Interaction Of Intense Laser Pulses Research Articles

Note: Diagnosing femtosecond laser-solid interactions with monochromatic K α imager and x-ray pinhole camera

An x-ray pinhole camera and a monochromatic K(α) imager are used to measure the interactions of intense femtosecond laser pulses with Cu foil targets. The two diagnostics give different features in the spot size and the laser energy scaling, which are resulted from different physical processes. Under our experimental conditions, the K(α) emission is mainly excited by the fast electrons transporting inside the cold bulk target. In contrast, the x-ray pinhole signals are dominated by the broadband thermal x-ray emission from the hot plasma at the front target surface.

Excitation of X rays by electrons accelerated in air in the wake wave of a laser pulse

The characteristics of X rays of a laser plasma generated in the interaction of a femtosecond pulse with solid targets in an air atmosphere have been investigated. It has been shown that the mechanism for the generation of X rays in the interaction of short intense laser pulses with solid targets in a gas atmosphere is attributed to the generation of fast electrons in the region of the filamentation of a laser pulse. It has been proven experimentally that under such conditions, the solid target irradiated by laser radiation of even a low density of about 1015 W/cm2 very efficiently emits ∼10-keV photons. It has been shown theoretically that the maximum energy of accelerated electrons can reach ɛmax ∼ 100–200 keV under these conditions. This means that the proposed method can provide characteristic radiation with the energy of photons much higher than 10 keV.

X-ray polarization spectroscopy from ultra-intense interactions

Detailed knowledge of fast electron energy transport following the interaction of ultrashort intense laser pulses is a key subject for fast ignition. This is a problem relevant to many areas of laser-plasma physics with particular importance to fast ignition and X-ray secondary source development, necessary for the development of large-scale facilities such as HiPER and ELI. Operating two orthogonal crystal spectrometers set at Bragg angles close to 45° determines the X-ray s- and p- polarization ratio. From this ratio, it is possible to infer the velocity distribution function of the fast electron beam within the dense plasma. We report on results of polarization measurements at high density for sulphur and nickel buried layer targets in the high intensity range of 1019 – 1021 Wcm−2. We observe at 45° the Ly-α doublet using two sets of orthogonal highly-orientated pyrolytic graphite (HOPG) crystals set in 1st order for sulphur and 3rd order for nickel.

Energetic electron and ion generation from interactions of intense laser pulses with laser machined conical targets

The generation of energetic electron and proton beams was studied from the interaction of high intensity laser pulses with pre-drilled conical targets. These conical targets are laser machined onto flat targets using 7–180 µJ pulses whose axis of propagation is identical to that of the main high intensity pulse. This method significantly relaxes requirements for alignment of conical targets in systematic experimental investigations and also reduces the cost of target fabrication. These experiments showed that conical targets increase the electron beam charge by up to 44 ± 18% compared with flat targets. We also found greater electron beam divergence for conical targets than for flat targets, which was due to escaping electrons from the surface of the cone wall into the surrounding solid target region. In addition, the experiments showed similar maximum proton energies for both targets since the larger electron beam divergence balances the increase in electron beam charge for conical targets. 2D particle in cell simulations were consistent with the experimental results. Simulations for conical target without preplasma showed higher energy gain for heavy ions due to ‘directed coulomb explosion’. This may be useful for medical applications or for ion beam fast ignition fusion.

Divergence of fast ions generated by interaction of intense ultra-high contrast laser pulses with thin foils

We propose an analytical model that analyzes the divergence of fast ion beams accelerated at the rear of thin foils irradiated with ultra-short intense laser pulses. We demonstrate the critical role played by the non-stationary character of the side components of the electric field, which is responsible for ion acceleration from the back of the foil. The model predictions are in very good agreement with 2D PIC simulations and with the experiments performed in the ultra-high-contrast regime as well.

Interaction of intense laser pulses with hydrogen atomic clusters

The interaction between intense femtosecond laser pulses and hydrogen atomic clusters is studied by a simplified Coulomb explosion model. The dependences of average proton kinetic energy on cluster size, pulse duration, laser intensity and wavelength are studied respectively. The calculated results indicate that the irradiation of a femtosecond laser of longer wavelength on hydrogen atomic clusters may be a simple, economical way to produce highly kinetic hydrogen ions. The phenomenon suggests that the irradiation of femtosecond laser of longer wavelength on deuterium atomic clusters may be easier than that of shorter wavelength to drive nuclear fusion reactions. The product of the laser intensity and the squared laser wavelength needed to make proton energy saturated as a function of the squared cluster radius is also investigated. The proton energy distribution calculated is also shown and compared with the experimental data. Our results are in agreement with the experimental results fairly well.

The long-term evolution of D_2 ^+ nuclear wave-packet with interaction of intense femtosecond laser pulse

We investigate the long-term evolution of D2+ nuclear wave-packet after interacting with a 5 fs, 800 nm laser pulse at an intensity of 0.5 x 10(14) W/cm2. The nuclear wave-packet evolves in field-free condition for very long time after the laser pulse. The collapse and revival of nuclear wave-packet is studied. The scale of the classical time (approximately 25 fs), the revival time (approximately 580 fs) and the super-revival time (approximately 12 ps) are determined from the simulation as well as the calculation. The constituents of long-standing nuclear wave-packet are also analyzed.

Dynamic Stark effect and the Coulomb-Volkov approximation

A modified Coulomb-Volkov theory is used to study the interaction of intense laser pulses with atomic Hydrogen. Photon energy greater than the ionization potential and non-perturbative conditions are considered. The dynamical Stark effect is accounted by introducing the initial state coupling to the remaining discrete and continuum atomic spectrum. Both the Stark shift and the decay of the initial state are studied. The ionization spectra, the Stark shift and the total ionization probability are compared with results obtained from numerical solution of the time dependent Schrödinger equation. Both results agree well.

Dynamics simulation on the interaction of intense laser pulses with atomic clusters

Under classical particle dynamics, the interaction process between intense femtosecond laser pulses and icosahedral noble-gas atomic clusters was studied. Our calculated results show that ionization proceeds mainly through tunnel ionization in the combined field from ions, electrons and laser, rather than the electron-impact ionization. With increasing cluster size, the average and maximum kinetic energy of the product ion increases. According to our calculation, the expansion process of the clusters after laser irradiation is dominated by Coulomb explosion and the expansion scale increases with increasing cluster size. The dependence of average kinetic energy and average charge state of the product ions on laser wavelength is also presented and discussed. The dependence of average kinetic energy on the number of atoms inside the cluster was studied and compared with the experimental data. Our results agree with the experimental results reasonably well.

Coulomb implosion mechanism of negative ion acceleration in laser plasmas

Coulomb implosion mechanism of the negatively charged ion acceleration in laser plasmas is proposed. When a cluster target is irradiated by an intense laser pulse and the Coulomb explosion of positively charged ions occurs, the negative ions are accelerated inward. The maximum energy of negative ions is several times lower than that of positive ions. We present the theoretical description and Particle-in-Cell simulation results of the Coulomb implosion mechanism, and show the evidence of the negative ion acceleration in the experiments on the high intensity laser pulse interaction with the cluster targets.

Experimental and theoretical study of Al plasma under femtosecond laser pulses

The amplitude and phase of the complex reflection coefficient of a weak probe laser pulse from strongly coupled Al plasma created on the surface of a metallic target by pump femtosecond laser pulses with intensities I ≲ 1015 W cm−2 were measured using femtosecond interference microscopy. A theoretical model developed for the interaction of intense ultrashort laser pulses with solid targets on the basis of a two-temperature equation of state for an irradiated substance was used for numerical simulations of the dynamics of the formation and expansion of the plasma. A comparison of the experimental data with the simulated results shows that the model is suitable up to I ∼ 1014 W cm−2. At higher intensities of the heating laser pulse, lower values of the reflection coefficient amplitude of Al plasma are observed in the experiment.

Soft x-ray emission, angular distribution of hot electrons, and absorption studies of argon clusters in intense laser pulses

High resolution soft x-ray spectra (3–40 nm) produced by the interaction of intense femtosecond laser pulses with argon clusters at an intensity of about 1×1016 W/cm2 are measured under different backing pressures. Soft x-ray spectra are strongly dependent on the initial size of cluster and plasma expanding process. We find that laser polarization has no evident effect on the soft x-ray emission from the lateral side. The measurement on angular distribution of hot electrons shows that the resonant absorption is the dominated mechanism. Both the x-ray yield and the laser energy absorption are more efficient when the radius of cluster is comparable with the plasma resonant length.

Intense femtosecond laser pulse interaction with ultra-low expansion glass: nonlinear responses and optical limiting effect

In this study, the nonlinear responses and optical limiting behaviour of titanium doped silicate glass (7.5TiO2–92.5SiO2), known as ultra-low expansion (ULE) glass, are investigated. The measurements were performed by use of the Z-scan technique and monitoring of the transmitted energy of 200 fs Ti:sapphire laser pulses at a central wavelength of 800 nm. The optical power limiting threshold was measured to be about 37 µJ (peak intensity of 0.55 × 1012 W cm−2) in this sample. The decrease of transmitted intensity was about 60% compared to the theoretical linear transmission for incident pulse energies of 320 µJ (peak intensity of 21 × 1012 W cm−2). For the prediction of the experimental nonlinear behaviour results, we used the three-photon absorption and three-photon-generated free-carrier absorption model within the sample.

Interaction of intense short laser pulses with positronium

The ionization of the positronium is studied in the over-the-barrier regime. The Volkov and momentum-space strong-field approximation are applied to describe the ionization of the hydrogen-like atomic systems. Classical trajectory Monte Carlo calculations were also performed and a good agreement with quantum-mechanical calculations were found. The scaling of the ionization probability density as function of the reduced mass of the target is analyzed. We find that the maxima of the ionization probability density is determined by laser field parameters, and it’s width can be characterised by the charge of the core and reduced mass of the target.

Thermal dynamics-based mechanism for intense laser-induced material surface vaporization

Laser material processing involving welding, ablation and cutting involves interaction of intense laser pulses of nanosecond duration with a condensed phase. Such interaction involving high brightness radiative flux causes multitude of non-linear events involving thermal phase transition at soild-liquid-gas interfaces. A theoretical perspective involving thermal dynamics of the vaporization process and consequent non-linear multiple thermal phase transitions under the action of laser plasma is the subject matter of the present work. The computational calculations were carried out where titanium (Ti) was treated as a condensed medium. The solution to the partial differential equations governing the thermal dynamics and the underlying phase transition event in the multiphase system is based on non-stationary Eulerian variables. The Mach number M depicts significant fluctuations due to thermal instabilities associated with the laser beam flux and intensity. A conclusive amalgamation has been established which relates material surface temperature profile to laser intensity, laser flux and the pressure in the plasma cloud.

Modified nanoplasma model for laser-cluster interaction

We present a modified nanoplasma model to describe the interaction of intense laser pulses with rare-gas clusters of nanometric size. Compared to previous models, the model relies on a better description of the collisional processes and allows one to obtain highly charged ion populations and associated x-ray spectra in accordance with experimental findings. We further study the x-ray emission dependence on pulse duration and cluster size and show that these parameters can be optimized to maximize $\text{He}\text{ }\ensuremath{\alpha}$ production.

High-intensity laser-plasma interactions in the refluxing limit

The interaction of intense laser pulses (5×1016W∕cm2<I<2×1019W∕cm2) with small-volume copper targets (20×20×2μm3<V<500×500×50μm3) is described. Electron refluxing causes Kα yields to be independent of laser intensity at I>1018W∕cm2, where the target is heated to less than 100eV. As target volume is reduced, efficient energy transfer by the refluxing electrons heats the targets to hundreds of eV. This affects L→K and M→K electronic transitions during ion de-excitation that probe the bulk plasma conditions during this rapid isochoric heating phase. Comparison of measured Kα yields in the cold-material limit with a Kα production model, and measured Kβ∕Kα variations with numerical target-heating calculations, provides a robust method for inferring the fast-electron energy content. Bulk electron temperatures of at least 200eV are inferred for the smallest mass targets.

Multistep Ionization of Argon Clusters in Intense Femtosecond Extreme Ultraviolet Pulses

The interaction of intense extreme ultraviolet femtosecond laser pulses (lambda = 32.8 nm) from the FLASH free electron laser (FEL) with clusters has been investigated by means of photoelectron spectroscopy and modeled by Monte Carlo simulations. For laser intensities up to 5x10(13) W/cm(2), we find that the cluster ionization process is a sequence of direct electron emission events in a developing Coulomb field. A nanoplasma is formed only at the highest investigated power densities where ionization is frustrated due to the deep cluster potential. In contrast with earlier studies in the IR and vacuum ultraviolet spectral regime, we find no evidence for electron emission from plasma heating processes.

A model for the efficient coupling between intense lasers and subwavelength grating targets

The interaction of an ultrashort intense laser pulse with a subwavelength rectangular grating target is studied by an analytical model and particle-in-cell simulations. Such interaction excites strong periodic charge separation at the grating surface. With the presence of formed electrostatic fields, the laser field can intensively heat electrons, causing near 100% light absorption. Its dependence on grating parameters is given. At low or moderate laser intensity, there are optimized sizes of grating gibbous cells and grooves as well as groove depths at which absorption maximums are found. At high intensity, the absorption is weakly affected by groove sizes and depths.

Interaction of intense ultra-short laser pulses with dielectrics

The interaction of intense ultra-short laser pulses with dielectrics is studied theoretically using a one-dimensional simulation model. The curl Maxwell's equations are solved coupled to the electron continuity, momentum and energy equations. What is new and innovative in our approach is the application of the ‘forest fire’ method to describe the optical field ionization. The cumulative action of the applied laser field and holes in the dielectric, created during ionization events, enhances the optical field ionization rate by orders of magnitude speeding up the electron avalanche development in the early times of the ionization dynamics. For peak laser intensity of about 1017 W m−2 (1013 W cm−2) the ‘forest fire’ model plays a major role in the ionization dynamics and increases the electron density by two orders of magnitude. The underlying mechanisms leading to electron avalanche in dielectrics are investigated for peak laser intensities between 1016 and 3 × 1018 W m−2 and laser pulse durations between 10 fs and 1 ps. It was established that the dominant process for electron multiplication depends only on the laser fluence: for F < 4 kJ m−2 (F < 0.4 J cm−2) the cumulative optical field ionization rate exceeds the collisional ionization rate, while for F > 4 kJ m−2 (F > 0.4 J cm−2) the collisional ionization is the dominant ionization mechanism. Since for laser pulse duration exceeding 20 fs the threshold for dielectric damage of SiO2 is at least Fthr > 15 kJ m−2 (Fthr > 1.5 J cm−2) (Lenzner et al 1998 Phys. Rev. Lett. 80 4076), the primary cause for dielectric damage is an electron avalanche due to collisional ionization.