harmonic generation is nowadays a topic of crucial interest since it allows the production of isolated attosecond pulses. Several efforts have been made so far for extending the generation of attosecond bursts beyond the 100 eV spectral region, in order to access to deeper electronic states in atoms and molecules. Recently, mid-infrared laser sources have demonstrated to be ideal candidates for pushing the high harmonic emission up to the soft X-rays. Here we show that it is possible to obtain continuous harmonic spectra, that are the signature of single attosecond pulse generation, by properly mixing two laser pulses, coming from optical parametric amplifiers, with wavelengths of 1.35 and 1.75 μm respectively, with either parallel or perpendicular polarizations. Our results demonstrate the generation of isolated attosecond burst driven by mid-infrared laser pulses of a duration larger than 30 fs.

A numerical method is presented for the study of time‐dependent phase‐matching (PM) in the process of obtaining isolated attosecond pulses from high‐harmonic generation (HHG) in noble gases. The method relies on the quantum trajectory analysis of individual electrons using the saddle‐point technique and can deal with arbitrary temporal variation of the driving field. Phase‐matching conditions at optical cycle level can be obtained and used for the investigation of attosecond pulse generation. The potentials of the method are presented through an example of isolated attosecond pulse generation in a polarization gate configuration.

We analyze the macroscopic effects which are responsible for producing clean isolated pulses lasting few hundreds of attoseconds when starting from multi-cycle fundamental pulses. In particular, we consider a polarization gating scheme and show that, at high fundamental peak intensities, in the range 0.7-1 PWcm(-2), it usually produces three-four main attosecond pulses of radiation at single dipole level, just located in the leading edge of the laser pulse. We describe the physical mechanisms contributing to the formation of a single attosecond pulse by using a three dimensional non-adiabatic model and a quantum trajectory phase calculation. An analysis of the scheme optimization and stability against various parameters is performed in view of an experimental scheme implementation. (C) 2009 Optical Society of America.

We model two-color high-order harmonic generation using a full 3-D nonadiabatic model and show that the presence of a weak perturbative near infrared pulse assisting a strong pulse of long wavelength increases the high-harmonic yield with one order of magnitude in the whole spectral range up to 160 eV. In addition, we demonstrate that it is possible to obtain a single attosecond pulse at the exit of the interaction region even by starting from two laser pulses of 50 fs each.

In this paper we present a theoretical and numerical tool for tracing quantum trajectories of electrons in interaction with arbitrary-shape laser pulses. The method calculates quantum trajectories using saddle-point analysis within the strong field approximation. The method is very general, thus it is applicable to arbitrarily shaped finite laser pulses. Example results and comparison with the classical trajectories are presented.

We investigate numerically the generation of water window x-rays under phase-matched conditions at very low ionization level. We show that at high-gas pressure and short-wavelength infrared driving pulse, phase matching condition is obtained only if one accounts for the effects ionization has on both driving and harmonic fields. Recent experimental results obtained in these conditions by Takahashi et al. [Phys. Rev. Lett. 101, 253901 (2008)] are also discussed.

Since its first observation almost three decades ago high-order harmonic generation (HHG) in gases became a reliable source of extreme ultraviolet (XUV) pulses, which gave the possibility to study electronic processes on their natural timescale [1, 2]. While the main building blocks of the experimental setups for gas HHG are the same in almost all cases, the focusing or medium geometry varies from realization to realization based on, for example, the available laser power [3, 4].

We experimentally demonstrate a new technique for the temporal gating of high-order harmonic generation, based on the mixing of two mid-infrared laser pulses coming from op- tical parametric amplifiers, at wavelengths of 1.35 and 1.75 μm, respectively. Both parallel and perpendicular configurations of the polarization directions were investigated. Results obtained in xenon and argon show continuous spectra with a significant cutoff extension, up to 160 eV. These outcomes demonstrate the possibility of extending the two-color gating technique for the generation of isolated attosecond bursts to a mid-infrared laser source and are, to our knowledge, the first experimental demonstration of the gating on a single half-cycle of high-order harmonics driven by laser pulses longer than 30 fs. Delay, fs

Experiments employing extreme ultraviolet sources based on high harmonic generation often suffer from photon flux limitations. We discuss current status and prospects for scaling such sources to higher repetition rate, pulse energy and average power.