Strong-field Processes Research Articles

Dynamical channel coupling in strong-field ionization of CO2

We present a theoretical study employing the time-dependent density functional theory (TDDFT) to explore the effects of angle-resolved channel coupling in strong field ionization of carbon dioxide (CO2) molecules. Our results reveal significant angular sensitivity of both the channel-resolved ionization probabilities and the effects of laser-induced channel couplings. By applying a linearly polarized two-color field scheme, we demonstrate the ability to significantly modify the strength of the laser-induced coupling, evidenced by the changes in the population distributions among the ionic states induced by the strong-field ionization. Importantly, the two-color field optimally modulates the coupling strength at the alignment angle where ionization of the highest occupied molecular orbital (HOMO) electrons is most efficient. This optimization is attributed to the reduction of the electron shielding effect. Our research provides valuable insights into the coherent manipulation of electron distribution within the cation, paving the way for the precise control of ultrafast electron dynamics during strong-field ionization processes.

Strong-field processes induced by an ultrashort linearly polarized pulse with two carrier frequencies

We perform a detailed semianalytical study of the characteristics of strong-field processes induced by an ultrashort linearly polarized pulse with two carrier frequencies. We show that the photoelectron spectra depend to a great extent on the absolute phases of the field components and on the duration of the pulse. By using the saddle-point method, we show that this dependence can be exploited to control the electron dynamics in the laser field. Besides the photoelectron spectra, we also investigate the spectra of high-order harmonics. Again, we find a strong dependence on the absolute phases and the duration of the pulse. We present the spectra using a false color scale in the absolute phase-harmonic energy plane, and show that particular regions in this plane can be assessed using the simple man's model founded on the classical solution of the Newton equation of motion. Finally, we investigate the symmetry properties of the photoelectron and high-order harmonic spectra with respect to the transformation which includes the change of the absolute phase, and compare them with those valid for a long driving pulse with a flat envelope. Published by the American Physical Society 2024

Computer Simulation of Explosive Emission Processes in Strong Electromagnetic Fields

The paper discusses the problem of computer simulation of explosive emission processes in strong electromagnetic fields. In this work, a numerical method for calculating electron emission from the surface of a metal cathode for the axial symmetric geometry of a technical system is proposed. For modelling, the particle method and grid calculations of electromagnetic fields based on Maxwell’s equations are applied. The approach uses the representation of large smoothed Gaussian particles and implements calculations of the electromagnetic fields on Cartesian spatial grids. The software realization is directed towards on parallel computing. The calculation of the emission process in a coaxial cylindrical diode with magnetic self-insulation with the use of developed technique was carried out. The process is considered for two situations: with and without the longitudinal plasma layer. The calculations show that the presence of plasma leads to an increase in the emission current. This result is consistent with the results of experiments.

Few-cycle optical vortices for strong-field physics

We report on the generation of optical vortices with few-cycle pulse durations, 500μJ per pulse, at a repetition rate of 1 kHz. To do so, a 25 fs laser beam at 800 nm is shaped with a helical phase and coupled into a hollow-core fiber filled with argon gas, in which it undergoes self-phase modulation. Then, 5.5 fs long pulses are measured at the output of the fiber using a dispersion-scan setup. To retrieve the spectrally resolved spatial profile and orbital angular momentum (OAM) content of the pulse, we introduce a method based on spatially resolved Fourier-transform spectroscopy. We find that the input OAM is transferred to all frequency components of the post-compressed pulse. The combination of these two information shows that we obtain few-cycle, high-intensity vortex beams with a well-defined OAM, and sufficient energy to drive strong-field processes.

On the status of channeling radiation and laser based radiation sources

A unified description of channeling radiation (CR) in oriented crystals (OC) and radiation in the field of an intense plane wave (laser radiation sources – LRS) is proposed in terms of two Lorentz-invariant parameters. The case of planar positron channeling (quasi-channeling) and linearly polarized laser wave is considered in detail up to TeV energies. The crucial difference between LRS and CR is that in the former case both invariants are independent, while in channeling they are linearly related to each other. This leads to a strong limitation on the range of possible values of these invariants in OC. On the other hand, OC make it possible to study QED processes in strong non-uniform external fields.

Simulation of Emission Processes in Strong Electromagnetic Fields

The problem of calculating the processes of electron emission from metal surfaces in strong electromagnetic fields is considered with allowance for relativistic effects. One of the methods of simulation in these processes is the particle method combined with grid calculation of fields on the basis of Maxwell’s equations. Similar techniques have been developed since the 1960s to the present. However, existing approaches have certain limitations. In this work, for an axisymmetric geometry of the generating system, a new numerical technique simulating the processes of electron emission from metal cathode surfaces is presented. The technique uses the representation of large smoothed Gaussian particles and implements the calculation of electromagnetic fields on Cartesian spatial grids. The software implementation is oriented to parallel computing. The aim of numerical experiments was to determine the parameters of electron emission. Diode and triode cylindrical systems were chosen as test problems. In numerical calculations, the spatiotemporal characteristics of relativistic electron beams generated by emission processes are obtained, including the reproduction of the Child–Langmuir current. The numerical technique developed has confirmed its correctness and efficiency.

Simulation of Emission Processes in Strong Electromagnetic Fields

Simulation of Emission Processes in Strong Electromagnetic Fields

SFQEDtoolkit: A high-performance library for the accurate modeling of strong-field QED processes in PIC and Monte Carlo codes

Strong-field quantum electrodynamics (SFQED) processes are central in determining the dynamics of particles and plasmas in extreme electromagnetic fields such as those present in the vicinity of compact astrophysical objects or generated with ultraintense lasers. SFQEDtoolkit is an open source library designed to allow users for a straightforward implementation of SFQED processes in existing particle-in-cell (PIC) and Monte Carlo codes. Through advanced function approximation techniques, high-energy photon emission and electron-positron pair creation probability rates and energy distributions are calculated within the locally-constant-field approximation (LCFA) as well as with more advanced models [Phys. Rev. A 99, 022125 (2019)]. SFQEDtoolkit is designed to provide users with high-performance and high-accuracy, and neat examples showing its usage are provided. In the near future, SFQEDtoolkit will be enriched to model the angular distribution of the generated particles, i.e., beyond the commonly employed collinear emission approximation, as well as to model spin and polarization dependent SFQED processes. Notably, the generality and flexibility of the presented function approximation approach makes it suitable to be employed in other areas of physics, chemistry and computer science. Program summaryProgram Title: SFQEDtoolkit (version 1.0)CPC Library link to program files:https://doi.org/10.17632/j7mhj3cg3m.1Developer's repository link:https://github.com/QuantumPlasma/SFQEDtoolkitLicensing provisions: GPLv3Programming language: C++, FortranNature of problem: The accurate simulation of strong-field QED processes in the interaction of particles or plasmas with strong electromagnetic fields is critical for present and future experiments with ultraintense lasers as well as to unveil the dynamics around compact astrophysical objects. State-of-the-art PIC and Monte Carlo codes implement strong-field QED processes by means of lookup tables, where the approximated function is evaluated on a fixed set of points. This typically results in numerical artifacts such as stairlike structures in the approximated function, and in a reduced efficiency when large size lookup tables are necessary. An easily implementable open-source accurate and efficient simulation tool is required to address problems at the high-intensity frontier of plasma physics and astrophysics.Solution method: Strong-field QED process rates and particle distributions are calculated by leveraging advanced approximation methods based on variable and function transformation, asymptotic expansions as well as Chebyshev polynomial expansions. This provides a numerically-stable and efficient approach that allows us to overcome all limitations of existing methods based on standard lookup tables.

Using analytically-corrected semiclassical rescattering model to study Coulomb impact in spiderlike photoelectron momentum distributions

We present a numerical investigation on the impact of the Coulomb interaction in the spiderlike photoelectron momentum distributions (PMDs) of a hydrogen atom ionized by an intense laser pulse. We have shown, by integrating analytical correction terms into the standard semiclassical rescattering model (SRM) to formulate the Coulomb analytically-corrected SRM (AC-SRM), that the interference fringes manifest a systematic shift along the transverse momentum direction upon considering the Coulomb action. A Coulomb correction to the SRM model has rarely been reported before. Analyses are made by varying physical quantities such as the carrier frequency and initial transverse velocity of the ionized electron. In particular, we decipher the impact of the Coulomb interaction with the classical-action phase map scheme, and demonstrate that this effect is more pronounced for smaller momenta in the spiderlike PMDs. It is proven that the presented AC-SRM is simple and effective in accounting for the Coulomb effect, as an alternative correction due to the Coulomb interaction to the standard SRM theory. Also, our phase map scheme is shown to be more powerful than the plain interference fringes in interrogating the Coulomb effect, especially for the first interference minimum. We anticipate that the present AC-SRM can be useful in investigating other strong field processes where the Coulomb interaction is considered.

High-order harmonic generation from a thin film crystal perturbed by a quasi-static terahertz field

Studies of laser-driven strong field processes subjected to a (quasi-)static field have been mainly confined to theory. Here we provide an experimental realization by introducing a bichromatic approach for high harmonic generation (HHG) in a dielectric that combines an intense 70 femtosecond duration mid-infrared driving field with a weak 2 picosecond period terahertz (THz) dressing field. We address the physics underlying the THz field induced static symmetry breaking and its consequences on the efficient production/suppression of even-/odd-order harmonics, and demonstrate the ability to probe the HHG dynamics via the modulation of the harmonic distribution. Moreover, we report a delay-dependent even-order harmonic frequency shift that is proportional to the time derivative of the THz field. This suggests a limitation of the static symmetry breaking interpretation and implies that the resultant attosecond bursts are aperiodic, thus providing a frequency domain probe of attosecond transients while opening opportunities in precise attosecond pulse shaping.

Ionization Dynamics for Atomic and Molecular Ions in Relativistic, Ultrastrong Laser Fields

The discovery of laser-driven rescattering and high harmonic radiation out to a maximum photon energy of 3.17 times the ponderomotive energy (Up ) laid the groundwork for attosecond pulse generation and coherent X-rays. As the laser field drives the interaction to higher energies, relativity and the Lorentz force from the laser magnetic field enter into the dynamics. We present the results of recent studies of laser rescattering, including these effects, to give a quantitative description of rescattering dynamics in the high-energy limit, ie, recollision energies of order 1,000 hartree (27 keV). The processes investigated include inner K- and L-shell excitation and the ultimate limit of high harmonic generation via rescattering bremsstrahlung. The results indicate the path to the frontier area of x-ray strong field processes.

Parametric waveform synthesis: a scalable approach to generate sub-cycle optical transients.

The availability of electromagnetic pulses with controllable field waveform and extremely short duration, even below a single optical cycle, is imperative to fully harness strong-field processes and to gain insight into ultrafast light-driven mechanisms occurring in the attosecond time-domain. The recently demonstrated parametric waveform synthesis (PWS) introduces an energy-, power- and spectrum-scalable method to generate non-sinusoidal sub-cycle optical waveforms by coherently combining different phase-stable pulses attained via optical parametric amplifiers. Significant technological developments have been made to overcome the stability issues related to PWS and to obtain an effective and reliable waveform control system. Here we present the main ingredients enabling PWS technology. The design choices concerning the optical, mechanical and electronic setups are justified by analytical/numerical modeling and benchmarked by experimental observations. In its present incarnation, PWS technology enables the generation of field-controllable mJ-level few-femtosecond pulses spanning the visible to infrared range.

All-optical nonlinear Breit-Wheeler pair production with γ -flash photons

High-power laser facilities give experimental access to fundamental strong-field quantum electrodynamics processes. A key effect to be explored is the nonlinear Breit-Wheeler process: the conversion of high-energy photons into electron-positron pairs through the interaction with a strong electromagnetic field. A major challenge to observing nonlinear Breit-Wheeler pair production experimentally is first having a suitable source of high-energy photons. In this paper we outline a simple all-optical setup which efficiently generates photons through the so-called $\ensuremath{\gamma}$-flash mechanism by irradiating a solid target with a high-power laser. We consider the collision of these photons with a secondary laser and systematically discuss the prospects for exploring the nonlinear Breit-Wheeler process at current and next-generation high-power laser facilities.

Rydberg state excitation in molecules manipulated by bicircular two-color laser pulses

Multiphoton resonant excitation and frustrated tunneling ionization, manifesting the photonic and optical nature of the driving light via direct excitation and electron recapture, respectively, are complementary mechanisms to access Rydberg state excitation (RSE) of atoms and molecules in an intense laser field. However, clear identification and manipulation of their individual contributions in the light-induced RSE process remain experimentally challenging. Here, we bridge this gap by exploring the dissociative and nondissociative RSE of H2 molecules using bicircular two-color laser pulses. Depending on the relative field strength and polarization helicity of the two colors, the RSE probability can be boosted by more than one order of magnitude by exploiting the laser waveform-dependent field effect. The role of the photon effect is readily strengthened with increasing relative strength of the second-harmonic field of the two colors regardless of the polarization helicity. As compared to the nondissociative RSE forming H2 * , the field effect in producing the dissociative RSE channel of ( H + , H * ) is moderately suppressed, which is primarily accessed via a three-step sequential process separated by molecular bond stretching. Our work paves the way toward a comprehensive understanding of the interplay of the underlying field and photon effects in the strong-field RSE process, as well as facilitating the generation of Rydberg states optimized with tailored characteristics.

Model of the aggregation process of fine ferromagnetic particles

The most important result of the effect of interparticle interactions of ferromagnetic particles in suspensions is the formation of aggregates and modification of their physical properties, which allows changing the kinetics of separation processes. The interest of the authors to this problem is dictated by both its theoretical content and purely applied aspects within mineral processing. Based on the block-structural approach to technological equipment modeling within the framework of the ideal mixing model, a mathematical model of processes occurring in the working volumes of apparatuses using magnetic forces to intensify the separation of mineral suspensions was created. The use of the MATHCAD software has allowed obtaining analytical solutions of the differential equations system of the model and predictions of the kinetics of aggregation of ferromagnetic particles and physical properties of the formed aggregates, such as coarseness, density, deposition velocity in the gravity field as functions of medium parameters and external magnetic fields. The analytical solutions are necessary for developing simulation models of magnetic separation processes in strong, weak, uniform and non-uniform magnetic fields, magnetic deslammers and filters.

Компьютерное моделирование процессов электронной эмиссии в сильных электромагнитных полях

The paper discusses the problem of computer simulation of electron emission processes in strong electromagnetic fields. In this work, a numerical method for calculating electron emission from the surface of a metal cathode for the axial symmetric geometry of a technical system is proposed. For modelling, the particle method and grid calculations of electromagnetic fields based on Maxwell's equations are applied. The approach uses the representation of large smoothed Gaussian particles and implements calculations of the electromagnetic fields on Cartesian spatial grids. The software realization is directed towards on parallel computing. The calculation of the emission process in a coaxial cylindrical diode with magnetic self-insulation with the use of developed technique was carried out. The process is considered for two situations: with and without the longitudinal plasma layer. The calculations show that the presence of plasma leads to an increase in the emission current. This result is consistent with the results of experiments.

Attaining a strong-field QED signal at laser-electron colliders with optimized focusing

Colliding bunches of high-energy electrons with intense laser pulses provides a basis for studying strong-field QED processes enabled by high values of quantum non-linearity parameter $\chi$. Nevertheless, the signal deconvolution is intricate due to probabilistic nature of the processes and shot-to-shot variation of the impact parameter, which disfavors the use of tight focusing. We propose a concept for distinguishing the signal of high-$\chi$ emissions that enables the use of optimal focusing to attain the highest $\chi \approx 5.25 \left(\varepsilon / (\text{1 GeV})\right)\left(P/(\text{1 PW})\right)^{1/2}\left((\text{1 }\mu\text{m})/\lambda\right)$ for a given electron energy $\varepsilon$, laser power $P$ and wavelength $\lambda$. Reaching such $\chi$ with f/2 focusing requires more than 10 times higher power.

Production of polarized particle beams via ultraintense laser pulses

High-energy spin-polarized electron, positron, and $$\gamma$$ -photon beams have many significant applications in the study of material properties, nuclear structure, particle physics, and high-energy astrophysics. Thus, efficient production of such polarized beams attracts a broad spectrum of research interests. This is driven mainly by the rapid advancements in ultrashort and ultraintense laser technology. Currently, available laser pulses can achieve peak intensities in the range of $$10^{22}$$ – $$10^{23}$$ $$\hbox {Wcm}^{-2}$$ , with pulse durations of tens of femtoseconds. The dynamics of particles in laser fields of the available intensities is dominated by quantum electrodynamics (QED) and the interaction mechanisms have reached regimes spanned by nonlinear multiphoton absorption (strong-field QED processes). In strong-field QED processes, the scattering cross-sections obviously depend on the spin and polarization of the particles, and the spin-dependent photon emission and the radiation-reaction effects can be utilized to produce the polarized particles. An ultraintense laser-driven polarized particle source possesses the advantages of high brilliance and compactness, which could open the way for the unexplored aspects in a range of researches. In this work, we briefly review the seminal conclusions from the study of the polarization effects in strong-field QED processes, as well as the progress made by recent proposals for production of the polarized particles by laser–beam or laser–plasma interactions.

Steering the longitudinal photoelectron momentum in the above-threshold ionization with two not-quite-collinear laser beams

Strong-field atomic processes, driven by long-wavelength laser beams, are known to be affected by magnetic forces. In such beams, the Lorentz force pushes the photoelectrons along the beam direction and prevents their rescattering or recombination with the parent ions. In high-order harmonic generation (HHG), therefore, the yield of energetic photons is markedly suppressed, rendering x-ray radiation sources from high harmonics so far impractical. To compensate these magnetic forces and to reenable HHG at long wavelengths, a setup of two not quite collinear beams has been suggested recently but not much analyzed beyond classical arguments and with respect to accessible laser parameters. Using the nondipole strong-field approximation, we here investigate when the longitudinal momentum of the photoelectrons vanishes and how this noncollinear setup explicitly depends on the wavelength and intensity of the driving beams. We also demonstrate that an optimal crossing angle ${\ensuremath{\delta}}_{0}$ between these beams always exists for which the fraction of the returning electrons is maximized. This rather simple steering of the longitudinal momentum will allow an efficient HHG with driving beams deep in the midinfrared.

Instrumentation challenges of the strong-field QED experiment LUXE at the European XFEL*

The LUXE experiment aims at studying strong-field QED in electron-laser and photon-laser interactions, with the 16.5 GeV electron beam of the European XFEL and a laser beam with power of up to 350 TW. The strong-field QED processes are expected to have production rates ranging from 10−3 to 109 per 10 Hz bunch crossing. Additionally, these measurements must be performed in a low-energy, high-radiation background. The LUXE experiment will utilise various detector technologies to overcome these challenges.