Showing papers on "Fluence published in 2002"

Metal ablation by picosecond laser pulses: A hybrid simulation

Abstract: We investigate picosecond laser ablation of metals using a hybrid simulation scheme. Laser energy input into the electron system and heat conduction within it are modeled using a finite-difference scheme for solving the heat conduction equation. Atom motion in the near-surface part ~72 nm! of the sample is modeled using molecular dynamics. Energy transfer between the electronic and atomic subsystems due to electron-phonon coupling is taken into account. For the special case of 0.5 ps UV laser irradiation of copper, we investigate the fluence dependence of the ablation yield, the temperature and pressure evolution in the target, and the ablation mechanism.

Fundamental photochemical approach to the concepts of fluence (UV dose) and electrical energy efficiency in photochemical degradation reactions

Abstract: For photochemical reactions in a quasi collimated beam, derivations are presented that introduce 'rate constants' based on the fluence (UV dose) received within the irradiated solution. These fluence-based 'rate constants' are shown to be fundamental and depend only on the quantum yield and the molar absorption coefficient at the irradiation wavelength. An experimental example is given, where the quantum yield for the photolysis of atrazine is determined to be 0.033. The new concepts are developed further to analyze the Figure-of-Merit Electrical Energy per Order (EEO), and it is shown that the EEO depends on the same fundamental photochemical parameters. An example of the photolysis of N-nitrosodimethylamine (NDMA) is presented, and it is shown that the EEO should decrease (increased electrical energy efficiency) as the radius of the UV reactor increases (increased path length), and should increase as the percent transmittance of the water decreases.

Femtosecond laser ablation ICP-MS

Abstract: Femtosecond laser ablation was investigated for direct solid sample chemical analysis. The phonon relaxation time in a solid is of the order of 100 fs, which is the same as the laser pulse duration. For such excitation, there should be little time for the matrix to experience a “temperature” during the laser pulse. If the surface explodes before the photon energy is dissipated as heat in the lattice, the ablation process should produce stoichiometric vapor (elemental fractionation should be negligible). Based on this hypothesis, NIST glasses were ablated using 100 fs laser pulses at 800 nm, with subsequent elemental analysis using the ICP-MS. Pb and U intensities, and Pb/U ratios in the ICP, were measured during repetitively femtosecond-pulsed ablation. These data show that fluence (laser energy/spot area) has a significant influence on the amount of mass ablated and on the degree of fractionation. An optimal fluence was found at which the fractionation index approached unity; negligible fractionation. Infrared femtosecond laser ablation produced similar characteristics to UV nanosecond laser ablation.

Ultrafast band-edge tuning of a two-dimensional silicon photonic crystal via free-carrier injection

Abstract: Ultrafast tuning of the band edge of a two-dimensional silicon/air photonic crystal is demonstrated near a wavelength of 1.9 mm. Changes in the silicon refractive index are optically induced by injecting free carriers with 800 nm, 300 fs pulses. The rise time of the shift occurs on the time scale of the pulse width apart from a small component associated with carrier cooling; the recovery time is related to electron-hole recombination. The band edge is observed to shift linearly with pump beam fluence, with a shift in excess of 30 nm for a pump beam fluence of 2 mJ cm 22 . A nonuniform spectral shift is attributed to finite pump beam absorption depth effects.

Energy spectra, angular spread, fluence profiles and dose distributions of 6 and 18 MV photon beams: results of Monte Carlo simulations for a Varian 2100EX accelerator

Abstract: The purpose of this study is to provide detailed characteristics of incident photon beams for different field sizes and beam energies. This information is critical to the future development of accurate treatment planning systems. It also enhances our knowledge of radiotherapy photon beams. The EGS4 Monte Carlo code, BEAM, has been used to simulate 6 and 18 MV photon beams from a Varian Clinac-2100EX accelerator. A simulated realistic beam is stored in a phase space data file, which contains details of each particle's complete history including where it has been and where it has interacted. The phase space files are analysed to obtain energy spectra, angular distribution, fluence profile and mean energy profiles at the phantom surface for particles separated according to their charge and history. The accuracy of a simulated beam is validated by the excellent agreement between the Monte Carlo calculated and measured dose distributions. Measured depth-dose curves are obtained from depth-ionization curves by accounting for newly introduced chamber fluence corrections and the stopping-power ratios for realistic beams. The study presents calculated depth-dose components from different particles as well as calculated surface dose and contribution from different particles to surface dose across the field. It is shown that the increase of surface dose with the increase of the field size is mainly due to the increase of incident contaminant charged particles. At 6 MV, the incident charged particles contribute 7% to 21% of maximum dose at the surface when the field size increases from 10 x 10 to 40 x 40 cm2. At 18 MV, their contributions are up to 11% and 29% of maximum dose at the surface for 10 x 10 cm2 and 40 x 40 cm2 fields respectively. However, the fluence of these incident charged particles is less than 1% of incident photon fluence in all cases.

A quantitative model of ultraviolet matrix-assisted laser desorption/ionization.

Abstract: A quantitative model of primary ionization in ultraviolet matrix-assisted laser desorption/ionization (UV-MALDI) is presented. It includes not only photochemical processes such as exciton pooling, but also the effects of the desorption event. The interplay of these two is found to be a crucial aspect of the MALDI process. The desorbing plume is modeled as an adiabatic expansion with entrained clusters. The parameters in the model are defined as much as possible via experiment or by analogy with known effects. The model was applied to the matrix 2,5-dihydroxybenzoic acid and found to reproduce the fluence dependence of the fluorescence yield and key features of the picosecond two-pulse ion generation efficiency curves. In addition, the model correctly predicts a fluence rather than irradiance threshold, the magnitude of the threshold, the magnitude of the ion yield, laser wavelength effects, plume temperatures, plume expansion velocities and the spot size effect.

Tantalum ions produced by 1064 nm pulsed laser irradiation

Abstract: A Q-switched Nd:YAG (yttrium aluminum garnet) laser (1064 nm wavelength) with a 9 ns pulse width, 1–900 mJ pulse energy, and 0.5 mm2 target spot, is employed to irradiate tantalum targets in vacuum. The irradiation produces a strong etching of the metal and forms a plasma in front of the target. The plasma contains neutrals and ions with a high charge state and a wide energy distribution. Time-of-flight measurements are presented for the ionic production. A cylindrical electrostatic ion analyzer permits to measure the yield and the charge state of the emitted ions and to extrapolate the ion energy distribution as a function of the laser fluence in the range 10–100 J/cm2. The measurements indicate that at high laser fluence the tantalum charge state may reach 8+ and the maximum ion energy about 6 keV. The ion energy distribution is presented as a function of the charge state. It follows approximately a “shifted Maxwellian distribution.” A better theoretical approach has been further developed considering t...

Visible luminescence from silicon surfaces microstructured in air

Abstract: We report visible luminescence from SiOx formed by microstructuring silicon surfaces with femtosecond laser pulses in air. Incorporation of oxygen into the silicon lattice occurs only where the laser beam strikes the surface. Laser microstructuring therefore offers the possibility of writing submicrometer luminescent features without lithographic masks. The amount of oxygen incorporated into the silicon surface depends on the laser fluence; the peak wavelength of the primary luminescence band varies between 540 and 630 nm and depends on the number of laser shots. Upon annealing, the intensity of the primary luminescence band increases significantly without any change in the luminescence peak wavelength, suggesting that the luminescence comes from defects rather than quantum confinement.

Pulsed laser-assisted surface structuring with optical near-field enhanced effects

Abstract: The effects of optical resonance and near field in the interaction of transparent particles on a substrate with laser light have been examined experimentally and theoretically. It is found that pits can be created at the contacting point between the particle and the metallic surface by laser irradiation (KrF,λ=248 nm) with a single pulse. The influence of the particle size and the laser fluence on the structuring of the surface has been investigated. The size of the particle ranges from 1.0 μm to 140 nm in diameter. The morphologies of the holes created have been characterized by an atomic force microscope and a scanning electron microscope. For constant laser fluence, the created hole is sensitive to the particle size. For higher-laser fluence, the corresponding hole becomes larger and deeper. With a low fluence of 300 mJ/cm2 and for 140 nm particles, the lateral dimensions of created pits can be down to 30 nm. With a high fluence of 750 mJ/cm2 and 1.0 μm particles, the diameter and the depth of created holes are about 350 and 100 nm, respectively. Theoretical calculations and an accurate solution of a boundary problem indicate that incident light could excite some resonance modes inside the particle and produce enhanced light intensities on the contacting area (substrate surface). The light intensity on the contacting area is nonuniform and sensitive to the particle size parameter. Experimental results are explained and are very consistent with those of theoretical calculations. The experimental results also provide direct evidence of the optical resonance and near-field effects in the interaction of transparent particles on the substrate with laser light.

Modifications of magnetic properties of Pt/Co/Pt thin layers by focused gallium ion beam irradiation

Abstract: We show how the magnetic properties of the Pt/Co ultrathin film structure can be modified and even controlled under uniform irradiation by Ga+ ions at low fluence in the 20–100 keV range. A systematic magneto-optical study is presented for the Pt/Co(1.4 nm)/Pt(111) ultrathin-film structure. At ion fluences below D=1014 Ga+/cm2, the coercive field is steadily reduced when increasing the fluence. At large fluences, in the range D=(5–10)×1014 Ga+ ions/cm2, the magnetization of the Co layer drops rapidly and the film finally becomes paramagnetic at room temperature for D>2×1015 Ga+ ions/cm2. We demonstrate that these magnetic changes are related to the effect of ion-induced collisional intermixing of the Co/Pt interfaces, leading to the formation of stable Co–Pt alloys with varying composition across the interfaces. A simple model is derived to relate the ion beam-induced mixing to the changes in magnetic properties. The present work allows us to gain a quantitative understanding of previous experiments using a focused Ga+ ion beam to pattern magnetic properties of similar samples at a sub-100 nm scale.

Flash lamp annealing with millisecond pulses for ultra-shallow boron profiles in silicon

Abstract: In this paper we report on recent results obtained from flash lamp annealing (FLA) for the formation of ultra-shallow junctions. Si(1 0 0) wafers were implanted at ultra-low energy (500 eV) with boron to a fluence of 10 15 ions/cm 2 . FLA was carried out at temperatures of 1100 and 1200 °C with a soak time of 20 ms. For comparison conventional rapid thermal annealing (RTA) was performed at 1100 and 1200 °C. The boron diffusion and the dopant activation were investigated by secondary ion mass spectroscopy (SIMS) and spreading resistance profiling (SRP). The activated doses after FLA were as high as 20% of the implanted dose and confined in a layer of 60 nm. The sheet resistances were comparable to those after RTA treatment.

Laser-induced high-quality etching of fused silica using a novel aqueous medium

Abstract: Laser-induced backside wet etching of fused-silica plates using an aqueous solution of naphthalene-1,3,6-trisulfonic acid trisodium salt (Np(SO3Na)3) is reported. A KrF excimer laser was employed as a light source. The etch rate varied greatly with the concentration of the solution and the laser fluence. For lower concentration solutions, the etch rate increased linearly with laser fluence. For highly concentrated solutions, however, the etch rate increased abruptly at higher fluence. Well-defined line-and-space and grid micropatterns were fabricated using a low etch rate. The etched surface was as flat as the surface of the virgin plates and the etched pattern was free of debris and microcracks. The formation and propagation of shockwaves and bubbles in the solution during the etch process were monitored. High pressure, as well as the high temperature generated by the photothermal process, plays a key role in the etching process.

Changes in the optical energy gap and ESR spectra of proton-irradiated unplasticized PVC copolymer and its possible use in radiation dosimetry

Abstract: The changes in the optical absorption spectra of unplasticized poly(vinyl chloride), UPVC films, irradiated with high-energy proton beam in the range 25–37 MeV and at a fluence 2.25×109 ions cm−2 have been studied. However, at the beam energy 25 MeV and with increasing the beam fluence up to 1015 cm−2, the experiment was performed again upon another series of films, where the films became darken. The dependence of the optical absorption on the beam fluence has been investigated in the fluence range of 1011–1015 ions cm−2. The changes in the band structure of the irradiated films were also measured spectrophotometrically using UV/VIS spectrophotometer. The results show that the tail width of the localized states in the band gap was decreased as the beam fluence increased which led to a decrease in the optical energy gap of the irradiated UPVC films. ESR and FTIR measurements for the samples were also undertaken. The results suggest the possible use of UPVC film as a dosimeter for H+ ion beam in the fluence range of this study (1011–1015 ions cm−2) by means of visible spectrophotometry and ESR techniques.

Cooperative photochemical reaction in molecular crystal induced by intense femtosecond laser excitation: Photochromism of spironaphthooxazine

Abstract: Photochromism of indolinospironaphthooxazine (SNO) microcrystalline powder was investigated by steady state and time-resolved diffuse reflectance spectroscopy using a femtosecond laser as a light source. When laser intensity is weak, the photoinduced ring-opening reaction in a picosecond time region and subsequent thermal ring-closure one in a nanosecond scale take place in the crystal, not leading to permanent photocoloration of the powder. The same sample shows photochromism upon intense femtosecond laser excitation. The absorption spectra after laser excitation are similar to those of the photocolored form in solution. The spectral shape is independent of the laser fluence, whereas the yield increases nonlinearly with the fluence. To reveal the photocoloration mechanism, we examined the excitation fluence dependence of the transient absorption spectra and elucidated the photocoloration behavior by femtosecond double pulse excitation varying the delay of the two pulses. These experimental results show t...

Axisymmetric modeling of femtosecond-pulse laser heating on metal films

Abstract: An axisymmetric, dual-hyperbolic, two-temperature model is developed to investigate the thermal response of ultrashort laser pulse interaction with a metal film. The absorbed laser energy is treated as a volumetric heat source, which includes the effect of the ballistic motion of the excited electrons into deeper parts of the material. The temperature-dependent thermophysical properties are incorporated to fully describe the thermal transport in electrons and the lattice. A fourth-order finite-difference algorithm is employed to solve the coupled nonlinear heat conduction equations. Both the electron and lattice temperatures in a gold film irradiated by Gaussian laser beams are computed and presented along the r and z axes, respectively. In addition, the suitability of using a one-dimensional, two-temperature model for predicting the damage threshold fluence is examined.

Effects of polymer properties on laser ablation behaviour

Abstract: An investigation aiming to seek a correlation between ablation rates and various polymer thermal properties, based on experimental ablation data generated for 14 polymers commonly used in microfluidics, is presented. A statistical analysis was carried out for laser fluence against various polymer descriptors and/or their combinations. The results of the analysis show a relatively high correlation coefficient of 0.82 for polymer ablation data when we compare fluence against the product of ablation rate and the difference between the glass transition temperature and room temperature. The effects of polymer properties are also illustrated by an investigation of ablation behaviour of DNQ/novolak thin films, which had been exposed to different levels of UV radiation prior to laser ablation, using atomic force microscopy. The surface characteristics of the thin films following laser irradiation are discussed in terms of differences in laser absorption and the glass transition temperature of the films. The results are consistent with the glass transition temperature being a critical factor affecting laser/polymer interaction.

Fluence correction factors in plastic phantoms for clinical proton beams

Abstract: In recent codes of practice for reference dosimetry in clinical proton beams using ionization chambers, it is recommended to perform the measurement in a water phantom. However, in situations where the positioning accuracy is very critical, it could be more convenient to perform the measurement in a plastic phantom. In proton beams" a similar approach as in electron beams could be applied by introducing fluence correction factors in order to account for the differences in particle fluence distributions at equivalent depths in plastic and water. In this work, fluence correction faciors as a function of depth were determined for proton beams with different energies using the Monte Carlo code PTRAN for PMMA and polystyrene with reference to water. The influence of non-elastic nuclear interaction cross sections was investigated. It was found that differences in proton fluence distributions are almost entirely due to differences in non-elastic nuclear interaction cross sections between the plastic materials and water. For proton beams with energies lower than 100 MeV, for which the contributions from non-elastic interactions become small compared to the total dose, the fluence corrections are smaller than 1%. For beams with energies above 200 MeV, depending on the cross sections dataset for non-elastic nuclear interactions, fluence corrections of 2-5% were found at the largest depths. The results could, with an acceptable accuracy, be represented as, a correction per cm. penetration of the beam, yielding values between 0.06% and 0.15 % per cm for PMMA and 0.06% to 0.20% per cm for polystyrene. Experimental information on these correction factors was obtained from depth dose measurements in PMMA and water. The experiments were performed in 75 MeV and 191 MeV non-modulated and range-modulated proton beams. From the experiments, values ranging from 0.03% to 0.15% per cm were obtained. A decisive answer about which dataset for non-elastic nuclear interactions would result in a better representation of the measurements could not be given. We conclude that below 100 MeV, dosimetry could be performed in plastic phantoms without a dramatic loss of accuracy. On the other hand, in clinical high-energy proton beams, where accurate positioning in water is in general not an issue, substantial correction factors would be required for converting dose measurements in a plastic phantom to absorbed dose to water. It is therefore not advisable to perform absorbed dose measurements nor to measure depth dose distributions in a plastic phantom in high-energy proton beams.

Measuring thermal effects in femtosecond laser-induced breakdown of dielectrics

Abstract: A quartz sample was submitted to femtosecond laser pulses with fluences up to 6×Fc, where Fc is the laser-induced breakdown fluence. A series of microthermocouples, deposited onto the surface, was used to measure heatings at the millisecond scale, at typically 50 μm from the laser impact. By using both the heat propagation equation and the Helmholtz equation to get the energy absorption profile, we estimate a minimum value of the temperature reached in the center of the crater. Even very close to Fc, this temperature is found to be ≳3000 °C, proving that heating effects play a major role in the breakdown of dielectrics.

Fabrication of SiO2-Humps on Silicone Rubber Using F2 Laser

Abstract: The surface of silicone rubber swelled and was modified to a SiO2 glass layer when irradiated by a 157-nm F2 laser. Ten minutes of irradiation of the laser with fluence of 14 mJ/cm2 operating at 20 Hz produced 3-µm-thick SiO2 humps, but the surface was not modified when a 193-nm ArF laser was used. High photon energy of the F2 laser may cause the two phenomena. This is a useful new technique which enables both processing and modification to be carried out at the same time.

Bimodal nanoparticle size distributions produced by laser ablation of microparticles in aerosols

Abstract: Silver nanoparticles were produced by laser ablation of a continuously flowing aerosol of microparticles in nitrogen at varying laser fluences. Transmission electron micrographs were analyzed to determine the effect of laser fluence on the nanoparticle size distribution. These distributions exhibited bimodality with a large number of particles in a mode at small sizes (3–6-nm) and a second, less populated mode at larger sizes (11–16-nm). Both modes shifted to larger sizes with increasing laser fluence, with the small size mode shifting by 35% and the larger size mode by 25% over a fluence range of 0.3–4.2-J/cm2. Size histograms for each mode were found to be well represented by log-normal distributions. The distribution of mass displayed a striking shift from the large to the small size mode with increasing laser fluence. These results are discussed in terms of a model of nanoparticle formation from two distinct laser–solid interactions. Initially, laser vaporization of material from the surface leads to condensation of nanoparticles in the ambient gas. Material evaporation occurs until the plasma breakdown threshold of the microparticles is reached, generating a shock wave that propagates through the remaining material. Rapid condensation of the vapor in the low-pressure region occurs behind the traveling shock wave. Measurement of particle size distributions versus gas pressure in the ablation region, as well as, versus microparticle feedstock size confirmed the assignment of the larger size mode to surface-vaporization and the smaller size mode to shock-formed nanoparticles.