Vacuum Laser Research Articles

NORMALIZED UNITS IN SIMULATION OF LASER/PARTICLE INTERACTIONS

An algorithm for simulation of laser/particle interactions using normalized units related to different laser frequencies was studied. Correct and wrong conversion methods between different normalized units were investigated in simulations of vacuum laser acceleration. The dynamic characteristics obtained with different processing methods differ greatly and some good acceleration results are actually no other than errors stemming from incorrect treatments of the normalized units.

Simulation prediction and experiment setup of vacuum laser acceleration at Brookhaven National Lab-Accelerator Test Facility

This paper presents the pre-experiment plan and prediction of the first stage of vacuum laser acceleration (VLA) collaborating by UCLA, Fudan University and ATF-BNL. This first stage experiment is a proof-of-principle to support our previously posted novel VLA theory. Simulations show that based on ATF's current experimental conditions the electron beam with initial energy of 15MeV can get net energy gain from an intense CO2 laser beam. The difference in electron beam energy spread is observable by the ATF beam line diagnostics system. Further, this energy spread expansion effect increases along with an increase in laser intensity. The proposal has been approved by the ATF committee and the experiment will be our next project.

Formation of TiO2 film with lower electrical resistance by aerosol beam and fiber laser irradiation

Titanium dioxide (TiO2) is a functional ceramic with unique photoconductive and photocatalytic properties. In our previous study, a TiO2 film was formed by aerosol beam irradiation. The films were darkened by femtosecond laser irradiation in air. Then electrical resistance of the darkened area on the film decreased. The heating process is also a useful method to vary the TiO2 film property. Local heating can be performed by using a continuous wave (CW) fiber laser. In this study, the film was irradiated with a commercial CW fiber laser in vacuum. Laser irradiated area on the film was also darkened after CW fiber laser irradiation. The electrical resistance of the darkened area on the films was decreased as laser fluence was increased. Electrical resistance of the darkened area after CW fiber laser irradiation in vacuum was much smaller than that after femtosecond laser irradiation.

Simulation of plume-plasma expansion with one-dimensional Particle-in-Cell

In this work we present the analysis of the dynamic of the expansion of Al Plasma produced by Nd:YAG laser (1064 nm, 500mJ, 9 ms, 10 Hz) in vacuum. To study the Coulomb interaction between the particles of the initial states of the plasma expansion, we use the one dimensional Particle-in-Cell method (PIC) and finite difference method. We considered an ideal model, that is, we assume that the plasma is in a local thermal equilibrium, the ablated particles have a fixed temperature and a constant evaporation flux (J) from the aluminium surface. To obtain more accurate results we use high computing exploiting the parallelization of this kind of algorithms. The mean velocity and particles densities are determined for different times of the expansion.

Surface modification of copper using high intensity, 1015 W/cm2, femtosecond laser in vacuum

A study of copper target surface modification with high intensity (1015W/cm2, fluence of ∼300J/cm2) Ti:sapphire laser, operating at 800nm wavelength and pulse duration of 60fs is presented. The Cu surface variations were studied in vacuum ambience. The copper target specific surface changes and phenomena observed are: (i) creation of craters (with extra-low diameters); (ii) formation of periodic surface structures especially expressed at the reduced intensity; (iii) presence of the “clean” surface in the irradiated zone (mainly absence of the oxide(s)), and (iv) occurrence of plasma in front the surface, emitting predominantly X-ray radiation (soft and hard). It can be concluded from this study that the reported laser intensities can effectively be applied for copper surface modification. The appearance of plasma in front of the target, irradiating in the X-ray/UV region, offers additional effect of surface purification facilitating contaminant-free conditions which is highly important for specific applications, e.g. in microelectronics. Generally, femtosecond laser surface modification of copper is non-contact and very rapid compared to traditional modification methods.

Microstructural characterization of plasma sprayed Al2O3-40 wt.% TiO2 coatings on high density graphite with different post-treatments

Graphite is one of the candidate materials proposed for application in pyrochemical reprocessing plants involving aggressive molten chloride environment. Post treatments are promising techniques for the improvement of properties of thermal spray coatings for different industrial applications. In the present work, the effect of post treatments like vacuum annealing (VA) and laser melting (LM) on the microstructure and chemical modification of plasma sprayed Al2O3-40wt.% TiO2 coatings over high density (HD) graphite substrates has been investigated. When compared with sprayed coatings (SC), VA coatings showed cluster morphology and LM coatings exhibited homogenous microstructure. On laser melted surfaces networks of cracks were observed. XRD studies showed that the metastable γ-Al2O3 phase present in the SC is transformed to stable α-Al2O3 after post treatments. In LM coatings Al2TiO5 phase was more predominant in contrast to SC and VA coatings. The microhardness enhancement was observed in case of LM coating compared to the VA and SC. Due to elimination of coating defects in LM samples, there is a considerable reduction in the surface roughness.

Experimental Observation of the 16-Electron Molecules SPN, SNP, and Cyclic PSN

16-Electron triatomic ring: novel thiazylidynephosphane (SNP) was produced by either flash vacuum pyrolysis (ca. 1000 °C) or laser photolysis (193 nm) of SP(N(3))(3). Its photointerconversion to cyclic thiazaphosphirine (cyc-PSN) and thiophosphoryl nitride (SPN) was found in Ar matrix at 16 K. Cyc-PSN is the first experimentally observed 16-electron cyclic triatomic molecule.

Laser Wavelength Dependence on Photochemical Surface and Interface Modifications of Aluminum Thin Films on Silica Glass

Photochemical surface and interface modifications of Al thin films on silica glass were successfully carried out using a 157 nm F2 laser for micropatterning. The surface modification phenomenon was discussed in relation to by changing the laser wavelength using a 193 nm ArF laser or a 266 nm neodymium-doped yttrium aluminum garnet (Nd:YAG) laser. The ArF laser could induce the surface modification of Al thin films to form a protective Al2O3 layer resistant to KOH aqueous solution, similarly to the F2 laser. However, the mechanical hardness of the ArF-laser-irradiated sample was clearly lower than that of the F2-laser-irradiated sample. The origin of the surface modification was examined by irradiating the F2 laser in vacuum. The interface modification phenomenon was analyzed by X-ray photoelectron spectroscopy in the three cases. The adhesion strengths of the samples were also compared. The 266 nm Nd:YAG laser was not effective for the present photochemical modifications.

Explicit general solutions to relativistic electron dynamics in plane-wave electromagnetic fields and simulations of ponderomotive acceleration

This study examines single-particle electron motions in both a plane electromagnetic wave and a Gaussian focus in vacuum. Exact, explicit analytic expressions for relativistic electron trajectories in a plane wave are obtained, using the proper time as a parameter, in the general case of arbitrary initial positions and velocities. It is shown that previous analyses can be completed using the proper-time parameter. The conditions under which localized oscillatory motions (‘figure-of-eight’ orbits) occur are derived from the new solutions. The general solutions are also connected with the figure-of-eight orbits by a Lorentz transformation. The analytic solutions for arbitrary initial conditions and an arbitrary initial field phase can be used to determine the ranges of electron ejection angle and emerging electron energy in a vacuum laser accelerator, in which electrons are ejected externally, and provide a basis for explaining the spectrum of nonlinear Thomson scattering radiation. Numerical solutions are used for electron motions in the focus of a Gaussian laser beam, and the mean motion allows one to test a new expression for the relativistic ponderomotive force. It is suggested that plane wave solutions can provide a basis for approximating the orbital motion of particles in Gaussian beams.

Late-time particle emission from laser-produced graphite plasma

We report a late-time “fireworks-like” particle emission from laser-produced graphite plasma during its evolution. Plasmas were produced using graphite targets excited with 1064 nm Nd: yttrium aluminum garnet (YAG) laser in vacuum. The time evolution of graphite plasma was investigated using fast gated imaging and visible emission spectroscopy. The emission dynamics of plasma is rapidly changing with time and the delayed firework-like emission from the graphite target followed a black-body curve. Our studies indicated that such firework-like emission is strongly depended on target material properties and explained due to material spallation caused by overheating the trapped gases through thermal diffusion along the layer structures of graphite.

Bimorph Cantilevers Actuated by Focused Laser from the Side

The thin and long bimorph cantilever is mechanically responded in motion depending on the position of the heating laser which is focused from the side. The bimorph cantilever was designed in order to detect the heat from a bio cell in liquid. In the calibration process of the cantilever using a focused laser in vacuum and air, we found the quick and interesting responses of bimorph cantilever, which is vibrated and statically deflected due to the position of the local heat source on the cantilever. The deflections of cantilever have been investigated with a developed model. The responses are expressed by the modeled system with heat loss condition into ambient. To define the temperature distributions on the cantilever using a model, the temperature increments at the end of cantilever are successfully estimated to be +20 K in air and +110 K in vacuum with a laser power of 0.17 mW.

Electrochemically induced nanocluster migration

In the presented study the influence of electrochemical treatments on size-selected Pt nanoclusters (NCs) supported on amorphous carbon is investigated by means of transmission electron microscopy (TEM). Well-defined Pt NCs are prepared by an ultra-high vacuum (UHV) laser vaporization source and deposited with low kinetic energy (≤10 eV/cluster) onto TEM gold grids covered by a thin (2 nm) carbon film. After transfer out of UHV Pt NCs are verified to be uniform in size and randomly distributed on the support. Subsequently, the TEM grids are employed as working electrodes in a standard electrochemical three electrode setup and the Pt nanoclusters are subjected to different electrochemical treatments. It is found that the NC arrangement is not influenced by potential hold conditions (at 0.40 V vs. RHE) or by potential cycling in a limited potential window ( V max = 0.55 V vs. RHE). Upon potential cycling to 1.05 V vs. RHE, however, the NCs migrate on the carbon support. Interestingly, migration in oxygen or argon saturated electrolyte leads to NC coalescence, a mechanism discussed for being responsible for performance degradation of low temperature fuel cells, whereas in carbon monoxide saturated electrolyte the Pt NC agglomerate, but remain separated from each other and thus form distinctive structures.

Simulations of vacuum laser acceleration: Hidden errors from particle’s initial positions

Simulation of vacuum laser acceleration, because of its scheme's simplicity, attracts many people involved in. However, how to put the particle in the initial positions in the field has not been considered seriously in some such schemes. An inattentive choice of electron's initial conditions may lead to misleading results. Here we show that arbitrarily placing the particle within the laser field leads to an overestimation of its energy gain, and offer suggestions for selecting appropriate initial conditions.

Low temperature synthesis of SiC films by vacuum laser ablation and their characterization

Thin films of silicon carbide were grown on Si(100) substrate by pulsed laser ablation of powder-pressed target. The influence of substrate temperature on both structure and surface morphology of SiC films was studied by scanning electron microscopy, X-ray diffraction, and IR spectroscopy.

Surface modification of bioresorbable polymer scaffolds by laser treatment

The effect of laser irradiation on the properties of the surface of films prepared from a bioresorbable polymer poly(hydroxybuturate) has been studied. To determine the spectral region of the polymer optimal for the effective action of radiation on electron molecular bonds, theoretical investigations have been performed, which have shown that, for modifying the surface of PHB scaffolds, it is expedient to use a vacuum laser at a wavelength of 160 nm. Using laser irradiation at a power from 3 to 30 W, a series of films with modified surface, from roughnesses to perforations, have been obtained. The microstructure and properties of the film surface depending on the mode of irradiation have been examined, and conditions have been found under which the contact marginal angles of film wetting with water can be decreased to 50° (compared with 76–80° in starting products). Thus, conditions of laser treatment of PHB scaffolds have been theoretically substantiated and experimentally realized that provide a beneficial effect on the properties of the surface without destroying the structure of the material.

Particle-in-cell modeling of relativistic laser–plasma interaction with the adjustable-damping, direct implicit method

Implicit particle-in-cell codes offer advantages over their explicit counterparts in that they suffer weaker stability constraints on the need to resolve the higher frequency modes of the system. This feature may prove particularly valuable for modeling the interaction of high-intensity laser pulses with overcritical plasmas, in the case where the electrostatic modes in the denser regions are of negligible influence on the physical processes under study. To this goal, we have developed the new two-dimensional electromagnetic code ELIXIRS (standing for ELectromagnetic Implicit X-dimensional Iterative Relativistic Solver) based on the relativistic extension of the so-called Direct Implicit Method [D. Hewett, A.B. Langdon, Electromagnetic direct implicit plasma simulation, J. Comput. Phys. 72 (1987) 121–155]. Dissipation-free propagation of light waves into vacuum is achieved by an adjustable-damping electromagnetic solver. In the high-density case where the Debye length is not resolved, satisfactory energy conservation is ensured by the use of high-order weight factors. In this paper, we first derive the electromagnetic direct implicit method as a simplified Newton scheme. Its linear properties are then investigated through numerically solving the relation dispersions obtained for both light and plasma waves, accounting for finite space and time steps. Finally, our code is successfully benchmarked against explicit particle-in-cell simulations for two kinds of physical problems: plasma expansion into vacuum and relativistic laser–plasma interaction. In both cases, we will demonstrate the robustness of the implicit solver for crude discretizations, as well as the gains in efficiency which can be realized over standard explicit simulations.

Acceleration of electrons and electromagnetic fields of highly intense laser pulses

AbstractThe acceleration of electrons by very intense lasers in vacuum is studied for different forms of laser pulses: finite plane and three-dimensional laser pulses. The latter case is treated by introducing an approximation of the laser frequency as a function of the Cartesian components of the wave number. Various examples which lead to high accelerations of electrons are given.

Pulsed Nd:YAG laser heating and treatment on multi-walled carbon nanotubes film

Multi-walled carbon nanotubes (MWCNTs) film surface was flashed with the second harmonic ( λ = 532 nm) of a single pulse of Nd:YAG laser in air, vacuum (0.03Pa) and nitrogen. When MWCNTs film is irradiated with a pulsed Nd:YAG laser of 23.7 mJ/cm 2, the calculated value of maximum surface temperature becomes 304 °C. The surface temperature increases from 595 °C to 2968 °C with the increase of laser power from 59.4 mJ/cm 2 to 469 mJ/cm 2. MWCNTs film was examined by Raman spectroscopy to clarify the effect of Pulsed Nd:YAG laser heating. Raman spectra of MWCNTs films before and after irradiation were measured. The intensity of the two characteristic Raman shifts I D (defect-induced mode: D-band) and I G (graphite-induced mode: G-band) was examined. Moreover, I G/ I D ratio and normalized D-band FWHM (Full Width at Half Maximum) of MWCNTs film were measured from Raman spectra and compared with the calculated results of maximum temperature of MWCNTs film surface. The purification and graphitization of as-prepared MWCNTs occurred after irradiation.

Laser oscillation in a strongly coupled single-quantum-dot–nanocavity system

The strong coupling of photons and matter1 in semiconductor nanocavities has been a test bed for cavity quantum electrodynamics2,3 (QED). Vacuum Rabi oscillation4,5,6,7,8—the coherent exchange of a single quantum between a single quantum dot (SQD) and an optical cavity—and highly efficient cavity-QED lasers9,10,11,12,13,14,15,16,17,18,19 have both been reported. The coexistence of vacuum Rabi oscillation and laser oscillation seems to be contradictory, but it has recently been predicted theoretically that the strong-coupling effect could be sustained in laser oscillation20. Here, we demonstrate the onset of lasing in the strong-coupling regime in an SQD–cavity system. A high-quality semiconductor optical nanocavity and strong SQD–field coupling enabled the onset of lasing while maintaining the fragile coherent exchange of quanta.

Effects of ambient pressure on Cu Kα X-ray radiation with millijoule and high-repetition-rate femtosecond laser

Hard Cu Kα X-ray radiation was generated with a millijoule and high-repetition-rate Ti: sapphire laser in air, helium or vacuum (2.7–1.3×104 Pa) ambient. The characteristic X-ray was obtained by focusing the 0.06–1.46 mJ/pulse, 100 fs, 1 kHz repetition femtosecond laser onto a solid copper target to a spot 4.8 μm in diameter. The relationship between Kα X-ray conversion efficiency and atmospheric conditions was explained with a simple electron collision model that suggested that the electron mean free path is an important parameter in the generation of ultrafast pulsed X-rays in any ambient condition. We also demonstrated a high-intensity X-ray source working in helium at atmospheric pressure.