Search Results (21)

We theoretically investigate high-order harmonic generation from ethylbenzene (C₈H₁₀), toluene (C₇H₈), and benzene (C₆H₆) molecules driven by a circularly polarized laser field using time-dependent density functional theory. By comparing the harmonic spectra of these structurally related molecules, we find that ethylbenzene, which features a larger molecular size due to the ethyl group, exhibits a higher harmonic cutoff and stronger harmonic intensity than toluene and benzene. Time-resolved electron density distributions, together with the probability current density analysis, indicate that under long-wavelength conditions (e.g., 1200 nm), the ethyl group in ethylbenzene and the methyl group in toluene significantly enhance the probability of ionized electrons from neighboring nuclei colliding with nearby nuclei, thereby leading to stronger harmonic emission, with ethylbenzene > toluene > benzene. In contrast, under short-wavelength conditions (e.g., 200 nm), the harmonic intensities of the three molecules show little difference, and the effects of the ethyl and methyl groups on the harmonic yield can be neglected. The influence of laser intensity and wavelength on high-order harmonic generation is further analyzed, confirming the robustness of the structural enhancement effect. Additionally, we study the harmonic ellipticity of ethylbenzene under different carrier-envelope phases, and find that while circularly polarized harmonics can be obtained, their spectral continuity is insufficient for synthesizing isolated circularly polarized attosecond pulses. This limitation is attributed to the broken ring symmetry caused by the ethyl substitution. Our findings offer insight into the relationship between molecular structure and harmonic response in strong-field physics, and provide a pathway for designing efficient circularly polarized attosecond pulse sources. Full article

Within the framework of classical dynamics, the impact of laser amplitude on the cross-collision between a linearly polarized intense laser pulse and a relativistic electron under tight focusing conditions was investigated via numerical simulation. As the laser amplitude intensifies, the z-axis oscillation trajectory of the electron elongates. The spatial radiation angular distribution of the electron transforms from a “hill shape” to a “comet shape”, and the radiation peak shifts toward the direction of smaller polar angle, with the radiation concentrating in the forward position. The time spectrum is symmetrical; the number of peaks is reduced from multiple peaks to three peaks; and the relative height of the main peak and secondary peaks increases, with the time distribution gradually concentrating, which can be regarded as an ultrashort attosecond single pulse. The spectrum exhibits a multi-peak distribution trend. When the laser amplitude is relatively strong, radiation with a more concentrated frequency range and better quality can be output. The above research findings are beneficial for generating X-rays of higher quality and can be applied in fields such as biomedicine and atomic physics. Full article

We present the Virtual Beamline (VBL) application, an interactive web-based platform for visualizing high-intensity laser–matter interactions using particle-in-cell (PIC) simulations, with future potential for experimental data visualization. These interactions include ion acceleration, electron acceleration, $\gamma$-flash generation, electron–positron pair production, and attosecond and spiral pulse generation. Developed at the ELI Beamlines facility, VBL integrates a custom-built WebGL engine with WebXR-based Virtual Reality (VR) support, allowing users to explore complex plasma dynamics in non-VR mode on a computer screen or in fully immersive VR mode using a head-mounted display. The application runs directly in a standard web browser, ensuring broad accessibility. VBL enhances the visualization of PIC simulations by efficiently processing and rendering four main data types: point particles, 1D lines, 2D textures, and 3D volumes. By utilizing interactive 3D visualization, it overcomes the limitations of traditional 2D representations, offering enhanced spatial understanding and real-time manipulation of visualization parameters such as time steps, data layers, and colormaps. Users can interactively explore the visualized data by moving their body or using a controller for navigation, zooming, and rotation. These interactive capabilities improve data exploration and interpretation, making VBL a valuable tool for both scientific analysis and educational outreach. The visualizations are hosted online and freely accessible on our server, providing researchers, the general public, and broader audiences with an interactive tool to explore complex plasma physics simulations. By offering an intuitive and dynamic approach to large-scale datasets, VBL enhances both scientific research and knowledge dissemination in high-intensity laser–matter physics. Full article

The chirped pulse amplification (CPA) systems based on transition-metal-ion-doped chalcogenide crystals are promising powerful ultrafast laser sources providing access to sub-TW laser pulses in the mid-IR region, which are highly relevant for essential scientific and technological tasks, including high-field physics and attosecond science. The only way to obtain high-peak power few-cycle pulses is through efficient laser amplification, maintaining the gain bandwidth ultrabroad. In this paper, we report on the approaches for mid-IR broadband laser pulse energy scaling and the broadening of the gain bandwidth of iron-doped chalcogenide crystals. The multi-pass chirped pulse amplification in the Fe:ZnSe crystal with 100 mJ level nanosecond optical pumping provided more than 10 mJ of output energy at 4.6 μm. The broadband amplification in the Fe:ZnS crystal in the vicinity of 3.7 μm supports a gain band of more than 300 nm (FWHM). Spectral synthesis combining Fe:ZnSe and Fe:CdSe gain media allows the increase in the gain band (~500 nm (FWHM)) compared to using a single active element, thus opening the route to direct few-cycle laser pulse generation in the prospective mid-IR spectral range. The features of the nonlinear response of carbon nanotubes in the mid-IR range are investigated, including photoinduced absorption under 4.6 μm excitation. The study intends to expand the capabilities and improve the output characteristics of high-power mid-IR laser systems. Full article

We investigate the production of an isolated attosecond pulse (IAP) via the phase-matching gating of high-harmonic generation with intense laser pulses. Our study is based on the integration of the propagation equation for the fundamental and generated fields with nonlinear polarisation found via the numerical solution of the time-dependent Schrödinger equation. We study the XUV energy as a function of the propagation distance (or the medium density) and find that the onset of the IAP production corresponds to the change from linear to quadratic dependence of this energy on the propagation distance (or density). Finally, we show that the upper limit of the fundamental pulse duration for which IAP generation is feasible is defined by the temporal spreading of the fundamental pulse during the propagation. This nonlinear spreading is defined by the difference in the group velocities for the neutral and photoionised medium. Full article

Atomic lifetime measurements span a wide range, from attoseconds to years. The frontier of exploratory lifetime measurements, presently, is in the long part of the above time range, with an eye on astrophysical problems. In a combination of review paper, tutorial, and Editorial, the physical environments and experiments are discussed, in which the results of such lifetime measurements matter. Although accurate lifetime measurement results are important for our understanding of atomic structure and dynamics, and for the diagnostics of various plasma environments, the order of magnitude is often precise enough to see why time resolution may be of interest in an experiment, from laser-produced plasmas of high densities to planetary nebulae of very low densities. Full article

Electron correlation describes the interaction between electrons in a multi-electron system. It plays an important role in determining the speed of relaxation of atoms and molecules excited by XUV/X-ray pulses, such as the argon decay rate. Most research on electron correlation has centered on the role of correlation in stationary states. A time-resolved experimental study of electron correlation is a grand challenge due to the required temporal resolution and photon energy. In this research, we investigated Auger decay in argon using 200-attosecond X-ray pulses reaching the carbon K-edge. At such a high photon energy, ionization occurs not only from the outer most levels (3s and 3p), but also from the 2p core shells. We have measured a lifetime of 4.9 fs of L-shell vacancies of argon in pump–probe experiments with a home-built high-resolution time-of-flight spectrometer. Full article

Mid-infrared (MIR) ultrashort laser pulses have a wide range of applications in the fields of environmental monitoring, laser medicine, food quality control, strong-field physics, attosecond science, and some other aspects. Recent years have seen great developments in MIR laser technologies. Traditional solid-state and fiber lasers focus on the research of the short-wavelength MIR region. However, due to the limitation of the gain medium, they still cannot cover the long-wavelength region from 8 to 20 µm. This paper summarizes the developments of 8–20 μm MIR ultrafast laser generation via difference frequency generation (DFG) and reviews related theoretical models. Finally, the feasibility of MIR power scaling by nonlinear-amplification DFG and methods for measuring the power of DFG-based MIR are analyzed from the author’s perspective. Full article

Ultrafast mid-infrared (mid-IR) lasers with a high pulse repetition rate are in great demand in various fields, including attosecond science and strong-field physics. Due to the lack of suitable mid-IR laser gain medium, optical parametric amplifiers (OPAs) are used to generate an ultrafast mid-IR laser. However, the efficiency of OPA is sensitive to phase mismatches induced by wavelength and temperature deviations from the preset points, which thus limits the pulse duration and the average power of the mid-IR laser. Here, we exploited a noncollinear phase-matching configuration to achieve simultaneously wavelength- and temperature-insensitive mid-IR OPA with a LiGaS₂ crystal. The noncollinearity can cancel the first-order dependence of phase matching on both wavelength and temperature. Benefitting from the thermal property of the LiGaS₂ crystal, some collinear phase-matching solutions derived from the first-order and even third-order wavelength insensitivity have comparatively large temperature bandwidths and can be regarded as approximate solutions with simultaneous wavelength and temperature insensitivity. These simultaneously wavelength- and temperature-insensitive phase-matching designs are verified through numerical simulations in order to generate few-cycle, high-power mid-IR pulses. Full article

In a recent work (Antonov et al., Physical Review Letters 123, 243903 (2019)), it was shown that it is possible to amplify a train of attosecond pulses, which are produced from the radiation of high harmonics of the infrared field of the fundamental frequency, in the active medium of a plasma-based X-ray laser modulated by a replica of the infrared field of the same frequency. In this paper, we show that much higher amplification can be achieved using the second harmonic of the fundamental frequency for modulating of a hydrogen-like active medium. The physical reason for such enhanced amplification is the possibility to use all (even and odd) sidebands induced in the gain spectrum in the case of the modulating field of the doubled fundamental frequency, while only one set of sidebands (either even or odd) could participate in amplification in the case of the modulating field of the fundamental frequency due to the fact that the spectral components of the high-harmonic field are separated by twice the fundamental frequency. Using the plasma of hydrogen-like C⁵⁺ ions with an inverted transition wavelength of 3.38 nm in the water window as an example, it is shown that the use of a modulating field at a doubled fundamental frequency makes it possible to increase the intensity of amplified attosecond pulses by an order of magnitude in comparison with the previously studied case of a fundamental frequency modulating field. Full article

Free-electron lasers (FELs) can produce radiation in the short wavelength range extending from the extreme ultraviolet (XUV) to the X-rays with a few to a few tens of femtoseconds pulse duration. These facilities have enabled significant breakthroughs in the field of atomic, molecular, and optical physics, implementing different schemes based on two-color photoionization mechanisms. In this article, we present the generation of attosecond pulse trains (APTs) at the seeded FEL FERMI using the beating of multiple phase-locked harmonics. We demonstrate the complex attosecond waveform shaping of the generated APTs, exploiting the ability to manipulate independently the amplitudes and the phases of the harmonics. The described generalized attosecond waveform synthesis technique with an arbitrary number of phase-locked harmonics will allow the generation of sub-100 as pulses with programmable electric fields. Full article

Chemical and physical processes in molecules can be controlled through the manipulation of quantum interferences between rotational, vibrational, and electronic degrees of freedom. Most of the past efforts have been focused on the control of nuclear dynamics. Even though electronic coherence and its coupling to nuclear degrees of freedom may profoundly affect the outcome of these processes, electron dynamics have received less attention. Proper investigation of electron dynamics in materials demands ultrafast sources in the visible, ultraviolet (UV), and extreme ultraviolet (XUV) spectral region. For this purpose, a few-cycle deep-UV and XUV beamlines have been constructed for studying ultrafast electron dynamics in molecules. To ensure the required high temporal resolution on the attosecond time scale, vibration isolation from environmental mechanical noise and active stabilization have been implemented to achieve attosecond timing control between pump and probe pulses with excellent stability. This is achieved with an actively phase-stabilized double-layer Mach-Zehnder interferometer system capable of continuous time-delay scans over a range of 200 fs with a root-mean-square timing jitter of only 13 as over a few seconds and ~80 as of peak-to-peak drift over several hours. Full article

In the past decade, mid-infrared (MIR) few-cycle lasers have attracted remarkable research efforts for their applications in strong-field physics, MIR spectroscopy, and bio-medical research. Here we present a review of MIR few-cycle pulse generation and amplification in the wavelength range spanning from 2 to ~20 μm. In the first section, a brief introduction on the importance of MIR ultrafast lasers and the corresponding methods of MIR few-cycle pulse generation is provided. In the second section, different nonlinear crystals including emerging non-oxide crystals, such as CdSiP₂, ZnGeP₂, GaSe, LiGaS₂, and BaGa₄Se₇, as well as new periodically poled crystals such as OP-GaAs and OP-GaP are reviewed. Subsequently, in the third section, the various techniques for MIR few-cycle pulse generation and amplification including optical parametric amplification, optical parametric chirped-pulse amplification, and intra-pulse difference-frequency generation with all sorts of designs, pumped by miscellaneous lasers, and with various MIR output specifications in terms of pulse energy, average power, and pulse width are reviewed. In addition, high-energy MIR single-cycle pulses are ideal tools for isolated attosecond pulse generation, electron dynamic investigation, and tunneling ionization harness. Thus, in the fourth section, examples of state-of-the-art work in the field of MIR single-cycle pulse generation are reviewed and discussed. In the last section, prospects for MIR few-cycle lasers in strong-field physics, high-fidelity molecule detection, and cold tissue ablation applications are provided. Full article

Attosecond science, beyond its importance from application point of view, is of a fundamental interest in physics. The measurement of tunneling time in attosecond experiments offers a fruitful opportunity to understand the role of time in quantum mechanics. In the present work, we show that our real T-time relation derived in earlier works can be derived from an observable or a time operator, which obeys an ordinary commutation relation. Moreover, we show that our real T-time can also be constructed, inter alia, from the well-known Aharonov–Bohm time operator. This shows that the specific form of the time operator is not decisive, and dynamical time operators relate identically to the intrinsic time of the system. It contrasts the famous Pauli theorem, and confirms the fact that time is an observable, i.e., the existence of time operator and that the time is not a parameter in quantum mechanics. Furthermore, we discuss the relations with different types of tunneling times, such as Eisenbud–Wigner time, dwell time, and the statistically or probabilistic defined tunneling time. We conclude with the hotly debated interpretation of the attoclock measurement and the advantage of the real T-time picture versus the imaginary one. Full article

Electron coherence is a fundamental quantum phenomenon in today’s ultrafast physics and chemistry research. Based on attosecond pump–probe schemes, ultrafast X-ray photoelectron imaging of molecules was used to monitor the coherent electron dynamics which is created by an XUV pulse. We performed simulations on the molecular ion H ${}_2^{+}$ by numerically solving time-dependent Schrödinger equations. It was found that the X-ray photoelectron angular and momentum distributions depend on the time delay between the XUV pump and soft X-ray probe pulses. Varying the polarization and helicity of the soft X-ray probe pulse gave rise to a modulation of the time-resolved photoelectron distributions. The present results provide a new approach for exploring ultrafast coherent electron dynamics and charge migration in reactions of molecules on the attosecond time scale. Full article