Showing papers on "Fluence published in 2006"

Space charge effects in photoemission with a low repetition, high intensity femtosecond laser source

Abstract: In this paper, we present experimental results on the effect of space charging in photoelectron spectroscopy from a surface using a pulsed and intense femtosecond light source. We particularly focus on a quantitative evaluation of the induced spectral broadening. Our results are compared with analytic calculations based on energy conservation considerations as well as with experimental results from measurements using picosecond pulses for the excitation process. As a measure of space charge effects, we monitored the angular and energy distributions of the photoemission from the occupied Shockley surface state of Cu(111) as a function of the total number N of the photoemitted electrons per laser pulse. Our results show that spectral distortions exist for the entire laser fluence regime probed. The energetic broadening of the surface state peak can be fitted with remarkable accuracy by a N dependence, in agreement with the theoretical predictions and different from the experimental picosecond results, where...

Morphology and microstructure in fused silica induced by high fluence ultraviolet 3ω (355 nm) laser pulses

Abstract: The morphology and microstructure induced in high quality fused silica by UV (355 nm) laser pulses at high fluence (10–45 J/cm 2 ) have been investigated using a suite of microscopic and spectroscopic tools. The laser beam has a near-Gaussian profile with a 1/e 2 diameter of� 0.98 mm at the sample plane and a pulse length FWHM (full width at half maximum) of 7.5 ns. The damage craters consist of a molten core region (thermal explosion), surrounded by a near concentric region of fractured material. The latter arises from propagation of lateral cracks induced by the laser-generated shock waves, which also compact the crater wall,� 10 lm thick and� 20% higher in density. The size of the damage crater varies with laser fluence, number of pulses, and laser irradiation history. In the compaction layer, there is no detectable change in the Si/O stoichiometry to within ±1.6% and no crystalline nano-particles of Si were observed. Micro(1–10 lm) and nano- (20–200 nm) cracks are found, however. A lower valence Si 3+ species on the top 2–3 nm of the compaction layer is evident from the Si 2p XPS. The results are used to construct a physical model of the damage crater and to gain critical insight into laser damage process. � 2005 Elsevier B.V. All rights reserved. PACS: 61.80.Ba; 61.72.Ji; 78.60.Hk; 76.30.Mi

Laser ablation of a platinum target in water. I. Ablation mechanisms

Abstract: This is the first in a series of three papers aimed at better understanding the processes that lead to nanomaterial formation during laser ablation of solid targets in liquids. Here we study the variation of the target surface morphology versus laser fluence and wavelength in order to suggest an ablation mechanism. A key finding is that an explosive ablation mechanism is prominent for a wide range of laser fluences for all wavelengths tested. Interestingly, however, ultraviolet (355nm) and infrared (1064nm) wavelengths show characteristically different explosive behaviors. In the infrared case, numerous large craters with diameters around 20μm form at localized points within the laser irradiated area. In contrast, ultraviolet ablation results in a striking transition to nanoscale surface roughness across the entire irradiated area. This texture is attributed to spinodal decomposition at the molten target surface. We propose that the wavelength and fluence dependence of the ablation craters can be explaine...

Controlled anisotropic deformation of Ag nanoparticles by Si ion irradiation

Abstract: The shape and alignment of silver nanoparticles embedded in a glass matrix is controlled using silicon ion irradiation. Symmetric silver nanoparticles are transformed into anisotropic particles whose larger axis is along the ion beam. Upon irradiation, the surface plasmon resonance of symmetric particles splits into two resonances whose separation depends on the fluence of the ion irradiation. Simulations of the optical absorbance unambiguously show that the anisotropy is caused by the deformation and alignment of the nanoparticles, and that both properties are controlled with the irradiation fluence.

Performance characteristics of ultra-violet femtosecond laser ablation inductively coupled plasma mass spectrometry at ∼265 and ∼200 nm

Abstract: The analytical figures of merit of ultra-violet femtosecond laser ablation inductively coupled plasma mass spectrometry (UV-fs-LA-ICP-MS) using the 3rd and 4th harmonics of Ti:Sapphire (∼265 and ∼200 nm, respectively) were explored. For this purpose, elemental ratios of aerosols produced by LA of silicate glass (SRM NIST 610) were studied under varying fluence conditions ranging from moderate values of 2 J cm−2 up to 30 J cm−2, taking into account e.g. laser-induced (66Zn/65Cu) and particle size-related (238U/232Th) phenomena. It could, for instance, be shown that signal ratios were less dependent on the wavelength or laser repetition rate chosen. Furthermore, fractionation indices defined using the temporal drift of elemental ratios over two equal parts of the acquired signal were subject to systematic changes for threshold-close fluences. As a consequence, corresponding 42Ca-normalized values were found to deviate by more than 20% from unity. In contrast, LA at higher fluences resulted in less pronounced discrepancies, falling below 5% even for the most critical elements such as 66Zn, 111Cd, and 208Pb. The complete suppression of particle size-related fractionation quantified on the basis of the 238U/232Th-system turned out to be highly consistent with the absence of μ-sized particles which were measured by optical particle counting (OPC). The relative fraction of particles >0.5 μm was determined to be less than 5%, independent on the wavelength, fluence, or laser repetition rate chosen. Moreover, our results indicate the occurrence of ICP-induced elemental fractionation during analysis due to an increased mass loading of the plasma source if medium or high fluences are applied. Using the 66Zn/65Cu-ratio as a thermometric probe, the change in plasma ionization temperature among low and high mass loading conditions was estimated to be −900 K. Nevertheless, UV-fs-LA-ICP-MS analysis of different matrices (silicate glass SRM NIST 610 and brass (Zn ∼20%)) performed within the high fluence range was found to enhance the accuracy for non-matrix-matched calibration. Evidence is given that the enhancement observed mainly depends upon the suppression of laser- and/or transport-induced fractionation.

SHI induced modification of ZnO thin film: Optical and structural studies

Abstract: Thermally evaporated ZnO thin films have been irradiated with 100 MeV Au8+ ions at different fluence from 5 × 1011 ions/cm2 to 5 × 1013 ions/cm2. The optical and structural properties of the irradiated and pristine films were studied using Fourier transform infrared spectroscopy (FTIR), UV–visible absorption spectroscopy, photoluminescence (PL), atomic force microscopy (AFM) and XRD. FTIR results showed that for low fluence, the transmittance of the film decreased and increased again at higher fluence but Zn–O bond remains unaffected by irradiation. As revealed from the absorption spectra, absorption edge is not changed by the irradiation but the optical absorption is increased. The AFM study of the films implied that roughness decreased at low fluence values up to 5 × 1012 ions/cm2 and at higher fluences the roughness increased.

Efficiency of silicon micromachining by femtosecond laser pulses in ambient air

Abstract: Femtosecond lasers have proven to be effective tools for precise micromachining. Taking advantage of the reduced heat diffusion and the sharp ablation threshold at comparatively low energy densities, subdiffraction limit sized craters have been machined on silicon wafers by single near infrared Ti:sapphire laser pulses using a high numerical aperture objective lens. Two different ablation regimes have been identified by varying the laser fluence. While two-photon absorption dominates in the low fluence regime, electronic diffusion is a major energy transport mechanism at higher laser fluences. Time-resolved pump-and-probe side-view imaging has been performed to investigate the energy coupling to the target specimen over a wide range of fluences (up to around 1000J∕cm2) at lateral beam dimensions of the order of micrometers. The decrease of the ablation efficiency in the high fluence regime (>10J∕cm2) is attributed to the strong interaction of the laser pulse with the laser-induced plasma.

Ultraviolet-infrared femtosecond laser-induced damage in fused silica and CaF2 crystals

Abstract: The damage in fused silica and CaF2 crystals induced by wavelength tunable femtosecond lasers is studied. The threshold fluence is observed to increase rapidly with laser wavelength lambda in the region of 250-800 nm, while it is nearly a constant for 800

Electronic excitation effects in CeO2 under irradiations with high-energy ions of typical fission products

Abstract: In order to understand the formation mechanism of a crystallographic re-structuring in the periphery region of high-burnup nuclear fuel pellets, named as “rim structure”, information on the accumulation process of radiation damage and fission products (FPs), as well as high-density electronic excitation effects by FPs, are needed In order to separate each of these processes and understand the high-density electronic excitation effects, 70–210 MeV FP ion (Xe 10–14+ , I 7+ and Zr 9+ ) irradiation studies on CeO 2 , as a simulation of fluorite ceramics of UO 2 , have been done at a tandem accelerator of JAEA-Tokai and the microstructure changes were determined by transmission electron microscope (TEM) Measurements of the diameter of ion tracks, which are caused by high-density electronic excitation, have clarified that the effective area of electronic excitation by high-energy fission products is around 5–7 nm ϕ and the square of the track diameter tends to follow linear function of the electronic stopping power ( S e ) Prominent changes are hardly observed in the microstructure up to 400 °C After overlapping of ion tracks, the elliptical deformation of diffraction spots is observed, but the diffraction spots are maintained at higher fluence These results indicate that the structure of CeO 2 is still crystalline and not amorphous Under ion tracks overlapping heavily (>1 × 10 15 ions/cm 2 ), surface roughness, with characteristic size of the roughness around 1 μm, is observed and similar surface roughness has also been observed in light-water reactor (LWR) fuels

Techniques for qualitative and quantitative measurement of aspects of laser-induced damage important for laser beam propagation

Abstract: Characterizing laser-induced damage in optical materials is important for laser design and operation. Previous methods of evaluating optical materials damage resistance to high-power laser irradiation have typically suffered from shot-to-shot uncertainties in laser energy output and/or have insufficient sensitivity. More importantly, such methods do not address the aspects of laser-induced damage important to laser beam propagation, namely the amount of light scattered by the damage. We present a method for the quantitative correlation of material modification on the surface or in the bulk of optical materials to laser parameters, which deconvolutes the effects of laser output instability. In image analysis, two images, one a fluence spatial profile and the other a visible light scatter image of the damage, are directly compared to extract scatter as a function of fluence. An automated microscope is used to record the location and number of bulk damage sites and determine a calibration factor between the scatter signal observed and damage density. We illustrate the method with a determination of both bulk damage density as a function of laser fluence and of a representative size distribution in a DKDP crystal. Our method is capable of determining damage densities with an absolute uncertainty of ±0.3 pinpoint damage sites per cubic millimetre (pp mm−3) in the range 1–100 pp mm−3 with our minimum detectable density being 0.01 pp mm−3. We also determined the pps produced by laser pulses of 351 nm, 3 ns light to have a mean diameter of 5.5 ± 2.5 µm (1/e2).

Ion beam enhanced etching of LiNbO3

Abstract: Single crystals of z - and x -cut LiNbO 3 were irradiated at room temperature and 15 K using He + - and Ar + -ions with energies of 40 and 350 keV and ion fluences between 5 × 10 12 and 5 × 10 16 cm −2 . The damage formation investigated with Rutherford backscattering spectrometry (RBS) channeling analysis depends on the irradiation temperature as well as the ion species. For instance, He + -irradiation of z -cut material at 300 K provokes complete amorphization at 2.0 dpa (displacements per target atom). In contrast, 0.4 dpa is sufficient to amorphize the LiNbO 3 in the case of Ar + -irradiation. Irradiation at 15 K reduces the number of displacements per atom necessary for amorphization. To study the etching behavior, 400 nm thick amorphous layers were generated via multiple irradiation with He + - and Ar + -ions of different energies and fluences. Etching was performed in a 3.6% hydrofluoric (HF) solution at 40 °C. Although the etching rate of the perfect crystal is negligible, that of the amorphized regions amounts to 80 nm min −1 . The influence of the ion species, the fluence, the irradiation temperature and subsequent thermal treatment on damage and etching of LiNbO 3 are discussed.

Erosion of Kapton ® H by Hyperthermal Atomic Oxygen

Abstract: Organic polymers are subject to erosion from ambient atomic oxygen in low Earth orbit. The linearity of the O-atom fluence dependence of Kapton ® H erosion and the dependence of Kapton H erosion yield on surface temperature have been investigated. Sample exposures were performed with a pulsed beam containing hyperthermal O atoms that were generated with a laser detonation source. After exposure, samples were removed from the chamber in which the exposures were performed, and postexposure analyses were performed: etch depth (profilometry) and surface topography (atomic force microscopy). A systematic set of exposures, which eroded room-temperature Kapton H from 1.4 to 25 μm, showed that the erosion yield of Kapton H is linearly dependent on O-atom fluence. This result helps validate the use of Kapton H mass loss (or erosion depth) as a linear measure of the O-atom fluence of a materials exposure. The erosion of Kapton H was strongly temperature dependent. At lower temperatures (<100◦C), the erosion yield appeared to be independent of sample temperature. However, above 100◦C, the erosion yield exhibited an Arrhenius-like temperature dependence, with an apparent activation energy of 0.31 eV. These observations suggest that O-atom-induced erosion of Kapton H proceeds through direct, nonthermal, gas-surface reactions and through reactions that depend on surface temperature.

Evolution of the Electron Yield Curves of Insulators as a Function of Impinging Electron Fluence and Energy

Abstract: Electron emission and concomitant charge accumulation near the surface of insulators is central to understanding spacecraft charging. A study of changes in electron emission yields as a result of internal charge buildup due to electron dose is presented. Evolution of total, backscattered, and secondary yield results over a broad range of incident energies are presented for two representative insulators, Kapton and Al2O3. Reliable yield curves for uncharged insulators are measured, and quantifiable changes in yields are observed due to <100-fC/mm2 fluences. Excellent agreement with a phenomenological argument based on insulator charging predicted by the yield curve is found; this includes a decrease in the rate of change of the yield as incident energies approach the crossover energies and as accumulated internal charge reduces the landing energy to asymptotically approach a steady state surface charge and unity yield. It is also found that the exponential decay of yield curves with fluence exhibit an energy-dependent decay constant alpha(E). Finally, physics-based models for this energy dependence are discussed. Understanding fluence and energy dependence of these charging processes requires knowledge of how charge is deposited within the insulator, the mechanisms for charge trapping and transport within the insulator, and how the profile of trapped charge affects the transport and emission of charges from insulators

Transmission electron microscopy investigation of the structural damage formed in GaN by medium range energy rare earth ion implantation

Abstract: The crystallographic nature of the damage created in GaN by 300keV rare earth ions has been investigated following implantation at room temperature by varying the fluence of Er, Eu, or Tm from 7×1013to2×1016at.∕cm2. There is a build up of point defects clusters, which increases in density and depth versus the ion fluence. When a threshold around 3×1015at.∕cm2 is reached, a nanocrystalline surface layer is observed. From the lowest fluence, we point out the formation of basal stacking faults, with a majority of I1. Their density also increases with the fluence, but it is seen to saturate at the onset of the observation of the surface nanocrystalline layer. Extrinsic E faults bounded by the c∕2 partials have also been identified; however, most of the E stacking faults transform to I1 which are noticed to fold easily from basal to prismatic planes by switching or not to the Drum atomic configuration.

Ion-irradiation-induced phase transformation in rare earth sesquioxides (Dy2O3,Er2O3,Lu2O3)

Abstract: Polycrystalline pellets of cubic C-type rare earth structure (Ia3¯) Dy2O3, Er2O3, and Lu2O3 were irradiated at cryogenic temperature (120K) with 300keV Kr++ ions to a maximum fluence of 1×1020Kr∕m2. Irradiated specimens were examined using grazing incidence x-ray diffraction and transmission electron microscopy. Ion irradiation leads to different radiation effects in these three materials. First, Dy2O3 begins to transform to a monoclinic B-type rare earth structure (C2∕m) at a peak dose of ∼5 displacements per atom (dpa), (corresponding to a fluence of 2×1019Kr∕m2). This transformation is nearly complete at a peak dose of 25 dpa (a fluence of 1×1020Kr∕m2). Er2O3 also transforms to the B-type structure, but the transformation starts at a higher irradiation dose of about 15–20 dpa [a fluence of about (6–8)×1019Kr∕m2]. Lu2O3 was found to maintain the C-type structure even at the highest irradiation dose of 25 dpa (a fluence of 1×1020Kr∕m2). No C-to-B transformation was observed in Lu2O3. The irradiation dose...

Study of damage in ion-irradiated α-SiC by optical spectroscopy.

Abstract: UV–visible absorption and Raman scattering spectroscopy were used to investigate the effects of 4 MeV Xe-ion and 4 MeV Au-ion irradiations on α-SiC single crystals The evolution of transmission spectra upon irradiation evidences an increase of the optical absorption The optical band-gap energy decreases versus fluence, which is linked to band-gap closure attributed to the creation of localized states into the forbidden energy band A strong effect of the irradiation temperature is observed as a result of dynamic annealing enhanced by the temperature increase The Urbach energy increases versus fluence due to disorder accumulation in the damaged layer Comparison of Urbach energy and disorder parameters extracted from Raman spectra shows that the Urbach energy is sensitive to the disorder induced by the accumulation of point defects

Etching and structural changes of polystyrene films during plasma immersion ion implantation from argon plasma

Abstract: Polystyrene films of 100 nm thickness were modified using plasma immersion ion implantation (PIII) with argon ions of energy 20 keV and fluences in the range 2 × 1014–2 × 1016 ions cm−2. The structure and properties of the films were determined by ellipsometry and FTIR spectroscopy, as well as AFM, wetting angle measurements, profilometry and optical microscopy. The effects of oxidation, carbonization, etching and gel-formation were observed. The etching rate was found to decrease with PIII fluence. The rates of degradation with increasing fluence of the aromatic and aliphatic parts of the polystyrene macromolecule were found to be similar. Oxidation of the polystyrene film ceases at fluences greater than 1015 ions cm−2. The surface morphology of the film did not change with PIII fluence. Washing with toluene produced surface wrinkling for low fluences up to 1015 ions cm−2 while at high fluences the modified films were stable.

Pulsed laser ignition of reactive multilayer films

Abstract: Nanostructured Al∕Pt multilayer films were ignited by single pulse irradiation from a Ti:sapphire femtosecond laser system. Critical ignition fluences (0.9–22J∕cm2) required to initiate a self-propagating reaction were quantified for different multilayer designs. Multilayers with smaller bilayer thickness required relatively lower fluence for ignition. Ignition threshold fluence was also found to be 1.4–3.6 times higher for Al-capped multilayers than for Pt-capped multilayers. Ablation threshold fluences were measured for Al (860±70mJ∕cm2) and Pt (540±50mJ∕cm2) and related to the observed difference in ignition fluences for Al- and Pt-capped multilayers.

Fabrication of buried channel waveguides in photosensitive glass using proton beam writing

Abstract: We report our results on the fabrication and characterization of buried, channel optical waveguides in photosensitive Foturan™ glass using a high energy proton beam. Waveguides were fabricated with varying fluence, and the propagation loss and refractive index change were measured. Near-field mode data measured at 632.8nm showed that waveguiding could be achieved for all fluences ranging from 1014to1016protons∕cm2. The maximum positive refractive index change of 1.6×10−3 was measured for the highest fluence. The waveguide propagation losses measured using the scattering technique were estimated to be in the range of 8.3–12.9dB∕cm, increasing with proton fluence.

Self-assembled and etched cones on laser ablated polymer surfaces

Abstract: At least two different routes lead to conical structures on laser ablated polymer surfaces. These were investigated by studying laser ablation on the surfaces of different classes of polymers. Cones appeared readily in strongly absorbing polymers such as poly(ethylene terephthalate) (PET) and polyimide (PI), but only within narrow laser parameters in nylon 6, and rarely in poly(chlorotrifluoroethylene), the last two being weak ultraviolet (UV) absorbers. Self-assembled, close-packed cones occurred in PET, in which heat generated due to absorbed laser energy creates a thin, chemically stable, viscoelastic, highly compliant layer (above the glass transition temperature). Surface structure in such polymers evolves from nodules through donuts into ripples and finally to cones as the energy deposited per unit area on the surface (total fluence) is increased using a combination of single pulse fluence and number of pulses. A phase transition from a ripple phase to a cone phase is thought to occur as the thickne...

Transition between nonthermal and thermal ablation of metallic targets under the strike of high-fluence ultrashort laser pulses

Abstract: The mutual transition dynamics between nonthermal and thermal dominant ablation processes is investigated in the hole-drilling and line-scribing experiments on aluminum samples with intense 50 fs to 24 ps laser pulses. It is found that a critical pulse width that separates the two different ablation regimes monotonically reduces with the increasing laser fluence. Theoretical analyses suggest that the complex interplay between photomechanical stress fragmentation and phase explosion could be responsible for these observations. A semiempirical transition law between the two ablation regimes is introduced, which is consistent with measured experimental data.

Energy Transport and Nanostructuring of Dielectrics by Femtosecond Laser Pulse Trains

Abstract: This study analyzes single burst ablation of dielectrics by a femtosecond pulse train that consists of one or multiple pulses. It is found that (1) there exist constant-ablation-depth zones with respect to fluence for one or multiple pulses per train and (2) for the same total fluence per train, although the ablation depth decreases in multiple pulses as compared to that of a single pulse, the depth of the constant-ablation-depth zone decreases. In other words, repeatable structures at the desired smaller nanoscales can be achieved in dielectrics by using the femtosecond pulse train technology, even when the laser fluence is subject to fluctuations. The predicted trends are in agreement with published experimental data.

Synergy between thermal spike and exciton decay mechanisms for ion damage and amorphization by electronic excitation

Abstract: A theoretical model is proposed to account for the damage and amorphization induced in LiNbO3 by ion bombardment in the electronic energy-loss regime. It relies on the synergy between the thermal spike generated by electron-phonon interaction and the nonradiative decay of localized self-trapped excitons. Calculations have been carried out to describe the effect of single impact as well as multiple impact high fluence irradiations. In the first case, the defect concentration profile and the radius of the amorphous tracks have been theoretically predicted and they are in good accordance with those experimentally determined. For high fluence irradiations 10 13 cm �2 the model predicts the formation of homogeneous amorphous surface layers whose thickness increases with fluence. The propagation of the crystalline-amorphous boundary has been determined as a function of irradiation fluence. Theoretical predictions are also in good agreement with experimental data on Si-irradiated 7.5 and 5 MeV LiNbO3 outside the region of nuclear collision damage.

Characterization of electron- or proton-irradiated Si space solar cells by THz spectroscopy

Abstract: The transmission or reflection measurement of p-type Si wafers with various resistivities was performed in the THz–IR frequency region, and the carrier concentration and mobility were evaluated by a simple Drude model. The range of electrical resistivity of 100–0.01 Ω cm and the range of carrier concentration of 1 × 1014 to 1 × 1019 cm−3 can be estimated non-destructively by this method. In addition, the annealing characteristics of electron- or proton-irradiated Si single crystals doped with 1015 cm−3 boron were characterized by this method in order to study the radiation damage of space solar cells. Combined with the results of far-IR and DLTS reported previously, THz spectroscopy can explain the recovery of carriers at high fluence samples, and the reverse annealing effect at low fluence samples as regards carrier scattering time in conjunction with the defect type.

F2-laser patterning of indium tin oxide (ITO) thin film on glass substrate

Abstract: This paper reports the controlled micromachining of 100 nm thick indium tin oxide (ITO) thin films on glass substrates with a vacuum-ultraviolet 157 nm F2 laser. Partial to complete film removal was observed over a wide fluence window from 0.49 J/cm2 to an optimized single pulse fluence of 4.5 J/cm2 for complete film removal. Optical microscopy, atomic force microscopy, and energy dispersive X-ray analysis show little substrate or collateral damage by the laser pulse which conserved the stoichiometry, optical transparency and electrical conductivity of ITO coating adjacent to the trenches. At higher fluence, a parallel micron sized channel can be etched in the glass substrate. The high photon energy and top-hat beam homogenized optical system of the F2 laser opens new means for direct structuring of electrodes and microchannels in biological microfluidic systems or in optoelectronics.