Laser Field Research Articles

Spin effect induced momentum spiral and asymmetry degree in pair production

Spin effect on the pair production under circularly polarized fields is investigated. Significantly different from what momentum spirals caused by two counterrotating fields with a time delay, we find for the first time that the spirals can also be induced due to the particle spin effect even if in a single field. We further examine the bichromatic combinational fields. The inhomogeneous spiral structures can be observed in the momentum spectrum; in particular, the spiral not only does exist in two cases of spin, but also is about 2 orders of magnitude amplified compared with that in the single field. Meanwhile, the spin asymmetry degree on the momentum distributions is investigated, and it was found that the effect of spin flip exists with an increased time delay between two fields. The spin asymmetry degree on the number density can reach to 98% in certain conditions. These results indicate that the signatures of created particles, especially the spiral structures, are strongly associated with information pertaining to the laser field as well as the created particle spin, which can deepen the understanding of vacuum pair production. Published by the American Physical Society 2024

Reducing the Cost of TD-CI Simulations of Strong Field Ionization.

Strong field ionization of molecules by intense laser pulses can be simulated by time-dependent configuration interaction (TD-CI) with a complex absorbing potential (CAP). Standard molecular basis sets need to be augmented with several sets of diffuse functions for effective interaction with the CAP. This dramatically increases the number of configurations and the cost of the TD-CI simulations as the size of the molecules increases. The cost can be reduced by making use of spin symmetry and by employing an orbital energy cutoff to limit the number of virtual orbitals used to construct the excited configurations. Greater reductions in the number of virtual orbitals can be obtained by examining their interaction with the absorbing potential during simulations and their contributions to the strong field ionization rate. This can be determined from the matrix elements of the absorbing potential and the TD-CI coefficients from test simulations. Compared to a simple 3 hartree cutoff in the orbital energies, these approaches reduce the number of virtual orbitals by 20-35% for neutral molecules and 5-10% for cations. As a result, the cost of simulations is reduced by 35-60% for neutral molecules. The number of virtual orbitals needed can also be estimated by second-order perturbation theory without the need for test simulations. The number of virtual orbitals can be reduced further by adapting orbitals to the laser field using natural orbitals derived from test simulations. This is particularly effective for cations, yielding reductions of more than 20%.

Amplification characteristics of N+2 lasing at multiple wavelengths generated by polarization-modulated femtosecond laser fields.

Multiple gain routes complicate the amplification behaviors of N2+ lasing. A direct comparison of the amplification processes of various lasing lines of N2+ is still lacking to date, mainly because the efficient generation of different lasing lines requires different experimental conditions. In this work, to overcome the limitation, we use an intense polarization-modulated femtosecond laser pulse to simultaneously produce high-intensity N2+ lasing signals at 391 nm and 428 nm, permitting us readily performing their time-domain characterizations. Our results show that the primary amplification of 428-nm lasing is always retarded by a few picoseconds with respect to the probe pulse while the amplification of 391-nm lasing can be basically completed within the probe pulse duration at a relatively high gas pressure, and besides, the time duration of the former sustains several times longer than that of the latter. These observations indicate the non-negligible contribution of the initial electronic coherence established by the pump laser, apart from the external triggering pulse. For the 391-nm lasing, the seed amplification and superradiance can fiercely interplay within the probe pulse duration. However, for the 428-nm lasing, its amplification is in essence Raman-assisted superfluorescence due to the lacking of the initial electronic coherence and a direct triggering pulse. Our findings not only shed light on the physics of N2+ lasing but also promote the relevant studies on the collective emissions in the cascaded multi-level molecular ion system.

Electron momentum correlation along laser magnetic field direction as a probe of nondipole effect in strong-field nonsequential double ionization.

The electric dipole approximation is commonly adopted in the theoretical investigation of light-atom/molecule interaction, wherein the magnetic component of the driving electromagnetic field is neglected. Our study highlights the significant role of the magnetic field effect in the recollision dynamics of nonsequential double ionization (NSDI) driven by a mid-infrared laser. Due to the magnetic component of the laser field, in the multiple-returning events, the tunneling electron with a large initial momentum along the laser magnetic field direction at some specific tunneling time is inefficient for NSDI. The corresponding footprint is revealed in the correlated electron momentum distribution along the magnetic field direction. Moreover, we show that this effect becomes more obvious with increasing laser wavelength, leading to a notable reduction in the NSDI yield. Our findings provide an alternative perspective for studying the recollision dynamics involving the magnetic field effect.

Investigation of the Relationship between Topographic and Forest Stand Characteristics Using Aerial Laser Scanning and Field Survey Data

The article thoroughly investigates the relationships between terrain features and tree measurements derived from aerial laser scanning (ALS) data and field surveys in a 1067-hectare forested area. A digital elevation model (DEM) was generated from ALS data, which was then used to derive additional layers such as slope, aspect, topographic position index (TPI), and landforms. The authors developed a mathematical procedure to determine the radii for the topographic position index. The canopy height model was created, and individual trees were segmented using a novel voxel aggregation method, allowing for the calculation of tree height and crown size. Accuracy assessments were conducted between ALS-derived data and field-collected data. Terrain variability within each forest unit was evaluated using characteristics such as standard deviation, entropy, and frequency. The relationships between tree height and the derived topographic features within forest subcompartments, as well as the correlation between the height yield map for the entire area and the TPI layer, were analysed. The authors found strong correlation between the topographic position index and tree heights in both cases. The presented remote-sensing-based methodology and the results can be effectively used in digital forest site mapping, complemented by field sampling and laboratory soil analyses, and, as final goal, in carbon stock assessment.

Unforeseen advantage of looser focusing in vacuum laser acceleration

Acceleration of electrons in vacuum directly by intense laser fields holds great promise for the generation of high-charge, ultrashort, relativistic electron bunches. While the energy gain is expected to be higher with tighter focusing, this does not account for the reduced acceleration range, which is limited by diffraction. Here, we present the results of an experimental investigation that exposed nanotips to relativistic few-cycle laser pulses. We demonstrate the vacuum laser acceleration of electron beams with 100s pC charge and 15 MeV energy. Two different focusing geometries, with normalized vector potential a0 of 9.8 and 3.8, produced comparable overall charge and electron spectra, despite a factor of almost ten difference in peak intensity. Our results are in good agreement with 3D particle-in-cell simulations, which indicate the importance of dephasing.

Time–energy distribution of photoelectron from atomic states with different magnetic quantum numbers in elliptically polarized laser fields

Abstract A Wigner-distribution-like (WDL) function based on the strong-field approximation (SFA) theory is used to investigate the ionization time of the photoelectron emitted from the initial states with different magnetic quantum number m in elliptically polarized electric fields. The saddle-point method is adopted for comparisons. For different m states, a discrepancy exists in the WDL distributions of the photoelectrons emitted in a direction close to the major axis of the laser field ellipse. Based on the saddle-point analysis, this discrepancy can be ascribed to the interference between electrons ionized from two tunneling instants. Our results show that the relationships between the tunneling instants and kinetic energy of photoelectrons are the same for different m initial states when the Coulomb potential is not considered. Our work sheds some light on the ionization-time information of electrons from different magnetic quantum states.

The long road to ignition: An eyewitness account

This paper reviews the many twists and turns in the long journey that culminated in ignition in late 2022 using the laser heated indirect-drive approach to imploding DT filled targets at the National Ignition Facility (NIF), located at the Lawrence Livermore National Laboratory (LLNL). We describe the early origins of the Laser Program at LLNL and key developments such as the paradigm shifting birth of high energy density physics (HEDP) studies with lasers, changes in choice of laser wavelength, and the development of key diagnostics and computer codes. Fulfilling the requirements of the multi-faceted Nova Technical Contract was a necessary condition for the approval of the NIF, but more importantly, the end of the Cold War and the cessation of nuclear testing were key catalysts in that approval, along with the ready-and-waiting field of HEDP. The inherent flexibility of the field of laser driven inertial confinement fusion played a fundamental role in achieving success at the NIF. We describe how the ultimately successful ignition target design evolved from the original “point design” target, through the lessons of experiment. All key aspects of that original design changed: The capsule's materials and size were changed; the hohlraum's materials, size, laser entrance hole size, and gas fills were also all changed, as were the laser pulse shapes that go along with all those changes. The philosophy to globally optimize performance for stability (by raising the adiabat and thus lowering the implosion convergence) was also key, as was progress in target fabrication, and in increasing NIF's energy output. The persistence of the research staff and the steadfast backing of our supporters were also necessary elements in this success. We gratefully acknowledge seven decades of researcher endeavors and four decades of the dedicated efforts of many hundreds of personnel across the globe who have participated in NIF construction, operation, target fabrication, diagnostic, and theoretical advances that have culminated in ignition.

Valley selections and control of topological phases by bicircular laser field in transition-metal dichalcogenides

The energy spectra of transition metal dichalcogenides are influenced by three orbitals, forming a three-band model with strong spin-orbit interaction. Analyzing these bands through a two-band projection suffices for exploring electronic and optical properties. We examine topological phases and the spin-valley Hall effect, derived from spin-valley Chern numbers. Spin-orbit interaction breaks inversion symmetry but preserves time-reversal symmetry, resulting in non-zero spin-valley-dependent Hall conductivities, crucial for both spin and valley Hall effects. Interestingly, direct optical manipulation of the valley degree of freedom can be achieved using a trefoil field generated by a bicircular field. Selectable valley conductivity occurs when the field orientation matches the lattice symmetry, leading to a quantum anomalous Hall effect without magnetization. This approach suggests the potential for valley selection via field orientation, with applications in valleytronics and spintronics devices, enabling qubit encoding.

Nonsequential double ionization of Ar in two-color inhomogeneous laser fields

With the classical ensemble method, we investigate the correlated electron dynamics in nonsequential double ionization (NSDI) of Ar in parallel two-color inhomogeneous laser fields with different inhomogeneity parameter ϵ. Numerical results show that at low laser intensities, the probability of NSDI is higher for two-color laser fields with a higher inhomogeneity parameter ϵ. At high laser intensities, however, the probability of NSDI decreases as ϵ increases. It is also shown that under low laser intensities, the two-color inhomogeneous laser fields drives the NSDI predominantly dominated by the RII channel, and the ratio of the RII channel increases as ϵ increases. More interestingly, an arc-like structure is found in the correlated electron momentum distributions for the two-color inhomogeneous laser fields. The underlying dynamics are revealed by tracing back to classical trajectories.

Subcycle dynamics of excitons under strong laser fields.

Excitons play a key role in the linear optical response of two-dimensional (2D) materials. However, their role in the nonlinear response to intense, nonresonant, low-frequency light is often overlooked as strong fields are expected to tear the electron-hole pair apart. Using high-harmonic generation as a spectroscopic tool, we theoretically study their formation and role in the nonlinear optical response. We show that the excitonic contribution is prominent and that excitons remain stable even when the driving laser field surpasses the strength of the Coulomb field binding the electron-hole pair. We demonstrate a parallel between the behavior of strongly laser-driven excitons in 2D solids and strongly driven Rydberg states in atoms, including the mechanisms of their formation and stability. Last, we show how the excitonic contribution can be singled out by encapsulating the 2D material in a dielectric, tuning the excitonic energy and its contribution to the high-harmonic spectrum.

Resonance-enhanced high-order harmonic generation by combination of an infrared laser field and a time-delayed attosecond XUV pulse

Resonance-enhanced high-order harmonic generation by combination of an infrared laser field and a time-delayed attosecond XUV pulse

The Orbital-Resolved Dissociative Ionization of the Molecular IBr in a Near-Infrared Femtosecond Laser Field

The Orbital-Resolved Dissociative Ionization of the Molecular IBr in a Near-Infrared Femtosecond Laser Field

Nonlinear Thomson scattering: velocity asymmetry inherent in electron figure-8 motion.

The individual polarization components of nonlinear Thomson scattering arise from the separate dimensions of electron figure-8 motion caused by a linearly polarized laser field. We present the first measurements of nonlinear Thomson scattering in both emission hemispheres. In the electron average rest frame, the shape of the electron figure-8 path is symmetric about the laser polarization dimension. However, the periodic electron velocity is intrinsically asymmetric. The full scattering emission pattern reveals this asymmetry and the direction that electrons move around the figure-8 path.

Analytically controlling the laser-induced electron phase in sub-cycle motion

We develop an approach to directly control the phase of an electron's sub-cycle motion in an intense laser field. The method is established by tuning a low-frequency electric field applied on a centrosymmetric gaseous target during its interaction with a few-cycle infrared laser pulse. We find a universal analytical relation between the low-frequency electric field and its induced harmonic frequency shift, derived by the strong-field approximation. This simple relation and its universality are confirmed numerically by directly solving the time-dependent Schrödinger equation. Moreover, we demonstrate the benefits of the discovered relation in applications, including continuously and precisely tuning XUV waves and developing a method of comprehensively sampling the THz pulse. Published by the American Physical Society 2024

Emission control of entangled electrons in photoionisation of a hydrogen molecule

For photo-dissociation of a single hydrogen molecule (H2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$H_2$$\\end{document}) with combined XUV and IR laser pulses, we demonstrate optical control of the emission direction of the photoelectron with respect to the outgoing neutral fragment (the H-atom). Depending on the relative delay between the two laser fields, adjustable with sub-femtosecond time resolution, the photoelectron is emitted into the same hemisphere as the H-atom or opposite. This emission asymmetry is a result of entanglement of the two-electron final-state involving the spatially separated bound and emitted electron.

Near-pure red emission of highly thermally stable Eu3+ doped multi-component transparent oxyfluoride aluminosilicate glass-ceramic for red lasers and trichromatic WLEDs

In this paper, Eu3+ doped multi-component transparent aluminosilicate and oxyfluoride aluminosilicate glasses were fabricated by the melt-quenching procedure, and the glass-ceramics containing LiAlSi2O6 microcrystals (LAS glass-ceramics) were produced by a two-step heat treatment process. The XRD, SEM, and EDS data demonstrate that spherical single-phase LiAlSi2O6 microcrystals have precipitated from the glass matrices. The microhardness and flexural strength of the glass-ceramics vary in 6.27–6.66 GPa and 89.18–155.75 MPa, respectively. Phonon sideband analysis reveals that the phonon energies for both fluorine-containing and fluorine-free LAS glass-ceramics are approximately 971 cm−1, and the multiphonon relaxation rates (Wmp/W0) of the former are lower than those of the latter. Given its lower Wmp/W0 values, the fluorine-containing LAS glass-ceramic exhibits a higher emission intensity and a longer lifetime. Under 394 nm excitation, the fluorine-containing LAS glass-ceramic presents a warm and nearly pure red emission, characterized by a correlated color temperature of less than 3300 K and a color purity approaching 100 %. Time-resolved emission spectra support the photoluminescent stability of the Eu3+ ions. According to the Judd-Ofelt theory and spectral results, the fluorine-containing LAS glass-ceramic possesses lower Ω2/Ω4 and asymmetry ratios, leading to a more symmetrical local environment for the Eu3+ ions. Benefiting from its enhanced red emission, the 5D0→7F2 transition in the fluorine-containing LAS glass-ceramic exhibits a higher branching ratio (69.28 % > 50 %), emission cross-section (8.94 × 10−22 cm2), and gain bandwidth (12.12 × 10−28 cm3). Additionally, temperature-dependent photoluminescence spectra substantiate the excellent thermal stability of the Eu3+ doped fluorine-containing LAS glass-ceramic. Specifically, the intensity of the 5D0→7F2 transition at 150 °C remains 80 % of the initial intensity. Therefore, the Eu3+ doped multi-component transparent oxyfluoride aluminosilicate glass-ceramic has a significant competitive advantage in the field of red lasers and trichromatic WLEDs.

Ultrashort-pulse-laser ablation of freestanding dielectric thin films

Ultrashort-pulse-laser ablation of dielectric thin films is strongly affected by the interference of the exciting laser pulse with itself, which causes the deposited energy to be confined to narrow regions equidistantly spaced along the propagation direction of the laser. We investigate how this affects the ablation mechanism of freestanding Al2O3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {Al}_2\\hbox {O}_3$$\\end{document} thin films by analyzing the laser-generated structures with several post-mortem imaging and spectroscopic techniques. Close to the ablation threshold, the laser-irradiated region exhibits surface blistering. Higher intensities cause layers of material to be removed corresponding to ejection of material initiated from the regions of high excitation. Significantly above the ablation threshold, the laser-generated structure is remarkably stable and consists of a membrane of thickness corresponding to the distance between neighboring interference maxima, which is uniquely determined by the central wavelength of the laser pulse and the refractive index of the film. The electronic excitation as a function of depth is simulated using a multiple-rate-equation model in combination with finite-difference-time-domain propagation of the laser field. The simulations confirm the strongly localized excitation and thus correlate well with the observed laser-generated structures.

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.

Non-resonant intense laser field and electric field effects on the optical characterization of Rosen-Morse quantum wells with different potential functions

Non-resonant intense laser field and electric field effects on the optical characterization of Rosen-Morse quantum wells with different potential functions