Values Of Laser Intensity Research Articles

The effect of different laser head angles and shielding gas supply systems to maximize the depth of penetration by minimizing the plasma shielding effect in fiber laser welding of 3-mm thick NiTinol sheet

Bead-on-plate laser welding of 3 mm thick NiTinol sheet was tried with the help of 2.3 kW Yb:Fiber laser. It was found that the laser power was insufficient to get the full penetration with narrow weld-bead by varying the scan speed when shielding gas was provided from the side of the bead and the laser head was kept perpendicular to the plate. It was observed that when gas was supplied from the top of the plate and moving with the laser head, fully penetrated bead was obtained at much lesser value of heat input (P/v) because of the reduction of length of plasma plume. It was also detected that with increase in rotation of laser head angle the depth of penetration and laser bead width decreases because of the increase in laser spot area, reduction of laser intensity and increase in length of plasma plume. The increase in length of plasma plume increased the plasma shielding effect and reduced the absorptivity of the laser in the NiTinol sheet. According to the study, the maximum depth of penetration at a constant laser heat input was obtained for minimum value of plasma plume length and maximum value of laser intensity.

Nonreciprocal transmission of electromagnetic waves with nonlinear active plasmonic metasurfaces

Nonreciprocity is important for optical information processing, full-duplex communications, and protection of sensitive laser equipment from unwanted reflections. However, it is very challenging to obtain strong nonreciprocal response in optical frequencies, especially when nanoscale configurations are considered. In this work, we solve this problem by demonstrating a compact bifacial plasmonic metasurface that acts as an ultrathin nonreciprocal transmission filter at near-infrared frequencies. The proposed nanostructure is simple to be practically implemented, since it is composed of two silver nanostripes with different dimensions placed on both sides of an ultrathin active dielectric spacing layer. The introduced gain leads to an exceptional point formation in the linear operation regime, where unidirectional perfect transmission due to loss compensation is achieved from the presented non-Hermitian parity-time symmetric nanoscale system. The demonstrated plasmonic metasurface breaks the Lorentz reciprocity law at the nanoscale when its nonlinear response is considered, mainly due to its spatially asymmetric geometry combined with enhanced nonlinearity. Strong and broadband nonreciprocity with unity transmission contrast from opposite directions is realized under relatively low-input laser intensity values. Furthermore, asymmetric nonlinear third-harmonic generation with a pronounced contrast is also illustrated by the same nonreciprocal metasurface design. The findings of this work can lead to the realization of various self-induced nonreciprocal nanophotonic configurations, such as integrated ultrafast switches, ultrathin protective coatings, and asymmetric directional imaging devices.

Relativistic Self-Focusing of Hermite-cosine-Gaussian Laser Beam in Collisionless Plasma with Exponential Density Transition

This work reveals an exploration of self-focusing of Hermite-cosine-Gaussian laser beam in a collisionless plasma under relativistic nonlinearity. Self-focusing along with self-trapping of Hermite-cosine-Gaussian laser beam are analyzed for different values of laser intensity, plasma density, and decentered parameters. Mathematical analysis displays that these parameters play a major role in achieving the stronger and earlier self-focusing. Further, a comparative study between self-focusing of Hermite-cosine-Gaussian laser beam with and without exponential density ramp profile is introduced. Plasma density transition with exponential profile is found to be more effective in order to have stronger self-focusing. The present analysis may lead to very useful applications in the field of efficient harmonic generation, laser driven fusion etc.

Combined Effect of Chirp and Exponential Density Ramp on Relativistic Self-focusing of Hermite-Cosine-Gaussian Laser in Collisionless Cold Quantum Plasma

This manuscript presents the dependence of exponential plasma density ramp on relativistic self-focusing of the Hermite-cosine-Gaussian (HcosG) chirped pulse laser in a collisionless cold quantum plasma. Self-focusing along with self-trapping of HcosG chirped pulse laser is studied for different values of laser intensity, decentered parameters, and plasma density. Numerical analysis reveals that these parameters play a key role in achieving the earlier and stronger self-focusing in cold quantum plasma. A comparative study of self-focusing of the HcosG chirped pulse laser with and without exponential density ramp profile is carried out. Exponential ramp profile is found to be more effective in achieving the stronger self-focusing of the HcosG chirped pulse laser. Combined effect of chirp and exponential plasma density ramp on relativistic self-focusing of HcosG laser in a collisionless cold quantum plasma is analyzed, and results reveal the early and strong self-focusing.

State transition control of emitter chain coupled to surface plasmon polariton

In this paper, the behavior of the coupled emitters inside a monochromatic laser field is investigated. The Hamiltonian of the system is obtained using the tensor product of the single emitter Hamiltonian and adding the interaction terms. The population of the states for each emitter is calculated by numerical solving of the time dependent Schrodinger equation. The results show that the transition occurs at a resonance frequency for any values of laser intensity. Also, the resonance frequency depends on the coupling strength of the system. Furthermore, the transition probability increases with increasing the laser intensity at each laser frequency and coupling strength.

Self-focusing and defocusing of Gaussian laser beams in collisional inhomogeneous plasmas with linear density and temperature ramps

Self-focusing and defocusing of Gaussian laser beams in collisional inhomogeneous plasmas are investigated in the presence of various laser intensities and linear density and temperature ramps. Considering the ponderomotive force and using the momentum transfer and energy equations, the nonlinear electron density is derived. Taking into account the paraxial approximation and nonlinear electron density, a nonlinear differential equation, governing the focusing and defocusing of the laser beam, is obtained. Results show that in the absence of ramps the laser beam is focused between a minimum and a maximum value of laser intensity. For a certain value of laser intensity and initial electron density, the self-focusing process occurs in a temperature range which reaches its maximum at turning point temperature. However, the laser beam is converged in a narrow range for various amounts of initial electron density. It is indicated that the σ2 parameter and its sign can affect the self-focusing process for different values of laser intensity, initial temperature, and initial density. Finally, it is found that although the electron density ramp-down diverges the laser beam, electron density ramp-up improves the self-focusing process.

Mitigation of stimulated Raman backscattering by elliptical laser beam in collisionless plasma

This paper presents theoretical and numerical study of the propagation of an elliptical laser beam in collisionless plasma with relativistic-ponderomotive nonlinearities and its effect on the excitation of electron plasma wave (EPW) and resulting stimulated Raman backscattering (SRBS) process. Nonlinear interaction between an intense elliptical laser beams with the electron plasma wave leads stimulated Raman backscattering. Following Wentzel Kramers Brillouin (WKB) and paraxial-ray approximations, non-linear differential equations for the beam width parameters of elliptical laser beam, electron plasma wave and back-scattered beam are set up and solved numerically. Numerical simulations have been carried out to investigate the effect of laser and plasma parameters on the focusing of elliptical laser beam in plasma and further its effect on the excitation of electron plasma wave and backreflectivity of stimulated Raman scattering (SRS). The focusing of an elliptical laser beam, EPW and scattered wave are reduced at higher values of laser intensity in plasma. The amplitude of excited electron plasma wave, which depends on the focusing of main laser beam and electron plasma wave reduce for higher values of incident laser intensity. The results indicate that the focusing of waves mitigates the backreflectivity of SRS. The results are also compared with only relativistic nonlinearity and the Gaussian profile of laser beam. It is observed that an elliptical laser beam mitigates the backreflectivity of SRS at higher values of laser intensity. This study is relevant to inertial confinement fusion scheme, where SRS plays very important role.

GeV electron acceleration by a Gaussian field laser with effect of beam width parameter in magnetized plasma

Electron acceleration due to a circularly polarized (CP) Gaussian laser field has been investigated theoretically in magnetized plasma. A Gaussian laser beam possesses trapping forces on electrons during its propagation through plasma. A single particle simulation indicates a resonant enhancement of electron acceleration with a Gaussian laser beam. The plasma is magnetized with an axial magnetic field in same direction as that of laser beam propagation. The dependence of laser beam width parameter on electron energy gain with propagation distance has been presented graphically for different values of laser intensity. Electron energy gain is relatively high where the laser beam parameter is at its minimum value. Enhanced energy gain of the order of GeV is reported with magnetic field under 20MG in plasma. It is also seen that the axial magnetic field maintains the electron acceleration for large propagation distance even with an increasing beam width parameter.

The influence of plasma shielding effect on laser-ablated copper samples: a focus on signal-to-background ratio and plasma expansion

AbstractThe influence of plasma shielding effect induced by ambient gas pressure and laser intensity on the laser-produced Cu plasma parameters, signal-to-background ratio (S/B) and expansion are experimentally and numerically investigated. A Q-switched Nd:YAG laser at 1064 nm at various intensities ranging from 2 to 7.1 GW/cm2 intensity (40–150 mJ) is used to produce Cu plasma in air, argon (Ar), helium (He), and neon (Ne) ambient gas at various pressures ranging from 5 to 1000 mbar. Laser-induced breakdown spectroscopy reveals that spectral radiation, S/B, electron temperature, number density, and front edge velocity of the plasma have an increasing trend up to a certain value of laser intensity and gas pressure. Afterwards, a saturation trend is achieved, which is attributed to the shielding and self-regulation effect. The numerical modeling of the laser-produced Cu plasma in the presence of air at atmospheric pressure is carried out using the MULTI radiation hydrodynamics code. We have shown that the feature of plasma shielding effect observed in the experiments can be reproduced using a continuum hydrodynamics model. Laser intensity at about 3.5 GW/cm2 is found to produce the highest S/B at 1000 mbar air. He, Ne, air, and Ar show the best S/B, respectively and the best S/B is found for air, Ar, He, and Ne at 10, 5, 10, and 20 mbar, respectively. The expansion of plasma plume is studied using a simple and effective technique based on probe laser absorption and scattering method. The plasma plume expansion through He, Ne, air, and Ar at 1000 mbar pressure has the highest velocity, respectively. The simulated results of strong shock wave model and Rankine–Hugoniot jump condition are fitted to the experimental data, which are then used to estimate the values of the ablation parameters.

Plasmonic Heating in Au Nanowires at Low Temperatures: The Role of Thermal Boundary Resistance.

Inelastic electron tunneling and surface-enhanced optical spectroscopies at the molecular scale require cryogenic local temperatures even under illumination-conditions that are challenging to achieve with plasmonically resonant metallic nanostructures. We report a detailed study of the laser heating of plasmonically active nanowires at substrate temperatures from 5 to 60 K. The increase of the local temperature of the nanowire is quantified by a bolometric approach and could be as large as 100 K for a substrate temperature of 5 K and typical values of laser intensity. We also demonstrate that a ∼3-fold reduction of the local temperature increase is possible by switching to a sapphire or quartz substrate. Finite element modeling of the heat dissipation reveals that the local temperature increase of the nanowire at temperatures below ∼50 K is determined largely by the thermal boundary resistance of the metal-substrate interface. The model reproduces the striking experimental trend that in this regime the temperature of the nanowire varies nonlinearly with the incident optical power. The thermal boundary resistance is demonstrated to be a major constraint on reaching low temperatures necessary to perform simultaneous inelastic electron tunneling and surface-enhanced Raman spectroscopies.

Controlling ultrafast currents by the nonlinear photogalvanic effect

We investigate the effect of broken inversion symmetry on the generation and control of ultrafast currents in a transparent dielectric (SiO2) by strong femtosecond optical laser pulses. Ab initio simulations based on time-dependent density functional theory predict ultrafast direct currents that can be viewed as a nonlinear photogalvanic effect. Most surprisingly, the direction of the current undergoes a sudden reversal above a critical threshold value of laser intensity of about W cm−2. We trace this switching to the transition from nonlinear polarisation currents to the tunnelling excitation regime. The latter is found to be sensitive to the relative orientation between laser polarisation and chemical bonds. We demonstrate control of the ultrafast currents by the time delay between two laser pulses. While two temporally separated laser pulses lead to currents along one direction their temporal overlap can reverse the current. We find the ultrafast current control by the nonlinear photogalvanic effect to be remarkably robust and insensitive to the laser-pulse shape and the carrier-envelope phase.

Noble gas clusters and nanoplasmas in high harmonic generation

We report a study of high harmonic generation from noble gas clusters of xenon atoms in a gas jet. Harmonic spectra were investigated as a function of backing pressure, showing spectral shifts due to the nanoplasma electrons in the clusters. At certain value of laser intensity this process may oppose the effect of the well-known ionization-induced blueshift. In addition, these cluster-induced harmonic redshifts may give the possibility to estimate cluster density and cluster size in the laser–gas jet interaction range.

Neutron yields of nuclear reactions induced by ion acceleration in carbon-deuterated plasma produced by ultra-intense lasers

Neutron yields of the nuclear reactions induced by accelerated deuterium or carbon ions when solid deuterated targets (CD or CD2) are irradiated by ultra-intense lasers have been investigated. The energies of the accelerated ions have been calculated at different laser intensities. Nuclear reactions that have threshold energies less than the maximum ion acceleration, in each case, have been studied in detail. The mechanisms of these reactions, at different ion energies, have been discussed and the relevant cross sections have been calculated to compare the neutron yield of each reaction with that of the fusion reaction D(d,n)3He. The study indicates that at a certain value of laser intensity, when the accelerated ion energy is sufficient, the energy spectrum of the neutron yields of the reactions D(d,np)D, 12C(d,n)13N, 12C(d,np)12C and D(12c,n)13N will overlap with the spectrum of the neutron yield of the D(d,n)3He reaction. Then, it is impossible to distinguish the different neutron yields experimentally despite the fact that the total neutron yield can be measured. However, by calculating the relative importance of the neutron yield of each reaction, the contribution of the fusion reaction to the total neutron yield can be estimated. Our calculations point out that the total neutron yield of the reactions 12C(d,n)13N and D(12c,n)13N can exceed the neutron yield of the D(d,n)3He reaction. Also, the total neutron yield of the breakup reactions D(d,np)D and 12C(d,np)12C is comparable with the neutron yield of the D(d,n)3He reaction and the produced neutrons will overlap the 2.45 MeV neutrons.

Complete transfer of populations from a single state to a preselected superposition of states using piecewise adiabatic passage: Experiment

We demonstrate a method of adiabatic population transfer from a single quantum state into a coherent superposition of states. The transfer is executed with femtosecond pulses, spectrally shaped in a simple and intuitive manner, which does not require iterative feedback-controlled loops. In contrast to nonadiabatic methods of excitation, our approach is not sensitive to the exact value of laser intensity. We show that the population transfer is complete, and analyze the possibility of controlling the relative phases and amplitudes of the excited eigenstates. We discuss the limitations of the proposed control methods due to the dynamic level shifts and suggest ways of reducing their influence.

Implementation of a double-scanning technique for studies of the Hanle effect in rubidium vapor

We have studied the resonance fluorescence of a room-temperature rubidium vapor exited to the atomic 5P3/2 state (D2 line) by powerful single-frequency cw laser radiation (1.25 W/cm^2) in the presence of a magnetic field. In these studies, the slow, linear scanning of the laser frequency across the hyperfine transitions of the D2 line is combined with a fast linear scanning of the applied magnetic field, which allows us to record frequency-dependent Hanle resonances from all the groups of hyperfine transitions including V- and Lambda - type systems. Rate equations were used to simulate fluorescence signals for 85Rb due to circularly polarized exciting laser radiation with different mean frequency values and laser intensity values. The simulation show a dependance of the fluorescence on the magnetic field. The Doppler effect was taken into account by averaging the calculated signals over different velocity groups. Theoretical calculations give a width of the signal peak in good agreement with experiment.

Structural changes in soot particles induced by diode laser irradiation

The effects of laser radiation on soot in a vacuum were studied by Raman spectroscopy and transmission electron microscopy. It was found that defective carbon onions could be formed simply by irradiation of focused common diode lasers with intensity much lower (10 3–10 6 W/cm 2) than those used in the previous work. A modified Melton model was introduced to analyze the formation of onions. The results show a threshold value of laser intensity (5.49 × 10 3 W/cm 2), above which sublimation and rearrangement play a dominant role in the formation of onions. Such onions can also be formed with laser intensities below the threshold since neighboring graphene layers may merge and grow by capturing interstitial carbon atoms. The deposited soot is damage resistant to the diode laser radiation in air due to the rapid formation of carbon onions.

Chemical composition of dome-shaped structures grown on titanium by multi-pulse Nd:YAG laser irradiation

The specific dome-shaped structures were grown by multi-pulse Nd:YAG (λ=1.064μm, τ=∼300ns, and ν=30kHz) laser irradiation of titanium targets in air at atmospheric pressure. The laser intensity values were chosen below the single-laser-pulse melting threshold of titanium. The chemical composition of the structures was studied as a function of laser pulse number as well as laser intensity, both at the outer surface layer and in depth. Micro-Raman spectroscopy, Auger electron spectroscopy (AES), and wavelength dispersive X-ray spectroscopy (WDX) were used as diagnostic techniques. Morphological investigations were performed by scanning electron microscopy. The obtained results revealed a lower oxygen concentration in the centre of the structures as compared to the borders and a lower concentration on the surface than in the depth. Moreover, it was found that the stoichiometry of the formed TiO2−x oxides increases from the structures centre towards the border and from the surface towards the depth.

NUMERICAL SIMULATION STUDY OF LASER-DRIVEN SHOCKWAVE PROPAGATION IN PLANAR Al FOIL TARGETS

The-numerical simulation study of an experimental result of laser-driven shock wave propagation in a planar aluminum foil target is performed using a one-dimensional radiation hydrodynamic code MULTI. The effect of spatial mesh size on numerical simulation resulits is found to be significant and optimum mesh size is obtained.-Series of simulations are performed using different sets of laser absorption coefficients and the most appropriate laser absorption coefficients value for two laser intensity values used in the experiment is obtained .The results suggest that shock velocities and peak shock pressures are in good agreement with the experimental results.

Laser-Induced Breakdown of Metal Vapor

The interaction of laser radiation in the intensity range G = 106 ÷ 1010 W/cm2 and at the wavelength λ = 1.06 μm with copper vapor is simulated, using a collision-radiation model describing the kinetics of nonequilibrium ionization and recombination. It is shown that the interaction of laser radiation with vapor can proceed in two qualitatively different ways, namely, prebreakdown and optical breakdown regimes. If the radiation intensity is insufficient to induce avalanche ionization, the system transfers to a stationary state characterized by one temperature, the equilibrium between the radiation and the vapor being absent. For higher Values of laser intensity, optical breakdown starts, i.e., the nonequilibrium transition state from a partially ionized vapor to a fully ionized plasma where Coulomb collisions are predominant. Simulations confirm experimental results that optical breakdown refers to a distinct threshold in the radiation intensity. In the macroscopic description of breakdown, the threshold intensity values depend on the ionization potential and the electronic state distribution of neutral atoms as well as on the initial temperature of the evaporated material and the laser pulse width.

Plasma heating by a strong laser field with statistically distributed parameters

The energy absorption rate by a classical homogeneous plasma irradiated by a strong fluctuating laser field via inverse bremsstrahlung is considered. A chaotic-field model is used and comparison is made with the fundamental model of a purely coherent field. In the present analysis, the emphasis is put on the interplay between the laser field statistics and the plasma electron energy distribution. Numerical calculations are concerned with the dependence of the energy absorption rates on laser intensity and frequency. Laser intensity values up to 4.6\1015 W/cm2 are considered. The multiphoton structure of the energy absorption is analysed as well. Concerning the joint influence of the radiation and particle statistics on the absorption rate, the basic result may be stated as follows. For situations where the particle thermal velocityv T is larger than the oscillatory velocityv 0 imparted by the field (v T ≥v 0, relatively weak field), the absorption rate is only weakly dependent on the field statistics. For situations, instead, whenv 0≫v T , which occurs for very high intensities, the reverse becomes true: now the initial particle velocity distribution plays the modest role of a velocity spread of an electron beam oscillating atv 0. In general, for very high intensities (v 0≫v T ), the energy absorption via bremsstrahlung becomes less effective because the high oscillatory velocityv 0 reduces the time available to electrons for the interaction with the ions, the third body which makes possible the exchange for energy between electrons and a radiation field. We report also, for the first time, results on the Marcuse effect for the case of a chaotic laser field, along with calculations of the absorption rate for a directed electron beam.