Gaussian Laser Pulse Research Articles

Laser intensity profile based terahertz field enhancement from a mixture of nano-particles embedded in a gas.

Nano-particle embedded system plays an importance in developing of future terahertz (THz) radiation source for real-world applications. The laser interactions with nanoparticle embedded system can produce a wide range of THz radiation due to plasma oscillations excitation. We investigate THz field generation from the laser-beat wave interaction with a mixture of spherical and cylindrical graphite nanoparticles in argon gas. Different laser intensity distributions such as Gaussian, cosh-Gaussian, flat-top and ring shape laser pulses have been studied in this work. The relevant plasmon resonance conditions with appropriate symmetry of spherical nanoparticles and cylindrical nanoparticles are discussed. THz field is enhanced upto the order of 10^{2} when the laser intensity redistributes along the polarization direction for a ring shape field envelope.

Magnetic field-assisted wakefield generation and electron acceleration by Gaussian and super-Gaussian laser pulses in plasma

This study focuses on electron acceleration and wakefield produced by Gaussian and Super-Gaussian (SG) laser pulses through plasma medium in the attendance of an external magnetic field. Two shapes of different laser spatial profiles Gaussian and SG are considered. Besides, governing equations for analysis of laser wakefield and electron acceleration by using of hydrodynamics fluid equations, Maxwell’s equations and perturbation technique for Gaussian and SG laser pulses in weakly nonlinear regime have been derived. In addition, influence of the external magnetic field strengths, laser intensity, the pulselength, the plasma electron density and the laser frequency on the wakefield and the electron energy gain have been investigated. Numerical results indicate that by increasing of external magnetic field, the amplitude of wakefield increases. Moreover, it is observed that wakefield amplitude excited by SG laser pulse in comparison with Gaussian laser pulse is larger when external magnetic field is increased. Furthermore, it is found that wakefield amplitude and shape of wakefield generation in within homogeneous plasma greatly are related to the laser profile type, selected laser parameters and external magnetic field. In addition, it is seen that influence of external magnetic field, while laser intensity enhances, on the electron energy gain in SG and Gaussian laser pulses is insignificant.

All-optical quasi-monoenergetic GeV positron bunch generation by twisted laser fields

Generation of energetic electron-positron pairs using multi-petawatt (PW) lasers has recently attracted increasing interest. However, some previous laser-driven positron beams have severe limitations in terms of energy spread, beam duration, density, and collimation. Here we propose a scheme for the generation of dense ultra-short quasi-monoenergetic positron bunches by colliding a twisted laser pulse with a Gaussian laser pulse. In this scheme, abundant γ-photons are first generated via nonlinear Compton scattering and positrons are subsequently generated during the head-on collision of γ-photons with the Gaussian laser pulse. Due to the unique structure of the twisted laser pulse, the positrons are confined by the radial electric fields and experience phase-locked-acceleration by the longitudinal electric field. Three-dimensional simulations demonstrate the generation of dense sub-femtosecond quasi-monoenergetic GeV positron bunches with tens of picocoulomb (pC) charge and extremely high brilliance above 1014 s−1 mm−2 mrad−2 eV−1, making them promising for applications in laboratory physics and high energy physics.

Morphology adjustable microlens array fabricated by single spatially modulated femtosecond pulse

AbstractSilica microlens arrays (MLAs) with multiple numerical-apertures (NAs) have high thermal and mechanical stability, and have potential application prospects in 3D display and rapid detection. However, it is still a challenge to rapidly fabricate silica MLAs with a larger range of NAs and how to obtain multiple NAs in the same aperture diameter. Here, a wet etching assisted spatially modulated femtosecond laser pulse fabricating technology is proposed. In this technology, Gaussian laser pulse is modulated in the axial direction to create a pulse with a large aspect ratio, which is used to modify the silica to obtain a longer modification distance than traditional technology. After that, a microlens with a larger NA can be obtained by etching, and the NA variable range can be up to 0.06–0.65, and even under the same aperture, the variable NA can range up to 0.45–0.65. In addition, a single focus is radially modulated into several focus with different axial lengths to achieve a single exposure fabricating of MLA with multiple NAs. In characterization of the image under a microscope, the multi-plane imaging characteristics of the MLA are revealed. The proposed technology offers great potential toward numerous applications, including microfluidic adaptive imaging and biomedical sensing.

Self-compression of a high-intensity laser pulse in a double-ionizing gas

The self-compression and spatiotemporal evolution of a Gaussian laser pulse propagating in a double-ionized helium gas are investigated. The numerical model is formulated by solving the nonlinear Schrödinger equation using the paraxial like approach. The beam width parameter and pulse width parameter are estimated to investigate the laser pulse advancement in a tunnel ionizing gas. Transverse focusing and longitudinal compression are examined by characterizing the beam spot size in space and time, incorporating the gas ionization processes, relativistic mass variation, and ponderomotive effects. The results show that the inclusion of laser-induced double ionization of helium gas modifies the plasma density, which significantly affects the laser pulse evolution. For intense laser pulse, relativistic-ponderomotive nonlinearity enhances the pulse compression and consequently the self-focusing of the laser pulse. The compression mechanism and the localization of the pulse intensity both are boosted by the modified electron density via a dielectric function. At a helium gas pressure of 1.4 bar, we observed that 100 fs long laser pulse with intensity I0=8.5×1016 W/cm2 is compressed to 20 fs and the initial laser spot size 10 μm focused to 2 μm. These results promise to be a method for the generation of table-top light sources for ultrafast high-field physics and advanced optics.

Coherence in macroscopic high harmonic generation for spatial focal phase distributions of monochromatic and broadband Gaussian laser pulses

Efficient application of ultrafast laser sources from high harmonic generation requires an understanding of how the spectrum can be controlled - the extent of the highest harmonics and the strength and cleanness of the harmonic lines. We study one important aspect in the coherent build-up of macroscopic high-order harmonic generation, namely the impact of different phase distributions in the focal area on the features of the generated radiation. Specifically, we compare the high harmonic signals for the commonly-used Gouy distribution of a monochromatic beam with those for the phase distribution of a short broadband Gaussian pulse. To this end, we apply a theoretical model in which the microscopic yields are obtained via interpolation of results of the time-dependent Schrödinger equation, which are then used in an individual-emitter approach to determine the macroscopic signals. Regions of poor and good coherent build-up as a function of the position of the gas jet are identified using measures for the strength of the harmonic lines and for the impact of off-harmonic radiation. While the largest extent of the spectra as well as the strongest contribution of off-harmonic radiation is found for positioning the gas jet after the focus for both distributions, the relative strength of the harmonics is overall weaker for the short Gaussian pulse distribution and the spectra differ for a gas jet positioned at the focus. These differences are mainly caused by the additional dependence of the focal phase in the transverse direction for the short Gaussian pulse distribution.

Proton Beam Generated in PW Laser Interaction With Nano-Structure Array for Proton Therapy

In comparison with large and expensive conventional linear and circular accelerator, miniaturized laser-driven proton accelerator provides the potential to reduce the overall costs. This could allow a widespread use of the superior properties of protons and ions in radiation therapy. However, the key to success is to improve the particle energy which is not high enough to meet the requirement for the treatment. With the development of high-repetition-rate petawatt class laser technology matures gradually, different acceleration mechanisms have been able to raise the energy of protons to close to 100MeV.To confirm the feasibility of proton beam generated in petawatt-laser irradiating nano-array target, three-dimensional particle-in-cell (3D-PIC) simulations were performed with the QED-PIC code EPOCH. The simulation box has a size of 30 μm(x) × 20 μm (y) × 20 μm (z), which is sampled by cells of 1500 × 400 × 400 with 125 pseudo-electrons, eight pseudo-carbons, and four protons in each cell. The Gaussian laser pulse propagates along the x-direction with y-direction polarization, a wavelength λ0 of 1μm, a peak power of 50 PW, and a duration of 30 fs in full width at half maximum (FWHM). With an initial focal spot radius σ = 3μm. In order to reduce the computational requirements. The array wires have an initial density of ne = 300nc (where nc = 1.1 × 1021 cm-3 is the critical plasma density) and are attached to a 4-μm-thick CH foil. In these simulations, periodic boundary conditions were employed for the laser field in the transverse directions and absorbing boundary conditions for the particles.Three-dimensional particle-in-cell simulation show that the laser-driven proton beam keep quite different characteristics compared with other accelerators proton beam, such as ultra-short beam duration (ns), high strength (107 / cm2 / pulse), large divergence angle (2°) and poly real-energetic spectrum. Meanwhile, we show that under optimal interaction conditions protons can be accelerated up to relativistic energies of 300 MeV by a petawatt laser field. The proton acceleration is due to the dragging Coulomb force arising from charge separation induced by the ponderomotive pressure ∼light pressure! of high-intensity laser.We investigate the impact of nano-structure array on the proton spectrum for different laser-plasma conditions. Our simulated data show that the nanostructures lead to a significant enhancement of absorption over the entire range of laser plasma conditions investigated. At conditions that do not allow for efficient laser absorption by plane targets, nano-structure array is found to significantly enhance the proton cut-off energy and conversion efficiency.S. Wu: None. Z. Wang: None. X. Sun: None. W. Wang: None. F. Xiao: None. L. Zhao: None.

Surface, microstructure, and tensile deformation characterization of LPBF SS316L microstruts micromachined with femtosecond laser

Considerable surface roughness, dimensional deviation, and non-uniform microstructure are a few of the characteristics found on thin or micro-scale features fabricated via laser powder bed fusion (LPBF) that yield inferior and/or inconsistent mechanical properties. Femtosecond laser micromachining can aid in fabricating micro-scale parts with ultra-high dimensional precision. In this work, the surface and tensile behavior of microstruts of 500μm nominal diameter micromachined with Gaussian laser pulses of <100fs duration are characterized. Roughness parameters such as Ra=0.9±0.2μm and Rz=3.4±1.3μm are achieved on the micromachined faces. Surface-associated grains are successfully ablated with negligible microstructural damage to the microstruts. As a result, the average uniform strain under quasi-static tensile loading is measured as 0.54±0.02 compared to 0.42±0.01 for the as-built microstruts. Uniform and non-uniform deformation strain portions are separated analytically and characterized primarily via in-situ imaging. Progressive degradation of the surface and dimensional variance is observed on the micromachined test specimens. Post necking initiation, ablation-associated asperities on the micromachined surfaces evolve into notches, leading to tensile failure.

Electron bunch charge enhancement in laser wakefield acceleration using a flattened Gaussian laser pulse

We demonstrate the charge enhancement of the electron bunch using a flattened Gaussian laser pulse in laser wakefield accelerations. In laser wakefield accelerations, the change in laser pulse shape affects the diffraction pattern, resulting in laser intensity variation during plasma interaction. We employ a flattened Gaussian laser pulse to excite wakefields for electron acceleration. The central flattened region of the flattened Gaussian laser pulse, having the same energy as the Gaussian pulse, helps to enhance the pulse intensity via self-focusing. Thus, the evolution of flat Gaussian pulse enhances self-injection, which leads a considerable amount of electrons into the accelerating fields. A linear theoretical model is adopted to understand the wakefield generation mechanism using Gaussian and flattened Gaussian pulses. In order to broaden our understanding, quasi-three-dimensional PIC simulations are also performed. Our results show that a flattened Gaussian laser pulse with N=2 (where N is the order of the super Gaussian shape) yields a good quality electron bunch with retaining a relatively higher charge (about 300 pC) with respect to the case of a Gaussian laser pulse. We also show a twofold increase in bunch peak current when a flattened Gaussian pulse is employed. The electron bunches with high charge and high current may be promising candidates to drive next-generation compact free-electron lasers with unique X-ray properties.

Investigation of self-focusing a Gaussian laser pulse in a weakly relativistic and ponderomotive regime inside a collisionless warm quantum plasma

This article investigates nonlinear self-focusing of an intense right hand circularly polarized Gaussian profile laser pulse in a weakly relativistic and ponderomotive regime inside a collisionless and unmagnetized warm quantum plasma. The nonlinear propagation equation for laser pulse in plasma has been derived. Then, the evolution differential equation for laser spot-size was obtained with considering the parabolic equation approach under the Wentzel-Kramers-Brillouin and paraxial ray approximations. This differential equation was solved numerically by fourth-order Runge-Kutta method. It is shown that our solution confirms the results of the self-focusing of the laser pulse in a weakly relativistic ponderomotive regime in cold quantum plasma in extreme conditions. Numerical results indicate that self-focusing of the laser pulse in the presence of relativistic and ponderomotive nonlinearity inside warm quantum plasma is improved in comparison with relativistic and ponderomotive cold quantum plasma.

Using deep learning for digitally controlled STIRAP

Stimulated Raman Adiabatic Passage (STIRAP) is a technique for preparing atoms and molecules in an arbitrary preselected coherent superposition of quantum states, ordinarily controlled by Gaussian laser pulses. We search novel pulse sequences by exploiting deep reinforcement learning algorithms in order to achieve fast and flexible solutions for integer and fractional STIRAP. By using the robustness of the PPO algorithm, we impose the agent to exploit only digital pulses corresponding to on/off states of the control radiation instead of continuous amplitudes. Such method allows to adapt to detuning of the energy levels and disturbances such as dephasing.

Saturation of electron emission with increasing ultrarelativistic intensity of the Gaussian laser pulse

Symmetric electron trajectories in the field of a Gaussian laser pulse are calculated as trajectories with compensation of the dynamic shock at the initial instant of motion. It is shown that the symmetric trajectories have an extremal property, namely, at a fixed peak intensity of the pulse, the time dependence of the electron emission intensity from a symmetric trajectory has the maximum width and amplitude compared to all other trajectories. Formation of paired peaks of electron emission intensity with changing laser pulse peak intensity is analyzed. It is found that the electron emission energy saturates with increasing peak intensity of the laser field in the ultrarelativistic region.

The Effect of Chirped Gaussian Laser Pulse on the Acceleration of Plasma Electrons in a Plasma Wakefield Accelerator in the Presence of a Planar Undulator Field

Improving the acceleration process in a very compact structure by using laser-plasma interactions has attracted various researchers’ attention in recent years. Novel ideas, technologies, and comprehensive development in the relevant physics enabled the expansion of plasma wakefield accelerators. In this study, the energy gain and acceleration of electrons in the plasma medium have been investigated through the wakefield generated by a chirped Gaussian laser pulse in the presence of a planar magnetic undulator field. The fourth-order Runge–Kutta method was applied to simulate the wakefield and to determine the electron energy gain. In order to improve the oscillatory velocity of electrons in a laser-plasma accelerator, a planar undulator magnetic field was used. The numerical results indicated that with increasing undulator field strength, the wakefield amplitude and the electron energy gain increased noticeably. In addition, increasing the undulator wavelength for constant values of the undulator field strength leads to an increase in the wakefield amplitude and electron energy gain. Further results revealed an optimized value of chirped parameter, in which the wakefield amplitude and the electron energy gain could be maximum.

High-quality GeV proton beam generation from multiple-laser interaction with double-layer target

A scheme for significantly enhancing the beam quality of laser-driven proton acceleration is proposed and investigated with particle-in-cell simulation: two linearly polarized Gaussian laser pulses obliquely irradiate a double-layer target that acquires a periodic surface structure that reduces the reflection, improves the focusing, and enhances the energy coupling of the third, or main, laser pulse that follows. The oblique pulses also provide some initial kinetic energy to the initially static target electrons and thus protons, which is very crucial for efficient acceleration. As a result, a proton beam of 1.15 GeV peak energy, very low energy spread ∼4%, and small divergence angle ∼5° can be obtained with laser intensities of 1021 W cm−2, which is significantly lower than that of the other recently proposed schemes.

Aberrations in (3+1)-dimensional Bragg diffraction using pulsed Laguerre-Gaussian laser beams

We analyze the transfer function of a three-dimensional atomic Bragg beamsplitter formed by two counterpropagating pulsed Gaussian laser beams. Even for ultracold atomic ensembles, the transfer efficiency depends significantly on the residual velocity of the particles as well as on losses into higher diffraction orders. Additional aberrations are caused by the spatial intensity variation and wavefront curvature of the Gaussian beam envelope, studied with (3+1)D numerical simulations. The temporal pulse shape also affects the transfer efficiency significantly. Thus, we consider the practically important rectangular-, Gaussian-, Blackman- and hyperbolic secant pulses. For the latter, we can describe the time-dependent response analytically with the Demkov-Kunike method. The experimentally observed stretching of the $\pi$-pulse time is explained from a renormalization of the simple Pendell\"osung frequency. Finally, we compare the analytical predictions for the velocity-dependent transfer function with effective (1+1)D numerical simulations for pulsed Gaussian beams, as well as experimental data and find very good agreement, considering a mixture of Bose-Einstein condensate and thermal cloud.

Pulse-length Effect on Laser Wakefield Acceleration of Electrons by Skewed Laser Pulses

Laser pulse-length-related control and optimization of electron acceleration from laser wakefield acceleration (LWFA) by a shaped laser pulse is proposed to investigate in this study. The laser pulse shape changes and becomes skewed due to the variation in refractive index during propagation in plasmas. The optimum laser pulse-length for excitation of plasma wave in linear regime $(a_{0} is $\tau _{L}=\lambda _{p} / \sqrt {2\pi c} $ , but, in nonlinear regime $(a_{0}>1)$ , is not quite certain. Here, laser strength parameter $(a_{0})$ is defined as the ratio $eE_{L}/\text {m}\omega \text {c}$ . In nonlinear case, the variation in pulse skewedness at an optimized laser pulse-length produces different evolution of the wakefield that might affect the electron beam parameters. The present investigation predicts an optimized laser pulse-length (with an optimize ratio of the leading to the trailing pulse edge duration) 35 fs and laser strength parameter $a_{0}=2$ to control the electron bunch charge, emittance, and energy wakefield. Injected bunch charge is enhanced about 20%, and beam emittance is reported to reduce about 40% by employing the skewed Gaussian laser pulse in place of Gaussian pulse for LWFA. Energy-spread of accelerated electron bunch is also reduced from 18% to 6.6% for $s_{\alpha }=0.45$ for 35-fs laser pulse. The electron bunch parameters can also be controlled and optimized using skewed laser pulse and optimized pulse-length.

Dynamics of the Creation of a Rotating Bose–Einstein Condensation by Two Photon Raman Transition Using a Laguerre–Gaussian Laser Pulse

We present numerical simulations to unravel the dynamics associated with the creation of a vortex in a Bose–Einstein condensate (BEC), from another nonrotating BEC using two-photon Raman transition with Gaussian (G) and Laguerre–Gaussian (LG) laser pulses. In particular, we consider BEC of Rb atoms at their hyperfine ground states confined in a quasi two dimensional harmonic trap. Optical dipole potentials created by G and LG laser pulses modify the harmonic trap in such a way that density patterns of the condensates during the Raman transition process depend on the sign of the generated vortex. We investigate the role played by the Raman coupling parameter manifested through dimensionless peak Rabi frequency and intercomponent interaction on the dynamics during the population transfer process and on the final population of the rotating condensate. During the Raman transition process, the two BECs tend to have larger overlap with each other for stronger intercomponent interaction strength.

Attosecond γ -ray vortex generation in near-critical-density plasma driven by twisted laser pulses

Ultra-short brilliant γ-rays have many potential applications in astrophysics, nuclear physics, and ultra-fast science. However, attosecond γ-ray beams with energy above 100 MeV are still very challenging. Here, we propose and numerically demonstrate an all-optical scheme to produce sub-GeV attosecond γ-rays in near-critical-density (NCD) plasma. When a left-hand circularly polarized Laguerre–Gaussian (LG) laser pulse irradiates the NCD plasma, dense attosecond electron bunches are produced, trapped by the LG laser, and accelerated to GeV energies. Subsequently, these electrons oscillate in the LG laser electric fields and emit a string of hundreds MeV attosecond (367 as) γ-ray pulses. Three-dimensional particle-in-cell simulations indicate that, at a laser intensity of 1022 W/cm2, the yield of γ-ray pulses with photon energies above 1 MeV is as high as 1013 with a peak angular momentum of 10−15 kg m2/s. This results in γ-ray vortex beams with an unprecedented peak brilliance of 1024 photons s−1 mm−2 mrad−2 per 0.1% bandwith at 1 MeV, which may offer a unique opportunity for diverse applications.

Isolated intense half-cycle attosecond pulse generation with orbital angular momentum

A relativistic linearly polarized Laguerre–Gaussian (LG) laser pulse interacting with a combination of gas plasma and an oblique foil target is proposed to generate an intense attosecond pulse carrying large orbital angular momentum (OAM). The LG laser pulse firstly produces an ultra-thin relativistic electron sheet (RES) in underdense plasma and transfers the OAM to the latter at the same time. When the RES passes through the oblique foil, it radiates an intense half-cycle attosecond pulse carrying large OAM. Three-dimensional particle-in-cell simulation confirms that an isolated ultra-intense half-cycle attosecond pulse with a duration of 542 as and a peak electric field of 5 × 1012 V m−1 is produced. The average OAM per photon of the attosecond pulse is about −1ћ. Such an intense, isolated attosecond pulse with the large OAM would provide new possibilities in attosecond scientific research.

Nonlinear Thomson scattering from a tightly focused circularly polarized laser with varied incident-pulse durations

In this paper, nonlinear Thomson scattering in an intense () tightly focused () Gaussian laser pulse is theoretically and numerically investigated to demonstrate varying spatial characteristics in the few-cycle and multi-cycle domains. We find that within ultrashort incident laser pulses, electrons have the potential to radiate a single attosecond pulse with a large signal-to-noise ratio, which turns into a train of attosecond pulses with a multi-cycle laser. Furthermore, a novel asymmetry phenomenon is discovered for the first time in the few-cycle domain whereby spatial radiation lasts for only one circle with apparent peaks and valleys, in contrast to the helical radiation pattern apparent in the multi-cycle domain, which gradually turns into a common fourfold-symmetry pattern. This means that the difference between tightly and non-tightly focused laser pulses can be bridged utilizing an appropriate incident pulse duration under specific conditions. The above difference is demonstrated again in the polar plane, where electrons change from a light-spot to a light-band radiation feature when the incident pulse duration increases.