Showing papers on "Fluence published in 2001"

Ultrashort-pulse laser ablation of indium phosphide in air

Abstract: Ablation of indium phosphide wafers in air was performed with low repetition rate ultrashort laser pulses (130 fs, 10 Hz) of 800 nm wavelength. The relationships between the dimensions of the craters and the ablation parameters were analyzed. The ablation threshold fluence depends on the number of pulses applied to the same spot. The single-pulse ablation threshold value was estimated to be φth(1)=0.16 J/cm2. The dependence of the threshold fluence on the number of laser pulses indicates an incubation effect. Morphological and chemical changes of the ablated regions were characterized by means of scanning electron microscopy and Auger electron spectroscopy.

Preparation of metal colloids by a laser ablation technique in solution: influence of laser wavelength on the ablation efficiency (II)

Abstract: Laser ablations of silver and copper targets in water were performed to prepare nanosize metal colloids. The influence of the laser wavelength, focusing conditions, and laser fluence on the ablation efficiency was studied. The relation between the ablation efficiency and wavelength varied with laser fluence. The ablation efficiency at shorter wavelengths was higher at low fluence, while the ablation efficiency at longer wavelengths was higher at high fluence. These findings were discussed in terms of the “intra-pulse” and “inter-pulse” self-absorptions of the incident laser light by colloidal particles.

Formation of conical microstructures upon laser evaporation of solids

Abstract: The formation and development of the large-scale periodic structures on a single crystal Si surface are studied upon its evaporation by pulsed radiation of a copper vapor laser (wavelength of 510.6 nm, pulse duration of 20 ns). The development of structures occurs at a high number of laser shots (∼104) at laser fluence of 1–2 J/cm2 below optical breakdown in a wide pressure range of surrounding atmosphere from 1 to 105 Pa. The structures are cones with angles of 25, which grow towards the laser beam and protrude above the initial surface for 20–30 μm. It is suggested that the spatial period of the structures (10–20 μm) is determined by the capillary waves period on the molten surface. The X-ray diffractometry reveals that the modified area of the Si substrate has a polycrystalline structure and consists of Si nanoparticles with a size of 40–70 nm, depending on the pressure of surrounding gas. Similar structures are also observed on Ge and Ti.

Ultrafast carrier relaxation in radiation-damaged silicon on sapphire studied by optical-pump–terahertz-probe experiments

Abstract: We investigate the relaxation of photogenerated carriers in radiation-damaged silicon on sapphire using a collinear optical-pump–terahertz-probe arrangement. Carrier densities greater than 1020 cm−3 are injected using 400 nm, 100 fs pump pulses, and the change in transmission of the terahertz-probe pulse is measured as a function of pump–probe delay. The time-dependent carrier density is deduced using a thin-film Drude model. A carrier mobility of 422±17 cm2/V s is measured, and single-exponential carrier relaxation times of 4 ps at low fluence and 6 ps at high fluence are observed.

Depth profiling of multi-layer samples using femtosecond laser ablation

Abstract: An all solid-state femtosecond laser (λ0 ∼775 nm, pulse duration ∼170 fs, maximum pulse energy ∼0.5 mJ) with a Gaussian beam profile was used for depth profiling of Cu–Ag and TiN–TiAlN multi-layers on silicon and iron substrates. Laser-induced breakdown spectroscopy (LIBS) in argon was used for characterisation of the Cu–Ag samples, while laser ablation in a vacuum with time-of-flight mass spectrometry (TOF-MS) was applied for the characterisation of the TiN–TiAlN samples. The thickness of the individual Cu and Ag layers was 600 nm. Each individual TiN and TiAlN layer was 280 nm thick. The LIBS experiment was performed in the pressure range 10–1000 mbar. Variation of the pulse fluence from 0.8 to 1.5 J cm−2 caused a change of the ablation rate from 15 to 30 nm per pulse. The first layers of Cu and Ag could be satisfactorily resolved by LIBS. In femtosecond laser ablation TOF-MS a lower fluence (about 0.3 J cm−2) than in LIBS could be applied. The TiN–TiAlN multi-structures were well resolved. The Gaussian-type beam of the femtosecond laser limited the contrast of the detected depth profiles in both schemes. The complementary sensing techniques enable study of technical and physical limitations in the use of femtosecond laser ablation.

Optical limitation induced by gold clusters: Mechanism and efficiency

Abstract: The optical limiting effect induced by gold clusters was measured as a function of excitation wavelength. The limiting effect is most efficient below 530 nm, and decreases towards the red. Two different behaviors are seen in the time-resolved signals in the nanosecond and picosecond ranges. These behaviors have different time delays for the amplitude maxima and for the fluence thresholds where nonlinear effects are observed. This suggests that two types of scattering centers are responsible for the optical limitation. The fast mechanism, which reaches a maximum amplitude in less than 1 ns, occurs at relatively high fluence and for short pulses and is assigned to the vaporization of metal particles. It is more pronounced for large clusters where the absorbed light energy is primarily highly confined. The slow mechanism, which develops in a few nanoseconds, is assigned to the energy transfer from the gold particles to the surrounding solvent and to the formation of solvent bubbles. At lower fluence and for smaller size clusters, only the second mechanism is observed in the nanosecond range, because the efficient dissipation of energy from the small clusters to the solvent, which produces bubbles, precludes metal-particle vaporization.

Growth of Laser Initiated Damage in Fused Silica at 527 nm

Abstract: The effective lifetime of optics is limited by both laser-induced damage and the subsequent growth of laser initiated damage sites. We have measured the growth rate of laser-induced damage in fused silica in both air and vacuum at 527 nm. For damage on the exit surface, the data shows exponential growth in the lateral size of the damage site with shot number. The exponential growth coefficient depends linearly on the laser fluence. The behavior at the fluence threshold for growth is contrasted to that observed at 351 nm. The growth rate was not significantly affected by either the wavelength of the initiating fluence or the presence of 10 torr of air as compared to vacuum. When the damage is located on the input surface, it has both a higher threshold for growth and does not grow exponentially.

Effect of Hydrogen on the Photoluminescence of Si Nanocrystals Embedded in a SiO 2 Matrix

Abstract: Hydrogen passivation of Si nanocrystals is shown to result in a redshift of photoluminescence (PL) emission spectra, as well as the more commonly observed intensity increase. The shift is reversible, with spectra returning to their unpassivated values as hydrogen is removed from the samples by annealing. The magnitude of the redshift also depends on the implant fluence employed for nanocrystal synthesis, increasing with increasing fluence or particle size. These data are shown to be consistent with a model in which larger crystallites are assumed to contain a greater number of nonradiative defects, i.e., the number of nonradiative defects is assumed to scale with the surface area or volume of a nanocrystal. Hydrogen passivation then results in a disproportionate increase in emission from larger crystallites, giving rise to an apparent redshift in the composite PL emission spectrum.

Time-resolved laser-induced incandescence and laser elastic-scattering measurements in a propane diffusion flame

Abstract: Laser-induced incandescence (LII) and laser elastic-scattering measurements have been obtained with subnanosecond time resolution from a propane diffusion flame. Results show that the peak and time-integrated values of the LII signal increase with increasing laser fluence to maxima at the time of the onset of significant vaporization, beyond which they both decrease rapidly with further increases in fluence. This latter behavior for the time-integrated value is known to be characteristic for a laser beam with a rectangular spatial profile and is attributed to soot mass loss from vaporization. However, there is no apparent explanation for the corresponding large decrease in the peak value. Analysis shows that the peak value occurs at the time in the laser pulse when the time-integrated fluence reaches approximately 0.2 J/cm2 and that the magnitude of the peak value is strongly dependent on the rate of energy deposition. One possible explanation for this behavior is that, at high laser fluences, a cascade ionization phenomenon leads to the formation of an absorptive plasma that strongly perturbs the LII process.

Energy-dependent anisotropic deformation of colloidal silica particles under MeV Au irradiation

Abstract: Spherical silica colloids with a diameter of 1.0 μm, made by wet chemical synthesis, were irradiated with 2–16 MeV Au ions at fluences in the range (2–11)×1014 cm−2. The irradiation induces an anisotropic plastic deformation turning the spherical colloids into ellipsoidal oblates. After 16 MeV Au irradiation to a fluence of 11×1014 cm−2, a size aspect ratio of 5.0 was achieved. The size polydispersity (∼3%) remains unaffected by the irradiation. The transverse diameter increases exponentially with ion fluence. By performing measurements as a function of ion energy at a fixed fluence, it is concluded that the transverse diameter increases linearly with the average electronic energy loss above a threshold value of ∼0.6 keV/nm. Nonellipsoidal colloids are observed in the case where the projected ion range is smaller than the colloid diameter. The data provide strong support for the thermal spike model of anisotropic deformation.

Modification of Co/Pt multilayers by gallium irradiation - Part 2: The effect of patterning using a highly focused ion beam

Abstract: The local and collective behavior of magnetic arrays fabricated by focused ion beam (FIB) patterning of a Co/Pt multilayer is described. The arrays comprised 1 μm nonirradiated square elements separated by narrow lines which were written using the FIB. While the square elements supported perpendicular magnetization, the ion fluence used to write the lines was chosen to make the local magnetization there lie in-plane. Lorentz microscopy showed that lines were approximately 60 nm wide and that the magnetization had the expected orientation. Application of fields perpendicular and parallel to the array showed that the magnetization in the square elements and in the lines could be controlled essentially independently of each other. Magneto-optic microscopy was used to study the behavior of the arrays as a whole. Frustrated checkerboard patterns were observed, whose detailed properties depended to an extent on the fluence used to write the lines.

Degradation of the optical properties of ZnO-based thermal control coatings in simulated space environment

Abstract: Optical absorption and photoluminescence spectroscopy have been used to investigate the physical changes induced by proton and electron irradiation in selected thermal control coatings. This study focused on a white paint and on its two components, a polydimethylsiloxane resin and zinc oxide powder. Samples were irradiated by either 45 keV protons (fluence up to 1016 protons cm-2) or 400 keV electrons (fluence up to 6×1015 electrons cm-2). In situ reflectance measurements were made during the test and were complemented after air introduction by photoluminescence characterization. The optical properties of the paint are essentially those of the ZnO pigment. The optical degradation of the material appears to be correlated with the density of defects created by ionization in a zone close to the surface of the paint and called the optical thickness of the material. Two degradation regions of the reflectance properties in the wavelength range 250-2500 nm have been identified: one in the infrared and one in the visible blue region. The degradation in the IR region disappears on exposure to air so that no complementary technique could be used for identifying its origin. The point defects responsible for the optical degradation of the paint in the blue region are singly ionized oxygen vacancies (F+ centres) either initially present in the material or induced by irradiation. Irradiation quenches the green photoluminescence emission. The latter would be excited by the recombination of doubly ionized oxygen vacancies with photoformed electrons of the conduction band.

Physical and chemical changes induced by 70 MeV carbon ions in polyvinylidene difluoride (PVDF) polymer

Abstract: Physical and chemical changes induced by 70 MeV carbon ions ( 12 C 5+ ) have been investigated in bulk polyvinylidene fluoride (PVDF) polymer. The induced changes have been studied with respect to their optical, chemical and structural response using UV-visible, FTIR and XRD techniques. The ion fluences ranging from 2.5×1011 to 9×10 13 ions cm −2 have been used to study the irradiation effects. It has been observed that at the fluence of 9×10 13 ions cm −2 the PVDF sample became brittle and practically it was not possible to handle it for any further measurements. The recorded UV-visible spectra show that the optical absorption increases with increasing fluence, indicating maximum absorption at 200 nm. An interesting feature of UV-visible spectra is that dips change into peaks and vice versa with increase of fluence. In the FTIR spectra, development of new peaks at 1714 and 3692 cm −1 along with disappearance of peaks at 2363 and 3025 cm −1 and shifting of peak at 2984–2974 cm−1 have been observed due to high energy irradiation, indicating the chemical changes induced by 12 C 5+ . The diffraction pattern of PVDF indicates that this polymer is semi-crystalline in nature; a large decrease in the diffraction intensity indicates decrease in crystallinity. Increase in crystallite size has also been observed due to heavy ion irradiation.

Photothermal conversion dynamics in femtosecond and picosecond discrete laser etching of Cu-phthalocyanine amorphous film analysed by ultrafast UV-VIS absorption spectroscopy

Abstract: Novel etching of Cu-phthalocyanine (CuPc) amorphous film which is characteristic of ultrashort laser irradiation was successfully confirmed by tuning Ti:Sapphire laser (780 nm) to 150 fs, 250 ps, or 100 ns, and the primary processes were investigated by fs pump-fs probe and ps pump-fs probe spectroscopic measurements. In the fs and ps laser ablation, we have found discrete laser etching in that the etch depth becomes constant and is independent of laser fluence above the ablation threshold, although gradual (normal) etching, in which the etch depth increase continuously with the fluence above ablation threshold, was observed in the ns laser ablation. The transient absorption spectral measurements reveal the nonlinear photothermal conversion processes, corresponding to exciton–exciton annihilation and cyclic multiphotonic absorption. Their time evolutions during and after the excitation pulse duration were considered and elucidated to depend strongly on the excitation pulse width. On the basis of these results, we discuss an ablation mechanism for the ps and fs ablation that the temperature elevation bringing about transient high pressure is responsible for discrete etching.

Ion-assisted deposition of MoSx films from laser-generated plume under pulsed electric field

Abstract: The thickness profiles and compositional distributions of MoSx films deposited from a plume generated by pulsed laser irradiation of the MoS2 target were investigated at a varying fluence and constant laser pulse energy. It was shown that films with stoichiometric composition were formed at sufficiently low fluence (near the ionization threshold), and increasing fluence caused intricate nonmonotonic variations of the compositional distribution. A substantial deviation of the film composition from stoichiometric and a significant radial gradient of the sulfur concentration over the substrate surface (1

Universal dynamics during and after ultrafast laser-induced semiconductor-to-metal transitions

Abstract: We observe common features in semiconductor-to-metal transitions induced by femtosecond laser pulses in crystalline GaAs, amorphous GaAs, and Sb-rich films of amorphous GeSb, by tracking ultrafast changes in the spectral dielectric function The dielectric function of the metal-like state reveals a decay in the plasma frequency with time after the transition In addition, the plasma frequency roughly decreases with increasing excitation fluence

Recrystallization in polyvinylidene fluoride upon low fluence swift heavy ion impact

Abstract: Thin films (9 μm) of polyvinylidene fluoride (PVDF) are irradiated by swift heavy ions (180 MeV Ag14+) in the fluence range 1×1010–1×1012ions/cm2 with an electronic linear energy transfer LET∼11 keV/nm. In sharp contrast to the previous results, the most characteristic crystalline asymmetric and symmetric “CH2” doublets (located at 3025 and 2985 cm−1), have shown remarkable increase in their respective Fourier transform infrared (FTIR) absorbance intensities upon low fluence ion impact (1010 ions/cm2). This increase in absorbance is in consonance with the simultaneous decrease of the transmission intensities of other crystalline bending vibration bands located at 532 (CF2 bending), 614, 796, and 975 cm−1 (all due to CH2 bending) at the similar ion fluence. It appears most probable from the results that, being a polar polymer, the molecular dipoles in PVDF forming a hydrogen bond network get realigned upon irradiation into a highly ordered state of chain molecules in the crystalline regions and create volu...

Spectral properties of proton irradiated gallium nitride blue diodes

Abstract: The permanent damage induced by 2 MeV proton irradiation at room temperature is reported for gallium nitride based blue emitting diodes (CREE model C430-DH85). Both optical and electrical device characteristics were measured. The I-V dependence was obtained as a function of temperature. At low voltages, the current is proportional to the exponential of the voltage at a constant temperature and the slope of the I-V curve is independent of temperature for the range 75-350 K, confirming the tunneling mechanism of the carrier injection. The room-temperature curve was studied as a function of 2-MeV proton irradiation in the fluence range 10/sup 11/ to 10/sup 15/ cm/sup -2/. It is hardly affected up to a fluence of 3/spl times/10/sup 12/ cm/sup -2/. Higher fluences do not affect the tunneling mechanism, but proton irradiation affects the saturation value of the current. The integrated electroluminescence versus voltage curves were obtained as a function of fluence, but the results were not amenable to a degradation constant interpretation. To gain insight into the degradation mechanism, the electroluminescence was analyzed spectrally and found to be the sum of the band-to-band transition in blue color at /spl ap/430 nm and a parasitic yellow band. The contribution of each transition was determined. The ratio of the contributions depends on driving current, temperature, and fluence. Treated individually, both the band-to-band and the yellow transition are related to fluence. The 2-MeV proton radiation damage constant is (7/spl plusmn/1)/spl times/10/sup -14/ cm/sup -2/ for the band-to-band and (2.0/spl plusmn/0.4)/spl times/10/sup -14/ cm/sup -2/ for the yellow transitions. The degradation of space charge recombination and diffusion of minority carriers cause the degradation of the electroluminescence. GaN light-emitting diodes (LEDs) are about two orders of magnitude more resistant to 2-MeV proton irradiation than GaAs LEDs.

Crystalline to amorphous transition and band structure evolution in ion-damaged silicon studied by spectroscopic ellipsometry

Abstract: Crystalline to amorphous transition and subsequent microstructural evolution in silicon induced by Ar+-ion implantation over a wide range of ion fluences (6×1013–1×1017 cm−2) have been investigated by spectroscopic ellipsometry. In the evaluation of the optical and microstructural properties of the damaged layer, the contribution of the surface overlayer to the measured dielectric spectra was separated by fitting a multilayer model with an effective medium approximation. The best fit to the dielectric spectra for disordered silicon could be obtained by taking our highest-fluence implanted (fluence=1×1017 ions/cm2) amorphous silicon (a-Si) data as reference data instead of a-Si data available in the handbook. The derivative spectra as a function of fluence show a distinct and sharp transition from the crystalline to amorphous phase. The threshold fluence for this transition is derived from fitting. Evaluation of standard sum rules and optical moments for imaginary part of the pseudodielectric function reveals no substantial change in various physical parameters below the transition indicating their insensitivity to point defects, while it shows a large change with fluence above the threshold for amorphization. The disorder induced changes in the effective dielectric constant, number of valence electrons per atom participating in optical transition, Penn gap energy, average bond length, coordination number, effective dispersion oscillator energy, an average strength of the interband optical transition with fluence is discussed on the basis of microstructural evolution and corresponding band structure modification. It is also shown that the dielectric functions of damaged silicon are well represented by a sum of six classical Lorentz oscillators. With increasing fluences, each of the oscillator amplitude decreases and linewidth increases except for the 3.3 eV transition which shows increasing amplitude with fluence. These results are discussed in the context of short-range order/disorder and effective band gap reduction along with flattening of the bands with increasing fluence above the amorphization threshold.

Ablation of aluminum thin films by ultrashort laser pulses

Abstract: In this study, various results are presented for laser ablation experiments on aluminum and silicon, made in ambient air by means of subpicosecond laser pulses. These results include threshold fluences for plasma formation and for the appearance of various spectral lines, and the single shot fluence required to remove aluminum layers of various specific thicknesses (ranging from 10 to 500 nm) deposited on a silicon substrate. The threshold fluence for plasma formation is of the order of 0.1 J/cm2. Threshold fluences for the appearance of the spectral lines considered vary from 0.1 to about 5 J/cm2. Finally, our results suggest that for high fluences, even for ultrashort laser pulses, the ablation depth is essentially determined by a long-range process, such as thermal conduction in the solid, rather than by the short range optical depth.

Polyimide surface modification by pulsed ultraviolet laser irradiation with low fluence

Abstract: Surface modification on a polyimide film by pulsed ultraviolet (UV) laser irradiation with a fluence below its ablation threshold was studied by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and UV-vis spectroscopy. It was observed that a photochemical reaction occurred and hydrophilic groups, such as —OH and —COOH, formed on the polyimide surface after irradiation. In addition, a ripple microstructure formed on the surface when the angle of incidence of the laser beam was 20–50o. The contact angle of the polyimide surface with water decreased and the adsorption ability of the surface to a water-soluble dye clearly increased after laser irradiation. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2739–2743, 2001

High fluence ion beam modification of polymer surfaces: EPR and XPS studies

Abstract: Polyethylene, polyamide-6 and polyimide foils implanted with 100 keV B + , P + and Sb+ ions to a fluence range of 10 15 –10 17 cm −2 have been studied using the electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) methods. The experimental data allow the comparison of the implantation-induced changes both in a given polymer foil under different ion beam regimes and in different polymers under similar ion-bombardment conditions. The high fluence implantation of boron ions, depositing energy mainly via electronic stopping, was found to be accompanied with the effective formation of π-bonded carbon-rich clusters. By contrast, heavier (phosphorus and antimony) ions, which deposit energy predominantly in nuclear collisions, produced a lower concentration of π-radicals and a less carbonised top surface layer. The peculiarities and main trends of the alterations of the polymer structure and composition induced via electronic and nuclear stopping have also been discussed.