Driving Laser Pulse Research Articles

Inline UV pulse synthesizer

We demonstrated an inline synthesizer for generating ultrashort pulses in the ultraviolet (UV) range. The inline UV pulse synthesizer comprised three nonlinear crystals located in the propagation path of the fundamental driving laser pulse. Second-harmonic signals with central wavelengths of 420, 375, and 345 nm were generated in turn in the three BBO crystals, resulting in a synthesized UV pulse subsequent to the final nonlinear crystal. Its temporal amplitude and phase could be manipulated easily by changing the relative positions of the crystals, allowing for flexibility of the waveform. The minimum pulse duration of the synthesized UV pulse was 4.7 fs, which was close to the Fourier-transform-limited pulse duration. This ultrashort UV pulse with 19 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu$$\\end{document}J energy can be utilized in various applications such as high harmonic generation and frustrated tunneling ionization.

Extraction of Molecular-Frame Electron-Ion Differential Scattering Cross Sections Based on Elliptical Laser-Induced Electron Diffraction.

We extracted the molecular-frame elastic differential cross sections (MFDCSs) for electrons scattering from N_{2}^{+} based on elliptical laser-induced electron diffraction (ELIED), wherein the structural evolution is initialized by the same tunneling ionization and probed by incident angle-resolved laser-induced electron diffraction imaging. To establish ELIED, an intuitive interpretation of the ellipticity-dependent rescattering electron momentum distributions was first provided by analyzing the transverse momentum distribution. It was shown that the incident angle of the laser-induced returning electrons could be tuned within 20° by varying the ellipticity and handedness of the driving laser pulses. Accordingly, the incident angle-resolved DCSs of returning electrons for spherically symmetric targets (Xe^{+} and Ar^{+}) were successfully extracted as a proof-of-principle for ELIED. The MFDCSs for N_{2}^{+} were experimentally obtained at incident angles of 4° and 7°, which were well reproduced by the simulations. The ELIED approach is the only successful method so far for obtaining incident angle-resolved ionic MFDCS, which provides a new sensitive observable for the transient structure retrieval of N_{2}^{+}. Our results suggest that the ELIED has the potential to extract the structural tomographic information of polyatomic molecules with femtosecond and subangstrom spatiotemporal resolutions that can enable the visualization of the nuclear motions in complex chemical reactions as well as chiral recognition.

Simulation Study on Attosecond Inverse Compton Scattering Source from Laser Wakefield Acceleration with Near-Threshold Ionization Injection

We present the generation of attosecond gamma rays via inverse Compton scattering within the framework of laser wakefield acceleration through 2D Particle-In-Cell simulations. Utilizing the near-threshold ionization injection mechanism, an attosecond micro-bunched electron beam characterized by a comb-like current density profile can be achieved with a linearly polarized laser at an intensity of a0 = 1.5. The micro-bunched beam provides a beam energy of approximately 300 MeV and achieves a minimum relative energy spread of about 1.64% after undergoing 2 mm of acceleration. In the inverse Compton scattering scheme, these attosecond electron micro-bunches interact with the reflected driving laser pulse, resulting in the attosecond gamma-ray radiation exhibiting similar structures. Individual spatial-separated gamma-ray pulses exhibit a length of approximately 260–300 as, with a critical energy of 2.0 ± 0.2 MeV. The separated attosecond gamma-ray source owns a peak brilliance of ~1022 photons s−1 mm−2 mrad−2 0.1% BW. This brilliance is competitive in a laboratory for multi-MeV γ-ray sources with a laser intensity of I = 5 × 1018 W/cm2. Such attosecond gamma-ray radiation offers promising applications requiring ultrashort X-ray/gamma ray sources.

Photoelectron momentum distributions of triatomic CO2 molecules by circularly polarized attosecond pulses

Abstract Molecular-frame photoelectron momentum distributions (MF-PMDs) have been studied for imaging molecular structures. We investigate the MF-PMDs of CO2 molecules exposed to circularly polarized (CP) attosecond laser pulses by solving the time-dependent Schrödinger equations based on the single-active-electron approximation frames. Results show that high-frequency photons lead to photoelectron diffraction patterns, indicating molecular orbitals, these diffraction patterns can be illustrated by the ultrafast photoionization models. However, for the driving pulses with 30 nm, a deviation between MF-PMDs and theoretically predicted results of the ultrafast photoionization models is produced because the Coulomb effect strongly influences the molecular photoionization. Meanwhile, the MF-PMDs rotates in the same direction as the helicity of driving laser pulses. Our results also demonstrate that the MF-PMDs in a CP laser pulse are the superposition of those in the parallel and perpendicular linearly polarized cases. The simulations efficiently visualize molecular orbital geometries and structures by ultrafast photoelectron imaging. Furthermore, we determine the contribution of HOMO and HOMO-1 orbitals to ionization by varying the relative phase and the ratio of these two orbitals.

Guided Mode Evolution and Ionization Injection in Meter-Scale Multi-GeV Laser Wakefield Accelerators.

We show that multi-GeV laser wakefield electron accelerators in meter-scale, low density hydrodynamic plasma waveguides operate in a new nonlinear propagation regime dominated by sustained beating of lowest order modes of the ponderomotively modified channel; this occurs whether or not the injected pulse is linearly matched to the guide. For a continuously doped gas jet, this emergent mode beating effect leads to axially modulated enhancement of ionization injection and a multi-GeV energy spectrum of multiple quasimonoenergetic peaks; the same process in a locally doped jet produces single multi-GeV peaks with <10% energy spread. A three-stage model of drive laser pulse evolution and ionization injection characterizes the beating effect and explains our experimental results.

Femtosecond Dynamics of Fast Electron Pulses in Relativistic Laser-Foil Interactions.

We report the femtosecond time-resolved dynamics of relativistic electron pulses in ultraintense laser-foil interactions, by characterizing the terahertz self-radiation with single-shot ultrabroadband interferometry. Experimental measurements together with theoretical modeling reveal that the electron pulses inherit the duration of the driving laser pulse. We also visualize the electron recirculation dynamics, where electrons remain trapped inside the self-generated electrostatic potential well and rebound back and forth around the thin foil for hundreds of femtoseconds. Our results not only demonstrate an insitu, real-time metrology scheme for electron bursts, but also have important implications for understanding and manipulating the time-domain properties of laser-driven particle and radiation sources.

Ultraviolet supercontinuum generation using a differentially-pumped integrated glass chip

We investigate the generation of ultrabroadband femtosecond ultraviolet (UV) radiation via third-order harmonic generation in highly confined gas media. A dual-stage differential-pumping scheme integrated into a glass microfluidic chip provides an exceptional gas confinement up to several bar and allows the apparatus to be operated under high-vacuum environment. UV pulses are generated both in argon and neon with up to ∼0.8 μJ energy and 0.2% conversion efficiency for spectra that cover the UVB and UVC regions between 200 and 325 nm. Numerical simulations based on the unidirectional pulse propagation equation reveal that ionization plays a critical role for extending the spectral bandwidth of the generated third-harmonic pulse beyond the tripled 800 nm driving laser pulse bandwidth. By delivering UV supercontinua supporting Fourier transform limits below 2 fs, as well as comparable pulse energies with respect to capillary-based techniques that typically provide high spectral tunability but produce narrower bandwidths, our compact device makes a step forward towards the production and application of sub-fs UV pulses for the investigation of electron dynamics in neutral molecules.

Correlations in strong-field-emitted ultrashort electron pulses from metal needle tips

When two electrons are emitted from a metal needle tip with the help of femtosecond laser pulses, they show a strong anticorrelation signal in the energy domain. Depending on the wavelength and intensity of the driving laser pulses, the electron emission process can be either in a perturbative regime, like single- or multi-photon photoemission, or in the strong-field regime, where emission is dominated by the instantaneous electric field of the laser pulse, or in the intermediate regime. Here, we report on the two-electron anticorrelation signal and how it evolves from the multiphoton toward the strong-field emission regime. We show that in both cases, the resulting anticorrelation signal can be well explained by semi-classical simulations using a point-particle model, thus the dynamics is dominated by the center-of-mass dynamics of the individual electrons. However, the actual emission process of multiple interacting electrons requires improved quantum mechanical models and therefore remains the subject of future work.This paper is part of the Special Topic Collection: papers from the 31th Annual International Laser Physics Workshop 2023 (LPHYS 2023).

Photoelectron jets and interference structures in ionization beyond dipole approximation: carrier- envelope phase effects and vortex streets.

We have analyzed photoelectron jet formation in strong-field ionization using the hydrodynamic picture of quantum mechanics. We showed that von-Kármán-like vortex streets emerge in between jets in photoelectron momentum distributions. The spatial orientation of the jets can be controlled by tailoring the carrier-envelope phase of the driving laser pulse. This indicates that it is possible to experimentally measure emitted photoelectrons off the optical axis, which opens up new possibilities for high-frequency laser pulse diagnostics.

Generation of ultra-intense vortex laser from a binary phase square spiral zone plate.

With the development of ultra-intense laser technology, the manipulation of relativistic laser pulses has become progressively challenging due to the limitations of damage thresholds for traditional optical devices. In recent years, the generation and manipulation of ultra-intense vortex laser pulses by plasma has attracted a great deal of attention. Here, we propose a new scheme to produce a relativistic vortex laser. This is achieved by using a relativistic Gaussian drive laser to irradiate a plasma binary phase square spiral zone plate (BPSSZP). Based on three-dimensional particle-in-cell (3D-PIC) simulations, we find that the drive laser has a phase difference of π after passing through the BPSSZP, ultimately generating the vortex laser with unique square symmetry. Quantitatively, by employing a drive laser pulse with intensity of 1.3 × 1018~W/cm2, a vortex laser with intensity up to 1.8 × 1019~W/cm2, and energy conversion efficiency of 18.61% can be obtained. The vortex lasers generated using the BPSSZP follow the modulo-4 transmutation rule when varying the topological charge of BPSSZP. Furthermore, the plasma-based BPSSZP has exhibited robustness and the ability to withstand multiple ultra-intense laser pulses. As the vortex laser generated via the BPSSZP has high intensity and large energy conversion efficiency, our scheme may hold potential applications in the community of laser-plasma, such as particles acceleration, intense high-order vortex harmonic generation, and vortex X/γ-ray sources.

Tunable multi-cycle terahertz pulse generation from a spintronic emitter

We demonstrate that a spintronic terahertz (THz) emitter can be driven by a chirped-pulse beating scheme to generate narrowband THz pulses, with continuous tuning of the frequency and linewidth by simply adjusting the laser chirp and/or the time delay between chirped pulses. As supported by model calculations, temporal shaping of the drive laser pulses can be exploited to manipulate the ultrafast demagnetization dynamics in the thin-film emitter, modulating the spin-polarized current in the ferromagnetic layer to access multi-cycle THz emission. Using a regenerative amplifier laser system with 50 fs transform-limited pulses chirped to 6 ps, we demonstrate narrowband THz generation over a frequency range from 0.4 to 2.3 THz, in addition to linewidths down to 40 GHz using 12 ps chirped pulses. Our proof-of-concept results pave the way to future narrowband THz sources with subgigahertz linewidth and center frequencies continuously tunable from 0.1 to 30 THz. By combining with the advantageous properties of spintronic THz emitters, from straightforward implementation to flexible polarization control, these sources open up opportunities for narrowband applications over the entire THz spectral range.

Precollision effect on the double ionization of an Ar atom driven by a two-color counter-rotating circularly polarized laser pulse

The double ionization process of an Ar atom driven by a two-color counter-rotating circularly polarized (TCCP) laser pulse is theoretically investigated. Classical trajectory analysis reveals that a significant portion of Nonsequential Double Ionization (NSDI) events occurs through precollision, rather than the well-known recollision mechanism. In the precollision-induced Nonsequential Double Ionization (PCDI) process, the two electrons exhibit a tendency for back-to-back emission, resulting in a lower electronic correlation effect, and the ion recoil momentum concentrated in the zero momentum region. On the other hand, in the recollision-induced Nonsequential Double Ionization (RCDI) process, the two electrons tend to emit in the same direction, leading to a higher electronic correlation effect, and the ion recoil momentum is situated far from the zero momentum region. Moreover, both PCDI and RCDI strongly depend on the parameters of the driving laser pulse. Therefore, the primary mechanism of double ionization can be experimentally probed by observing the corresponding ion recoil momentum.

Control of Spatiotemporal Localization of Infrared Pulses in Gas-Filled Capillaries Using Weak Ultraviolet Pulses

Manipulation of intense pulse propagation in gas-filled capillaries is desirable for various high-field applications. Tuning the parameters of the driving laser pulse and the working gas is the conventional approach, and it provides limited capability of control. Here we demonstrate through numerical simulations a practical scheme to control the propagation of intense pulses. A weak ultraviolet pulse is launched into a capillary with a negative delay with respect to a main infrared pulse. The pulses begin to temporally overlap due to dispersion. As the main pulse self-compresses, the control pulse is strongly red-shifted due to cross-phase modulation. The frequency shifts of the two pulses mitigate pulse walk-off and allow an efficient coupling, substantially extending the effective interaction length. This interesting phenomenon may benefit applications such as high-order harmonic generation.

Generation of high-order harmonics and attosecond pulses in the water window via nonlinear propagation of a few-cycle laser pulse.

We theoretically and computationally study the generation of high-order harmonics in the water window from a semi-infinite gas cell where a few-cycle, carrier-envelope-phase-controlled 1.7-µm driving laser pulse undergoes nonlinear propagation via optical Kerr effect (self-focusing) and plasma defocusing. Our calculation shows that high harmonic signals are enhanced for extended propagation distances and furthermore, isolated attosecond pulses in the water window can be generated from the semi-infinite gas cell. This enhancement is attributed mainly to better phase matching for extended propagation distances achieved via nonlinear propagation and resulting intensity stabilization.

Thomson scattering measurements of electron temperature and electron density in laser-driven Gd plasmas

In this study, we demonstrate the laser intensity scaling of electron temperature in Gd laser-produced plasmas (LPPs) through experiment and simulation. The spatial and temporal profiles of electron density ne and temperature Te in Gd LPPs were directly measured during a drive laser pulse duration of 7 ns at a laser wavelength of 1064 nm using collective Thomson scattering. We found that the measured maximum Te value in Gd LPPs increases with increasing laser intensity IL , with the dependence , in the IL range of 1010–. Radiation-hydrodynamic simulation code of the STAR-2D was performed under identical conditions to those used in the experiment, and extended further to higher laser intensities of up to ; the simulated Te was found to be in good agreement with the experimental data over the used IL range. The simulation indicates that higher overall maximum Te typically exists 50–70 above the target and modifies the dependence as . Experiments reveal that a laser intensity of is required to achieve the optimum condition ( 100 eV and ion charge states , at –) for efficient beyond extreme ultraviolet (EUV) light source. In addition, two spot sizes (150 and 250 in diameter) were used to study the effect of the spot size on the Te profile. Our experimental findings, supported by the simulations, show that larger spots can create uniform Te profiles, higher maximum Te , and larger high-Te regions in LPPs. The results and scaling laws presented in this study provide important information for developing more powerful and energy-efficient EUV light sources.

Effect of pulse duration on the above-threshold ionization of a hydrogen atom irradiated by a 400 nm intense laser.

The photoelectron emission spectra generated by the interaction between ultrashort intense laser pulses and atoms can reveal the ultrafast dynamics of electrons. By using the numerical solution of the time-dependent Schrödinger equation in momentum space, the photoelectron emission spectra of atoms irradiated by 400 nm intense lasers with different durations of the pulse has been investigated. In the photoelectron emission spectrum, in addition to the above-threshold ionization peaks due to ionization interference in multiple cycles and the sideband peaks mainly due to the interference of ionized electrons at different moments along the rising edge of the laser pulse envelope, additional peaks of photoelectron emission whose intensity appears to oscillate with the increasing duration of the laser pulse can also be observed. Based on strong-field approximation and the population's analysis of the bound state, it is found that these photoelectron peaks originate from the ionization of the excited state and the oscillations of these peaks are due to the superposition of their peak energy positions with the sideband energy positions. Furthermore, it is demonstrated that the energy positions of the maximum intensity of the photoelectron emission spectra move towards the higher energy end as the duration of the driving laser pulse extends. This phenomenon can be attributed to the fact that the main moment of ionization of atoms changes with the increasing duration of the driving laser pulse, thus allowing the real-time ionization of atoms to be probed using photoelectron emission spectra.

High-Intensity Harmonic Generation with Energy Tunability Produced by Robust Two-Color Linearly Polarized Laser Fields

By using the numerical solution of the time-dependent Schrödinger equation, we theoretically explored the high-order harmonic generation process under the interaction of high-intensity two-color ultrashort driving laser pulses with atoms. The symmetry of the electric field of the laser pulse will be broken. The producing electric field was controlled at the subcycle level by an IR laser and its second harmonic, which has the unique characteristic that two sequential half-cycles become distinct, rather than merely opposite in sign. Compared with the case of the atom in the fundamental laser pulse, the harmonic efficiency showed an increase of 1∼2 orders of magnitude at specific harmonic order with this combined pulse action. Through the theoretical analysis with the “three-step model”, it was demonstrated that the enhancement of the harmonic intensity is due to the fast ionization of electrons at the ionization moment and the short time from ionization to recombination of ionized electrons. In addition, effects of the peak field amplitude ratio, the full width at half maximum, the phase delay of the two-color pulses, the laser intensity and ionization probability on the harmonic efficiency enhancement were also investigated.

Influence of chirp and carrier-envelope phase on noninteger high-harmonic generation

High harmonic generation (HHG) is a versatile technique for probing ultrafast electron dynamics. While HHG is sensitive to the electronic properties of the target, HHG also depends on the waveform of the laser pulse. As is well known, (peak) positions, $\omega$, in the high-harmonic spectrum can shift when the carrier envelope phase (CEP), $\varphi$ is varied. We derive formulae describing the corresponding parametric dependencies of CEP shifts; in particular, we have a transparent result for the (peak) shift, $d\omega/d\varphi = {-} 2 \bar{\mathfrak f}' \omega/\omega_0$, where $\omega_0$ describes the fundamental frequency and $\bar{\mathfrak f}'$ characterizes the chirp of the driving laser pulse. We compare the analytical formula to full-fledged numerical simulations finding only 17 % average relative absolute deviation in $d\omega/d\varphi$. Our analytical result is fully consistent with experimental observations.

Direct spectral measurements of midinfrared radiation from a laser wakefield accelerator

The formation of plasma waves during laser wakefield acceleration (LWFA) produces time-dependent frequency shifts in the driving laser pulse, extending its spectral content to the infrared. We present direct spectral measurements of multi-mJ infrared pulses from an LWFA. Experimental beam profile measurements and three-dimensional particle-in-cell simulations suggest high focusability of these pulses. Particle-in-cell simulations demonstrate that the measurement of side-scattered infrared radiation may serve as a diagnostic of plasma waves and the efficiency of spectral broadening during LWFA.

High harmonic generation near a bow-tie nanostructure: sensitivity to carrier envelope phase and plasmonic inhomogeneity

High harmonic generation (HHG) from atoms near a plasmonic nanostructure interacting with a relatively low intensity driving laser field is a promising candidate for table top attosecond pulse source. The effect of carrier envelope phase (CEP) of the few cycle driving pulse on inhomogeneous high harmonics generation is well studied in literature, for example, the harmonic cut-off can be efficiently controlled by tuning the CEP. Here, we show selective enhancements of harmonic spectra due to half-cycle cutoff (HCO) which is highly sensitive to the CEP, in both spatially homogeneous and inhomogeneous driving laser fields. Essentially the selective enhancement of spectral structures results from contributions of both short and long trajectories in certain HCO regions. Compared to the homogeneous HHG in the presence of inhomogeneity, these enhanced groups eventually merge to the background with the increase of the strength of inhomogeneity. This limits the maximum possible tunability of selective enhancement. Further, near cut-off harmonics can be a good candidate to produce isolated attosecond pulses, with substantial control via CEP of the driving laser pulse along with the strength of inhomogeneity.