Interaction Of Laser Pulses Research Articles

Features of noncollinear second harmonic generation by infrared laser pulses with opposite frequency chirps

The features of noncollinear second harmonic generation during the interaction of broadband laser pulses with opposite frequency chirps are considered. The impact of spectral phase mismatch of the interacting pulses on spectrum width and the efficiency of conversion to the noncollinear second harmonic is analyzed on an example of lithium triborate and beta-barium borate crystals.

Subcycle-resolved strong-field tunneling ionization: Identification of magnetic dipole and electric quadrupole effects

Interaction of a strong laser pulse with matter transfers not only energy but also linear momentum of the photons. Recent experimental advances have made it possible to detect the small amount of linear momentum delivered to the photoelectrons in strong-field ionization of atoms. Linear momentum transfer is a unique signature of the laser-atom interaction beyond its dipolar limit. Here, we present a decomposition of the subcycle time-resolved linear momentum transfer in term of its multipolar components. We show that the magnetic dipole contribution dominates the linear momentum transfer during the dynamical tunneling process while the post-ionization longitudinal momentum transfer in the field-driven motion of the electron in the continuum is primarily governed by the electric quadrupole interaction. Alternatively, exploiting the radiation gauge, we identify nondipole momentum transfer effects that scale either linearly or quadratically with the coupling to the laser field. The present results provide detailed insights into the physical mechanisms underlying the subcycle linear momentum transfer induced by nondipole effects.

Theoretical Approach for Electron Dynamics and Ultrafast Spectroscopy (EDUS).

In this manuscript, we present a theoretical framework and its numerical implementation to simulate the out-of-equilibrium electron dynamics induced by the interaction of ultrashort laser pulses in condensed-matter systems. Our approach is based on evolving in real time the density matrix of the system in reciprocal space. It considers excitonic and nonperturbative light-matter interactions. We show some relevant examples that illustrate the efficiency and flexibility of the approach to describe realistic ultrafast spectroscopy experiments. Our approach is suitable for modeling the promising and emerging ultrafast studies at the attosecond time scale that aim at capturing the electron dynamics and the dynamical electron-electron correlations via X-ray absorption spectroscopy.

Femtosecond laser melting NbMoTaW refractory high entropy alloy: A micro-scale thermodynamic simulation

Refractory high entropy alloys (RHEA) exhibit excellent material properties, such as high strength, high hardness, high melting point and high temperature oxidation resistance. Ultrafast laser fabricating RHEAs is potential widely used in extreme environments at high temperature. In the present study, a multiscale simulation of the femtosecond laser pulse melting NbMoTaW RHEA film was performed to observe the transient interaction process between femtosecond laser and RHEA at the atomic scale. The temperature dependent electron thermophysical parameters (electron heat capacity, electron thermal conductivity and effective electron–phonon coupling factor) were calculated from density functional theory. The two temperature model coupling molecular dynamics was used to reveal the temperature change, the density variation and the pressure evolution during the interaction of a single pulse laser with the NbMoTaW RHEA. The simulated ablation and melting depths varied differently with laser fluence parameters. The ablation depth increases with the laser fluence, while the non-removed melting depth varies little with laser energy beyond the ablation threshold. The research reveals the mechanisms in melting process at atomic-level, and the results are helpful to the fabrication of RHEA.

Generating High Efficiency Terahertz Radiation From Relativistic Nano-Bunches in the Interaction of Ultrarelativistic Lasers With Thin Solid Targets

The interaction of femtosecond laser pulses with thin solid targets is one of the most efficient methods to obtain terahertz (THz) radiation. In the specific conditions, the electron of plasma surface distribution became so localized and nano-bunches are formed which causes coherent synchrotron emission (CSE). In this mechanism which occurs when the relativistic laser is obliquely incident on plasma, the plasma electrons gather in a very dense and thin layer of nanometer width. The electrons of nano-bunch can emit intense coherent emissions in different ranges of electromagnetic radiation. In this study, producing high-efficiency THz radiating from relativistic nano-bunches is discussed. The effect of plasma and nano-bunch parameters, the mobility of ions, the initial thermal velocity of electrons, and the speed of electrons are investigated. According to the results, the direct relation between the width of the nano-bunch and the efficiency is seen. The nonlinear relation between the initial thermal velocity of electrons and the THz yield is seen. In the desirable conditions, a high THz yield of up to 37% is obtained from nano-bunches with the initial thermal velocities as 0.3 of light speed.

Mathematical formalism of femtosecond laser-deoxyribonucleic acid interaction: thermal evolution

A novel analytical formalism is proposed based upon Quantum heat transport equation in order to describe the femtoseconds/picoseconds laser pulses interaction with the Deoxyribonucleic acid (DNA). The formalism generates solutions based upon inputs as: voltage, laser beam intensity and laser - DNA interaction time. Thermal waves induced inside irradiated DNA are defined and accounted for. Analytical simulations show that the optimum regime of laser - DNA interaction was reached for a potential carrier generated at the interface equal to 3.5 × 10−3 eV. It has to be mentioned that the formalism breaks down if the potential carrier generated at the interface is inferior to 10−2 eV. Accordingly, for pulse duration inferior to 1 ps, the laser beam spatial-temporal distribution has an essential role in defining the shape and magnitude of the thermal distribution within the irradiated DNA strands.

Towards bright gamma-ray flash generation from tailored target irradiated by multi-petawatt laser

One of the remarkable phenomena in the laser-matter interaction is the extremely efficient energy transfer to gamma -photons, that appears as a collimated gamma -ray beam. For interactions of realistic laser pulses with matter, existence of an amplified spontaneous emission pedestal plays a crucial role, since it hits the target prior to the main pulse arrival, leading to a cloud of preplasma and drilling a narrow channel inside the target. These effects significantly alter the process of gamma -photon generation. Here, we study this process by importing the outcome of magnetohydrodynamic simulations of the pedestal-target interaction into particle-in-cell simulations for describing the gamma -photon generation. It is seen that target tailoring prior the laser-target interaction plays an important positive role, enhancing the efficiency of laser pulse coupling with the target, and generating high energy electron-positron pairs. It is expected that such a gamma -photon source will be actively used in various applications in nuclear photonics, material science and astrophysical processes modelling.

Physics-based computational modeling and time-resolved imaging of plasma plume generated by nanosecond laser interaction with a bed of micro metallic powder

Although continuous lasers see frequent applications in selective laser melting or sintering, lasers with short pulse durations (such as those with nanosecond scale durations) possess potential advantages that may include good spatial resolutions and small heat-affected zones. Previous work on selective laser sintering with nanosecond lasers has been reported, particularly for laser micro sintering. At a sufficiently high intensity, a nanosecond (ns) laser pulse can generate a plasma plume from its irradiated metallic powder bed surface. The plasma plume evolution, such as the pressure it generates on the surface of the powder bed, may significantly influence the sintering process in the powder bed. However, physics-based modeling work for plasma generated by nanosecond-pulsed laser interaction with a metallic micro powder bed has been seldomly seen in literatures according to the knowledge of the authors’. Such modeling work has been reported in this paper for a ~4-ns laser pulse interaction with a cobalt micro powder bed, integrated with time-resolved plasma imaging using an intensified CCD (ICCD) camera with nanosecond scale gate widths for the model validation. For the conditions investigated, the model-predicted plasma plume evolutions agree reasonably well with the ICCD imaging results for the given period of comparison. This has reasonably supported the hypothesis posed in this paper (which has been rarely tested according to the knowledge of the authors’) that a short ns laser pulse-induced plasma from a metallic micro powder bed below the critical temperature can be reasonably well described by solving gas dynamic equations in the gaseous phase together with solving the heat transfer equation in the powder bed condensed phase, where the coupling is via the Knudsen layer (KL) relations for vaporization at the interface between the two phases. The model calculations show that in comparison with the bulk cobalt situation, the ns laser pulse can induce more significant surface vaporization from the cobalt powder bed, leading to a plasma plume with typically higher peak temperatures and densities in the simulated period. The plume can generate a short total pressure pulse with a ~491-MPa peak magnitude on the surface of the powder bed.

Laser ion acceleration from tailored solid targets with micron-scale channels

Laser ion acceleration is a promising concept for the generation of fast ions using a compact laser-solid interaction setup. In this study, we theoretically investigate the feasibility of ion acceleration from the interaction of petawatt-scale laser pulses with a structured target that embodies a micron-scale channel filled with relativistically transparent plasma. Using 2D and 3D particle-in-cell (PIC) simulations and theoretical estimates, we show that it is possible to generate GeV protons with high volumetric charge and quasimonoenergetic feature in the energy spectrum. Optimal parameters of the target are obtained using 2D PIC simulations and interpreted on a basis of an analytical two-stage ion acceleration model. 3D PIC simulations and realistic preplasma profile runs with 2D PIC show the feasibility of the presented laser ion acceleration scheme for the experimental implementation at the currently available petawatt laser facilities.

Enhanced betatron x-ray emission in a laser wakefield accelerator and wiggler due to collective oscillations of electrons

We report x-ray emission caused by betatron motion of electrons in a laser wakefield accelerator driven by high intensity ($\ensuremath{\sim}4\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$) laser pulse interaction with a 4-mm long helium gas-jet and study the role of collective oscillation of electrons as well as plasma density. The collective oscillations of electrons, inferred from the observed wavy pattern in the electron beam spectrum (up to 300 MeV) led to enhance peak x-ray flux which was found to be further enhanced at higher plasma densities. A similar correlation between the enhancement in x-ray emission and collective oscillation was also observed in nitrogen gas target. Possible physical pictures for the above observations are also suggested and discussed. For the helium gas jet target, the typically observed x-ray source brightness is $\ensuremath{\ge}1\ifmmode\times\else\texttimes\fi{}{10}^{20}\text{ }\text{ }\mathrm{photons}/\mathrm{s}\text{ }{\mathrm{mm}}^{2}\text{ }{\mathrm{mrad}}^{2}$ in 0.1% BW, with a critical energy of $35\ifmmode\pm\else\textpm\fi{}5\text{ }\text{ }\mathrm{keV}$. We have also demonstrated the application of the betatron x-ray source in high-resolution imaging.

Electron acceleration by relativistic pondermotive force in the interaction of intense laser pulse with an axially inhomogeneous underdense plasma

In this paper, electron acceleration by relativistic pondermotive force has been studied in the interaction of intense laser pulse with an axially inhomogeneous underdense plasma. The results show that the relativistic ponderomotive nonlinearity causes obvious beam self-focusing and a highly energetic electron can be achieved in the plasma, moreover, the high-energy electron would be automatically ejected by the ponderomotive force without adding any additional extractor. In addition, it is found that the real part and imaginary part of Lorentz factor can directly affect the amplitude and change the trend of electron acceleration, and electrons acceleration can be strengthened when we can elect properly the laser intensity with higher momentum and smaller beam width, but nonuniformity of plasmas can slightly decrease the electron acceleration. The results can provide useful physical support for novel design of table accelerator and the optimized parameters of electron acceleration in the plasma.

Calculation of high-order harmonic generation of atoms and molecules by combining time series prediction and neural networks.

High-order harmonic generation (HHG) from the interaction of ultra-intense laser pulses with atoms is an important tabletop short-wave coherent light source. Accurate quantum simulations of it present large computational difficulties due to multi-electron multidimensional effects. In this paper, the time-dependent response of hydrogen atoms is calculated using a time-series prediction scheme, the HHG spectrum is reconstructed very accurately. The accuracy of the forecasting is further improved by using a neural network scheme. This scheme is also applied to the simulation of the harmonic emission on multi-electron systems, and the applicability of the scheme is confirmed by the harmonic calculation of complex systems. This method is expected to simulate the nonlinear dynamic process of multi-electron atoms and molecules irradiated by intense laser pulses quickly and accurately.

Hot electron and x-ray generation by sub-ns kJ-class laser-produced tantalum plasma

This paper presents experimental investigation of temperature scaling and threshold of instability in hot electron and bremsstrahlung radiation from the interaction of sub-nanosecond and kilo-joule class laser pulses with a tantalum foil target at Prague Asterix Laser System. The laser intensity was varied between and W · cm−2 at the target focus. The energy distribution functions of electrons were measured by an angular array of magnetic spectrometers indicating the electron temperature in the range between 30 keV and 70 keV. The bremsstrahlung spectrum was characterized using a scintillator-based calorimeter. In particular, we show the laser-energy scaling of the total flux of hot electrons in the forward and backward directions with respect to the laser vector, the conversion efficiency of the laser energy to the energy carried by hot electrons, and the temperature of hot electrons as well as the unfolded bremsstrahlung temperature using a Monte Carlo code consistent with signals of the scintillator detector. The scaling shows that the electron flux increases discontinuously with increasing laser intensity from W · cm−2 with consequent instability in the production of hot electrons and bremsstrahlung radiation.

Powerful laser-produced quasi-half-cycle THz pulses.

The Maxwell equations-based 3D-analytical solution for the terahertz (THz) half-cycle electromagnetic wave transition radiation pulse has been found. This solution describes generation and propagation of transition radiation into free space from laser-produced relativistic electron bunch which crosses a target-vacuum interface as a result of ultrashort laser pulse interaction with a thin high-conductivity target. The analytical solution found complements the theory of laser initiated transition radiation. It describes the THz wave half-cycle pulse at the arbitrary distance from a target surface including near-field zone rather than its standard far-field characterization. The analytical research has also been supplemented with the 3D simulations using the finite-difference time-domain method, which makes it possible for description of much wider spatial domain as compared to that from the particle-in-cell approach. The presented result sheds light fundamentally on the interference of the electron bunch field and the generated THz field of broadband transition radiation from laser-plasma interaction. The latter is studied for a long time in the experiments with solid density plasma and the theory developed may inspire to targeted measurements and investigations of unique super intense half-cycle THz radiation waves near the laser target.

Dynamics of plasmas produced by a laser pulse, inside a dense gaseous target, formed in an ambient gas

In this work, the time development of plasmas produced by interaction of laser pulses, with a nitrogen gas, was investigated. The interaction took place inside a small portion of dense nitrogen gas injected temporarily into a chamber filled with the gas under low pressure. High-temperature plasmas produced directly by the laser pulse were a source of soft x rays and charged particles, ionizing and exciting the surrounding gas. In this way, low-temperature plasmas were produced. The formation of high-temperature plasmas was studied using soft x-ray spectroscopy and x-ray streak imaging. Low-temperature plasmas formed at various distances from the laser focus were investigated using an optical streak camera. Interpretation of the experimental data was supported by numerical modeling of the laser-produced plasma hydrodynamics. It was concluded that depending on the distance from the focal spot, the formation of the low-temperature plasmas was dominated by ion streams or by soft x-ray radiation.

Spin-polarized proton acceleration by an intense laser pulse with a foil-gas target

A novel scheme for obtaining high-energy polarized proton beams by the interaction of a femtosecond laser pulse with a foil-gas composite target has been proposed. The carbon plasmas heated by the laser pulse expand toward the prepolarized HCl gas and excite shock waves in the gas target, reflecting and accelerating spin-polarized protons. According to the results from particle-in-cell simulations with the addition of spin dynamics, protons of several MeV are produced with the polarization rate remaining above 90% in the high energy region. The simulation results show that a large number of the reflected protons are subjected to a weak azimuthal magnetic field and with less depolarization. The intensity of laser pulses and the thickness of foils also affect the strength of the azimuthal magnetic field, which affects the depolarization of the proton beams.

High order modes of intense second harmonic light produced from a plasma aperture

Because of their ability to sustain extremely high-amplitude electromagnetic fields and transient density and field profiles, plasma optical components are being developed to amplify, compress, and condition high-power laser pulses. We recently demonstrated the potential to use a relativistic plasma aperture—produced during the interaction of a high-power laser pulse with an ultrathin foil target—to tailor the spatiotemporal properties of the intense fundamental and second-harmonic light generated [Duff et al., Sci. Rep. 10, 105 (2020)]. Herein, we explore numerically the interaction of an intense laser pulse with a preformed aperture target to generate second-harmonic laser light with higher-order spatial modes. The maximum generation efficiency is found for an aperture diameter close to the full width at half maximum of the laser focus and for a micrometer-scale target thickness. The spatial mode generated is shown to depend strongly on the polarization of the drive laser pulse, which enables changing between a linearly polarized TEM01 mode and a circularly polarized Laguerre–Gaussian LG01 mode. This demonstrates the use of a plasma aperture to generate intense higher-frequency light with selectable spatial mode structure.

Ultrafast pulsed laser stealth dicing of 4H-SiC wafer: Structure evolution and defect generation

Ultrafast pulsed laser for wafer processing has been promising in the semiconductor industry due to its high precision and low thermal effect. The microstructure and defect during the stealth dicing process is critical to controlling the desired quality of the wafer. The structural evolution and defect formation mechanisms during stealth dicing of 4H-SiC with femtosecond and picosecond lasers were investigated. In order to understand the ultrafast pulsed laser interaction with 4H-SiC wafer, we studied the laser intensity dependence of the nonlinear refractive index, the propagation of laser beam focusing in 4H-SiC, the laser absorption, electron concentration evolution and energy deposition. The effect of two significant processing factors was discussed, including laser fluence and laser scribing times. The propagation paths and energy deposition of femtosecond and picosecond lasers in the internal processing of 4H-SiC were calculated by finite element method. The evolution of electrons temperature, lattice temperature and free electron concentration in the material during laser processing was analyzed. Femtosecond laser stealth dicing had higher processing accuracy due to minimal thermal effects. The defect induced by femtosecond laser was dominated by nanovoids and micro-cracks. The defect structure was affected by laser processing parameters such as scanning passes and laser power density. Multi-focusing of ultrafast pulsed lasers in transparent materials was generally considered to be a dynamic balance of nonlinear self-focusing and plasma defocusing. Picosecond laser processing resulted a higher processing efficiency under the same laser energy density comparing with femtosecond laser processing. However, defects during picosecond laser dicing were dominated by thermal ablation, which impaired the processing accuracy.

Few-cycle optical field breakdown and damage of gallium oxide and gallium nitride

Both gallium oxide and gallium nitride have great potential for use as high power transparent conducting oxide materials for a wide range of optoelectronic applications. It is, therefore, important to determine the dynamic optical field breakdown for these materials. Here, we report laser damage thresholds for Ga2O3 and GaN using 9 fs few-cycle pulses with a center wavelength near 760 nm. We determine laser-induced damage thresholds (LIDTs) for both single pulse and multi-pulse exposures, with multi-pulse LIDT showing significant reduction compared to single pulse LIDT (in some cases, >50%), possibly due to laser-induced defects. We show that Ga2O3 and GaN have an ultrafast optical field breakdown of 23 and 18 GV/m when nearly polarized along their [010] and [111] planes, respectively, extending their suitability for high power applications to the ultrashort pulse regime. To accurately characterize the excited carrier density criteria in which apparent laser damage occurs, carrier excitation dynamics for the entire laser pulse interaction were simulated using a modified Keldysh ionization model. For the measured single-shot threshold fluences, the plasma critical density was surpassed by 2 orders of magnitude and 2D finite-difference time-domain simulations were employed to understand the pulse interaction near the surface.

Pulsed neutron source from interaction of relativistic laser pulse with micro-structure assisted pitcher–catcher target

Neutron detection technology is a powerful method for characterizing fundamental and industrial materials, such as magnetic materials, nanomaterials, polymers, and biological substances. A compact pulsed neutron source is desired, as the traditional way of producing high-yield neutrons depends mainly on large-scale accelerators or fission reactors. Here, we propose a scheme to generate a high-yield pulsed neutron source by using a 100 fs relativistic laser pulse interacting with a micro-structure assisted pitcher–catcher target. Three-dimensional particle-in-cell and Monte Carlo hybrid simulations demonstrate that an energetic deuterium ion beam with a cutoff energy of 75 MeV and a 7.23 laser-to-deuterium energy conversion efficiency can be obtained with a laser pulse of intensity W cm−2, duration 330 fs, power 34 TW and energy 6.7 J. When they strike the following lithium fluoride converter of thickness 2 cm, a large number of neutrons are thus produced via a 7Li(d,n) nuclear reaction. The neutron yield is up to 109 and its pulse duration is as short as 20 ps. This scheme could be realized in laboratories with current hundreds-of-terawatt or multi-petawatt laser facilities.