Strong-field Physics Research Articles

Characterizing post-compression of mJ-level ultrafast pulses via loose focusing in a gas cell

The ability to generate high-intensity ultrashort laser pulses is a key driver for advancing the strong-field physics and its applications. Post-compression methods aim to increase the peak intensity of amplified laser pulses via spectral broadening through self-phase modulation (SPM), followed by temporal pulse compression. However, other unavoidable nonlinear self-action effects, which typically occur parallel to SPM, can lead to phase distortions and beam quality degradation. Here we study the ability to compress high-energy pulses by loose focusing in a noble gas to induce nonlinear spectral broadening, while limiting unwanted nonlinear effects such as self-focusing. We introduce ptychographic wavefront sensor and FROG measurements to identify the regimes that optimize pulse compression while maintaining high beam quality. Using a 700 mbar argon-filled double-pass-based scheme, we successfully compress 2 mJ, 170 fs, 1030 nm laser pulses to ∼35 fs, achieving 90% overall flux efficiency and excellent stability. This work provides guidelines for optimizing the compressed pulse quality and further energy scaling of double-pass-based post-compression concepts.

Generation of sub-100 fs vortices from a Kerr-lens mode-locked Hermite-Gaussian Yb:CALGO oscillator.

Mode-locked oscillators with high-order transverse modes are excellent platforms for generating femtosecond optical vortices with high average power and good propagation stability. These have important applications in diverse fields such as optical communication, strong-field physics, and laser processing. So far, generating vortex pulses with ultrashort pulse duration remains a challenge. In this Letter, we report a Kerr-lens mode-locked Yb:CALGO laser oscillator delivering Hermite-Gaussian (HG) pulses with a pulse duration of 86 fs using a non-collinear pumping technique. 91 fs optical vortex pulses were generated by using a cylindrical-lens mode converter. To the best of our knowledge, this is the shortest pulse duration ever obtained from a diode pumped solid-state mode-locked oscillator with a pure high-order Hermite-Gaussian mode. The phase structures of the generated femtosecond vortices are characterized.

Ultrafast 550-W average-power thin-disk laser oscillator

SESAM modelocked oscillators are interesting for applications in strong-field physics such as high-harmonic generation and attosecond science at high repetition rates or frequency combs in the ultraviolet. Here we present a SESAM modelocked ultrafast thin-disk laser oscillator providing 550W of average output power with 852fs pulses at 5.5MHz repetition rate. To reach this significant power scaling, a replicating cavity design for modelocked oscillators is utilized. The oscillator delivers 103 MW of peak power with a pulse energy of 100 µJ at a beam quality of M2<1.2, with a high optical-to-optical efficiency of 35%. The advances in SESAM design and manufacturing that enabled this result are discussed, as well as practical challenges when scaling oscillators to the kW-class. When combined with established pulse compression technologies, this oscillator can enable simpler systems by avoiding the complexity of chirped pulse amplifier chains. Additionally, high power oscillators support a much lower noise floor due to the reduced influence of shot noise, which may provide a route to more sensitive pump-probe measurements.

Enhanced high harmonic efficiency through phonon-assisted photodoping effect

High-harmonic generation (HHG) has emerged as a central technique in attosecond science and strong-field physics, providing a tool for investigating ultrafast dynamics. However, the microscopic mechanism of HHG in solids is still under debate, and it is unclear how it is modified in the ubiquitous presence of phonons. Here we theoretically investigate the role of collectively coherent vibrations in HHG in a wide range of solids (e.g., hBN, graphite, 2H-MoS2, and diamond). We predict that phonon-assisted high harmonic yields can be significantly enhanced, compared to the phonon-free case – up to a factor of ~20 for a transverse optical phonon in bulk hBN. We also show that the emitted harmonics strongly depend on the character of the pumped vibrational modes. Through state-of-the-art ab initio calculations, we elucidate the physical origin of the HHG yield enhancement – phonon-assisted photoinduced carrier doping, which plays a paramount role in both intraband and interband electron dynamics. Our research illuminates a clear pathway toward comprehending phonon-mediated nonlinear optical processes within materials, offering a powerful tool to deliberately engineer and govern solid-state high harmonics.

Deciphering the role of three-color synthesized laser pulse parameters in N2 ionization and attosecond pulse shaping

Photoionization of atoms or molecules is a fundamental process in strong-field physics, essential for generating high-order harmonics and producing attosecond pulses. Here, we investigate how a three-color laser field, consisting of wavelengths at 800 nm and its second and third harmonics, photoionizes N2 molecules and evaluates its effect on the properties of attosecond pulses. It has been discovered that by altering the relative phase, these harmonics periodically influence molecular ionization, increasing the discreteness and strength of the attosecond pulse trains. Three-color field driving dramatically raises the pulse peak while decreasing the width of the strongest peak pulse, whereas two-color field driving just slightly enhances the pulse intensity and duration. Furthermore, the effectiveness of harmonic production is significantly increased by varying the laser intensity ratios. Even though the efficiency and pulse intensity of the three-color field are limited by conversion efficiency, attosecond pulses with higher intensities can be produced with proper parameter tuning at lower ionization rates. These findings offer a significant foundation for precisely controlling attosecond pulse generation. Ocis codes320.7110; 320.7120.

TW-scale few-cycle short-wave infrared laser based on nonlinear compression in a large-core gas-filled hollow core fiber

In this Letter, we present the generation of terawatt-scale few-cycle short-wave infrared (SWIR) lasers using nonlinear compression in a large-core gas-filled hollow core fiber. Through the experimental verification, we investigate the energy scaling properties and nonlinear pulse propagation in the argon-filled hollow core fiber. At static pressure, the system delivers pulses with 5.55 mJ/9.04 fs at a central wavelength of 1.45 μm, resulting in a peak power of about 0.5 TW. Subsequently, based on the chirped input pulses and pressure gradient, the system delivers terawatt-scale two-cycle SWIR pulses with 9.52 mJ/10.65 fs, resulting in a record peak power of about 0.7 TW. This study marks a crucial advancement in the field of SWIR ultra-intense, ultrashort pulse nonlinear interaction platforms. This high-energy, few-cycle SWIR source has significant applications in high-intensity THz radiation, x-ray generation, and other fields of strong-field physics.

Estimation of electric field in intermediate-energy heavy-ion collisions

We estimate the spacetime profile of the electric field in head-on heavy-ion collisions at intermediate collision energies sNN=O(3–10GeV). Using a hadronic cascade model (JAM; Jet AA Microscopic transport model), we numerically demonstrate that the produced field has strength eE=O((30–60MeV)2), which is supercritical to the Schwinger limit of QED and is non-negligibly large compared even to the hadron/QCD scale, and survives for a long time τ=O(10fm/c) due to the baryon stopping. We show that the produced field is nonperturbatively strong in the sense that the nonperturbativity parameters (e.g., the Keldysh parameter) are sufficiently large. This is in contrast to high-energy collisions sNN≳100GeV, where the field is extremely short-lived and hence is perturbative. Our results imply that the electromagnetic field may have phenomenological impacts on hadronic/QCD processes in intermediate-energy heavy-ion collisions and that heavy-ion collisions can be used as a new tool to explore strong-field physics in the nonperturbative regime. Published by the American Physical Society 2024

Field-resolved space–time characterization of few-cycle structured light pulses

Accompanied by the rapid development of ultrafast laser platforms in recent decades, the spatiotemporal manipulation of ultrashort laser pulses has attracted much attention due to the potential for cutting-edge applications of structured light, including optical tweezers, optical communications, super-resolution imaging, time-resolved spectroscopy in molecules and quantum materials, and strong-field physics. Today, techniques capable of characterizing the full spatial, temporal, and polarization state properties of structured light are strongly desired. Here, we demonstrate a technique, termed 3D TIPTOE, for characterizing structured mid-infrared waveforms, which uses only a two-dimensional silicon-based image sensor as both the detector and the nonlinear medium. By combining the advantages of the sub-cycle time resolution afforded by nonlinear excitation and the spatial resolution inherent to the two-dimensional sensor, the 3D TIPTOE technique allows full characterization of structured electric fields, significantly reducing the complexity of detection compared to other techniques. The validity of the technique is established by measuring both few-cycle Bessel–Gaussian pulses and radially polarized femtosecond vector beams.

The hybrid anti-symmetrized coupled channels method (haCC) for the tRecX code

We present a new implementation of the hybrid antisymmetrized Coupled Channels (haCC) method in the framework of the tRecX (Scrinzi, 2022 [6]). The method represents atomic and molecular multi-electron functions by combining CI functions, Gaussian molecular orbitals, and a numerical single-electron basis. It is suitable for describing high harmonic generation and the strong-field dynamics of ionization. Fully differential photoemission spectra are computed by the tSurff method. The theoretical background of haCC is outlined and key improvements compared to its original formulation are highlighted. We discuss control of over-completeness resulting from the joint use of the numerical basis and Gaussian molecular orbitals by pseudo-inverses based on the Woodbury formula. Further new features of this tRecX release are the iSurff method, new input features, and the AMOS gateway interface. The mapping of haCC into the tRecX framework for solving the time-dependent Schrödinger equation is shown. Use, performance, and accuracy of haCC are discussed on the examples of high-harmonic generation and strong-field photo-emission by short laser pulses impinging on the Helium atom and on the linear molecules N2 and CO. Program summaryProgram title: tRecX — time-dependent Recursive indeXing (tRecX=tSurff+irECS)CPC Library link to program files:https://doi.org/10.17632/m9g2jc82sw.1Developer's repository link:https://gitlab.physik.uni-muenchen.de/AG-Scrinzi/tRecXLicensing provisions: GNU General Public License 2Programming language: C++External libraries: Eigen, arpack, lapack, blas, boost, FFTW (optional)Journal Reference of previous version: A. Scrinzi, Comp. Phys. Comm., 270:108146, 2022.Does the new version supersede the previous version: YesReasons for the new version: Major new functionality: haCC — hybrid antisymmetrized coupled channels methodSummary of revisions: Main additions are haCC and iSurff. Code usage and compilation were improved.Nature of problem: tRecX is a general solver for time-dependent Schrödinger-like problems, with applications mostly in strong field and attosecond physics. There are no technical restrictions on the spatial dimension of the problem with up to 6 spatial dimensions realized in the strong-field double ionization of Helium. Gaussian-based quantum chemical multi-electron atomic and molecular structure can be combined with the numerical basis. A selection of coordinate systems is available and any Hamiltonian involving up to second derivatives and arbitrary up to three dimensional potentials can be defined on input by simple scripts.Solution method: The method of lines is used with spatial discretization by a flexible combination of one dimensional basis sets, DVR representations, discrete vectors, expansions into higher-dimensional eigenfunctions of user-defined operators, and Gaussian based molecular orbitals. Multi-electron Gaussian-based CI (configuration interaction) functions for neutrals and ions are combined with the numerical basis. Photo-emission spectra are calculated using the time-dependent surface flux method (tSurff) in combination with infinite range exterior complex scaling (irECS) for absorption. The code is object oriented and makes extensive use of tree-structures and recursive algorithms. Parallelization is by MPI. Code design and performance allow use in production as well as for graduate level training.

Dense Polarized Positrons from Beam-Solid Interaction.

Relativistic positron sources with high spin polarization have important applications in nuclear and particle physics and many frontier fields. However, it is challenging to produce dense polarized positrons. Here we present a simple and effective method to achieve such a positron source by directly impinging a relativistic high-density electron beam on the surface of a solid target. During the interaction, a strong return current of plasma electrons is induced and subsequently asymmetric quasistatic magnetic fields as high as megatesla are generated along the target surface. This gives rise to strong radiative spin flips and multiphoton processes, thus leading to efficient generation of copious polarized positrons. With three-dimensional particle-in-cell simulations, we demonstrate the production of a dense highly polarized multi-GeV positron beam with an average spin polarization above 40% and nC-scale charge per shot. This offers a novel route for the studies of laserless strong-field quantum electrodynamics physics and for the development of high-energy polarized positron sources.

High harmonic generation in condensed and engineered materials: introduction

The emerging field of high harmonic generation in condensed matter systems lies at the confluence of strong-field physics, ultrafast optics, and nanotechnology and offers numerous avenues for fundamental research and applications. The goals of this JOSA B feature issue on high harmonic generation in condensed and engineered materials are to facilitate interaction between the different communities and to provide an up-to-date snapshot of the current status of this rapidly developing interdisciplinary field at the frontier of condensed materials and ultrafast physics.

SLIMP 2.0: A new version of strong laser interaction model package for atoms and molecules, now with molecular orbital tomography based on high-order harmonic spectra

A new version of strong laser interaction model package for atoms and molecules is presented. Its capacities are enhanced by incorporating modules related to molecular orbital tomography (MOT) in length and velocity forms based on high-order harmonic spectra (HHS). This advanced package enables two-dimensional and three-dimensional MOT of both symmetric and asymmetric molecules using HHS, providing an efficient means for both experimental and theoretical investigations of molecular orbital structures. In addition, it can calculate the precise theoretical orbitals corresponding to those reconstructed by MOT. Furthermore, the package can calculate the transition dipole moment (TDM) in the plane wave approximation and allow for reference MOT based on TDM. It can evaluate the quality of reconstructed orbitals based on HHS and reveal the physical and numerical effects on MOT. This package is suitable for MOT under conditions of driving laser at different intensities and wavelengths, and can be applied to different molecules with various structures. It is designed to facilitate easy expansion for additional capabilities. Program summaryProgram Title: SLIMP, version 2.0CPC Library link to program files:https://doi.org/10.17632/jgcb7m7wwv.1Licensing provisions: GPLv3Programming language: Fortran 90Journal reference of previous version: Comput. Phys. Commun. 192 (2015) 330-341.Does the new version supersede the previous version?: Yes.Reasons for the new version: Molecular orbital tomography (MOT) [1] based on high-order harmonic spectra (HHS) [2] is an important research topic in the field of strong-field physics. It provides an efficient way to probe molecular orbital structures experimentally, which was not considered in the former version of SLIMP [3] and is now included in SLIMP 2.0. The new version package allows performing two-dimensional (2D) and three-dimensional (3D) MOT [4] based on HHS obtained from experiments or simulations, providing reliable detections for many orbitals of interest, either symmetric or asymmetric molecules.Summary of revisions: Extending capacities focus on enabling performing 2D and 3D MOT on symmetric and asymmetric molecules based on HHS, and together the relevantly precise theoretical orbital output, calculation of transition dipole moment (TDM) and reference MOT based on TDM. For 2D MOT, MOT in both length and velocity forms are implemented.Nature of problem: To perform MOT based on HHS, the user needs general reconstruction programs, taking symmetric and asymmetric molecules, 2D and 3D orbitals, and MOT in length and velocity forms into account. In addition, evaluation of imaging quality is required, which can reveal the physical and numerical effects on reconstructions.Solution method: The 2D symmetric and asymmetric, length- and velocity-form, and 3D reconstruction algorithms are implemented based on Fortran 90. Furthermore, this package provides a reference MOT based on directly calculated TDM, served as a comparable result to the reconstructed orbital based on HHS. The comparison between reconstructions based on HHS and TDM can reveal the effects on MOT in physical and numerical aspects.Additional comments including restrictions and unusual features: This package does not include MOT of asymmetric molecules in velocity form. For the 2D MOT based on HHS, the reconstructed 2D projection of gerade, ungerade orbital or the gerade and ungerade components of asymmetric orbital must have the axial symmetry about the x- and z-axis. For the 3D MOT, the reconstructed 3D orbital has to be cylindrically symmetric.

Efficient single-cycle mid-infrared femtosecond laser pulse generation by spectrally temporally cascaded optical parametric amplification with pump energy recycling.

We proposed spectrally temporally cascaded optical parametric amplification (STOPA) using pump energy recycling to simultaneously increase spectral bandwidth and conversion efficiency in optical parametric amplification (OPA). Using BiB3O6 and KTiOAsO4 nonlinear crystals, near-single-cycle mid-infrared (MIR) pulses with maximum energy conversion efficiencies exceeding 25% were obtained in simulations. We successfully demonstrated sub-two-cycle, CEP-stable pulse generation at 1.8 µm using a four-step STOPA system in the experiment. This method provides a solution to solve the limitations of the gain bandwidth of nonlinear crystals and the low conversion efficiency in broadband OPA systems, which is helpful for intense attosecond pulse generation and strong laser field physics studies.

Advances in the development of few-cycle gigawatt-peak-power mid-IR optical parametric amplifiers pumped by Cr:Forsterite laser using non-oxide nonlinear crystals

We demonstrate an experimental and theoretical comparison of non-oxide LiGaS2, HgGa2S4, and AgGaS2 crystals performance for wavelength conversion into the near and mid-IR range 1.5–8 μm in optical parametric amplifier pumped by Cr:Forsterite laser, delivering 100 fs pulses at 1.24 μm. It is shown that exceptionally high total energy conversion efficiency into the idler (4–5 μm) and signal (1.65–1.8 μm) waves up to 18% can be achieved using the HGS crystal, providing high nonlinearity, while the LGS crystal is more preferable for generating few-cycle mid-IR pulses due its unique dispersive properties. Our source features high peak power in gigawatt regime (0.4–2.4 GW) with pulse duration below 80 fs and optical synchronization with high harmonic generation (HHG) and THz beamlines, which is ideal for pump-probe experiments of nonlinear and strong-field physics.

Gain Measurement of ZnGeP2 Optical Parametric Oscillator Pulses in a High-Pressure CO2 Amplifier

Laser pulse amplification by a high-pressure CO2 amplifier in the long-wave infrared (LWIR) spectral range is a feasible technology for strong-field physics research. Crystals such as ZnGeP2 (ZGP) have high nonlinear coefficients and transmittance in the LWIR region, with spectral widths of generated pulses closely matching the gain spectrum of high-pressure CO2 amplifiers. Therefore, ZGP optical parametric oscillation (OPO) may allow higher-efficiency energy extraction in amplifiers, improving the output characteristics of LWIR amplification systems. In this study, the gain measurement of ZGP OPO pulses amplified by a high-pressure CO2 amplifier was carried out for the first time. Single-detector acquisition was utilized to achieve a unified sensor responsivity, and a laser signal-triggered function generator was used to synchronize the seed pulse and amplifier. Six-pass amplification was performed successively, yielding an amplification factor of 4.5 for the peak power and a maximum coefficient of 0.42% cm−1 for the small-signal gain. The gain and loss effect during small-signal amplification were discussed. The potential capability of acquiring ultra-short pulses with ZGP OPO pulses was also explored with the FFT function of MATLAB software.

Integrating artificial intelligence into the simulation of structured laser-driven high harmonic generation

High harmonic generation (HHG) stands as one of the most complex processes in strong-field physics, as it enables the conversion of laser light from the infrared to the extreme-ultraviolet or even the soft x-rays, enabling the synthesis and control of pulses lasting as short as tens of attoseconds. Accurately simulating this nonlinear and non-perturbative phenomena requires the coupling the dynamics of laser-driven electronic wavepackets, described by the three-dimensional time-dependent Schrödinger equation (3D-TDSE), with macroscopic Maxwell’s equations. Such calculations are extremely demanding due to the duality of microscopic and macroscopic nature of the process, thereby requiring the use of approximations. We develop a HHG method assisted by artificial intelligence that facilitates the simulation of macroscopic HHG within the framework of 3D-TDSE. This approach is particularly suited to simulate HHG driven by structured laser pulses. In particular, we demonstrate a self-interference effect in HHG driven by Hermite-Gauss beams. The theoretical and experimental agreement allows us to validate the AI-based model, and to identify a unique signature of the quantum nature of the HHG process.

Envelope effect of Rydberg States Generation in Strong Laser Pulses

Rydberg atoms are important building blocks for quantum technologies, exploited to new applications in quantum computing, quantum communication and quantum sensing due to their unique tunable quantum properties.Besides the widely-used few-photon resonant excitation for the specific Rydberg state, multiple Rydberg states can be populated coherently and effciently through the frustrated tunneling ionization or the Coulomb potential recapture effect in strong laser field. The Rydberg states excitation in strong field provides an opportunity to realize the ultrafast quantum control on Rydberg atom and bridge the strong field physics and quantum information technology. <br>Using the Classical Trajectory Monte Carlo method and Qprop package to solve Time-Dependent increases with the parameter of the asymmetric laser envelope. Based on the Quantitative Rescattering theory, the calculated time-dependence of recapture rate is negatively related to the laser envelope and the residual laser interaction time, which is termed as envelope effect. Combined with the carrier-wave effect, an analytic formula is proposed to calculate the Rydberg states population:$Y(t) \propto W_0(t) \frac{t-\tau+c}{f(t)} \cos (\omega t+\phi)$ This results open the door to enhance the Rydberg states generation using the laser envelope control, benefiting the future quantum technology based on the Rydberg states generated in the strong laser field.(a) Laser electric fields with the same pulse duration τ = 10T<sub>0</sub> and different asymmetric parameters α. Black solid line, red dashed line and blue dashed-dotted line are for α = -0.6, 0, 0.6 respectively. (b) The yields of Rydberg states change with the asymmetric parameter under different laser pulse duration. Black solid line, red dashed line and blue dashed-dotted line are for τ = 1010T<sub>0</sub>, 1510T<sub>0</sub>, 2010T<sub>0</sub> respectively. Schrödinger Equation, we calculate the population of Rydberg states. Our results show that the population.

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.

Nonlinear compression of naturally down-chirped superradiance pulses from a free-electron laser oscillator by thick germanium plates.

Naturally down-chirped superradiance pulses, with mirco-pulse energy, peak wavelength, and micropulse duration of 40 µJ, 8.7 μm, and 5.1 optical cycles, respectively, emitted from a free-electron laser (FEL) oscillator were nonlinearly compressed down to 3.7 optical cycles using a 30-mm-thick Ge plate. The peak power enhancement owing to nonlinear compression was found to be 40%. The achieved peak power and pulse duration were comparable to those of recently developed high-intensity and few-cycle long-wavelength infrared sources based on solid-state lasers. FEL oscillators operating in the superradiance regime can serve as unique tools for studying strong-field physics in long-wavelength infrared regions.

High-order harmonic interferometry for the study of intensity-dependent dipole phase

Abstract High-order harmonics are ideal probes to resolve the attosecond dynamics of strong field recollision process. In this work, an easy-to-implement phase mask is utilized to covert the Gaussian beam to TEM01 transverse electromagnetic mode, allowing the realization of two source interferometry of high-order harmonics. We experimentally measured the intensity-dependence of dipole phase directly with high-order harmonic interferometry, in which the driving laser intensity can be precisely adjusted. The classical electron excursion simulations reproduce the experimental findings quite well, demonstrating that Coulomb potential plays subtle roles on the movement of electrons for harmonics near the ionization threshold. This work is of great importance for precision measurements of ultrafast dynamics in strong-field physics.