Showing papers on "Fluence published in 1989"
TL;DR: Calculations and measurements reveal that very large errors can result if the optical properties are inaccurate, if anisotropic scattering is neglected, or if the finite size of the irradiating light beam is not taken into account.
Abstract: The propagation of light in tissue may be calculated by exact transport theory, or the approximate diffusion theory, provided the optical properties are known at the source wavelength. Optical properties for the exact methods are the absorption coefficient, scattering coefficient, and angular distribution of scattering. Appropriate properties for diffusion theory are the diffusion length and diffusion coefficient (corrected for anisotropic scattering). Computer programs and analytical solutions (for some simple geometries) exist, but the optical properties have to be determined experimentally and are not well defined as yet. The radiant energy fluence rate and the diffuse transmittance and reflectance have been measured in several tissues and in a few geometries, but there are gaps in the data as a function of wavelength. Calculations and measurements reveal that very large errors can result if the optical properties (for example, the diffusion length) are inaccurate, if anisotropic scattering is neglected, or if the finite size of the irradiating light beam is not taken into account. Furthermore, the radiant energy fluence and transmittance are perturbed by local regions of lesser or greater absorption, although recovery of the fluence and transmittance occurs beyond some three diffusion lengths.
340 citations
IBM1
TL;DR: When a pulse (~ 14 nsec half width) of laser radiation of 193 nm wavelength with a fluence above a threshold value falls on a polymer film, the material at the irradiation site is spontaneously etched away to a depth of 1000 A or more.
Abstract: When a pulse (~ 14 nsec half-width) of laser radiation of 193 nm wavelength with a fluence above a threshold value falls on a polymer film, the material at the irradiation site is spontaneously etched away to a depth of 1000 A or more.[1,2] This process has been called Ablative Photo Decomposition’ [3]. The excimer laser which is the source of the 193 nm radiation is capable of providing radiation at other wavelengths such as 249 nm, 308 nm, and 351 nm. Spontaneous etching of the polymer films by the laser beam has been observed at all of these wavelengths [4–7].But there are quantitative differences in the etching process at different wavelengths and with different polymers.
269 citations
TL;DR: In this paper, the authors describe the evolution of far-UV laser photoablation of polymers using a dynamic model which states that the irradiated interface moves at a rate proportional to the difference between, the intensity reaching it, and the ablation threshold intensity I t.
Abstract: The evolution since 1982, of far-UV laser photoablation of polymers is described. The experimental data
can be fitted by using a dynamic model which states that the irradiated interface moves at a rate
proportional to the difference between, the intensity reaching it, and the ablation threshold intensity I t .
The screening effect of the ablated gaseous products is taken into account. The experimental etch depth
versus fluence, obtained with our new quartz crystal microbalance technique, can be fitted by adjusting
two parameters of this model; the mean absorption coefficient of the products β and the so-called ablation
rate constant k, which is the etch rate for I=I t + 1 MW/cm2. These two parameters are wavelength
dependent. The model allows also the calculation of the dose of absorbed energy as a function of depth in
the material. For each layer of the ablated depth, the dose absorbed before and after (reabsorption of the
gas products) ablation are distinguished. The dose of reexcitation varies highly with fluence and the
distribution of gas products broadens accordingly. However, primary products need to be studied. Surface
products are formbd on the final surface, during the postablation phase of the pulsed irradiation. The
probable mechanism of the evolution of excitation energy is discussed, with stress on the elementary steps
leading to heat production.The perspectives of evolution of etching and patterning polymer surfaces by far-UV photoablation are
high in basic research and technology.
171 citations
TL;DR: The study investigates the dependence of the patterns that are produced on Ge and Al by a KrF 249-nm laser on incident fluence and both the polarization and angle of incidence of the light and reports surface structures that are consistent with the idea of capillary waves being launched and subsequently frozen on the surface of the material.
Abstract: We report the first systematic investigation into the properties of ultraviolet-laser-induced periodic surface structures that can be produced on nominally smooth surfaces. The study investigates the dependence of the patterns that are produced on Ge and Al by a KrF 249-nm laser on incident fluence and both the polarization and angle of incidence of the light. At high fluence we show that the results are consistent with the assumption that the surface melts uniformly, whereas the patterns formed at low fluence can be explained on the basis of the localized melting of the surface. For P-polarized light we find two dominant patterns, one perpendicular to the polarization and one parallel to it. For S-polarized light we report the first observation of two new patterns, one parallel and one perpendicular to the polarization, of which the latter is explained by invoking a new formation mechanism. The results show that for P-polarized light at large angles of incidence the ripple spacing shows large deviations from its previously expected value. The results are shown to be in excellent agreement with a previously developed first-principles theory. In addition, we report surface structures that are consistent with the idea of capillary waves being launched and subsequently frozen on the surface of the material.
118 citations
TL;DR: This study shows that irradiance within tissues can greatly exceed the irradiance of incident collimated light, an effect that should be accounted for in photomedical dosimetry or research.
Abstract: The integrated irradiance (energy fluence rate) within tissue can exceed the incident irradiance due to backscattered and multiply reflected light near the sample surface. This was studied quantitatively using pulsed photothermal radiometry, which measures blackbody radiation emitted by a sample during and after absorption of an optical pulse. Aqueous gels containing absorbing dye with or without various scattering materials were studied using a fast sensitive IR detector system and 1-μs tunable pulsed dye laser. For nonscattering samples, the temperature transient (T-jump) due to absorption of a laser pulse was consistent with Beer’s law for homogeneous absorbing media. When scattering was present, increases of up to almost an order of magnitude in the T-jump were observed. For a given absorption coefficient, there was a proportional relationship between the increase in the T-jump and the sample’s diffuse reflectance. A model describing the reflectance of diffuse radiation at the sample boundary was derived to explain this result. To test the model, the refractive index was varied with air as the external medium and was also matched to that of BaF2 as the externalmedium. The subsurface fluence is, to a reasonable approximation, given by E ≅ E0(1 + 2bR), where E0 is the incident fluence of an infinitely wide collimated beam, b is a coefficient strongly dependent on only the refractive index, and R is the measured diffuse reflectance of the sample. This study shows that irradiance within tissues can greatly exceed the irradiance of incident collimated light, an effect that should be accounted for in photomedical dosimetry or research.
77 citations
TL;DR: The relationship between etch depth and fluence is broadly similar to that observed for excimer laser etching but with a less well-defined threshold as discussed by the authors, which is consistent with the rate of thermal decomposition of PET.
Abstract: Films of polyethylene terephthalate (PET) can be successfully etched with 9 μm radiation from a pulsed TEA CO2 laser. The relationship between etch depth and fluence is broadly similar to that observed for excimer laser etching but with a less well-defined threshold. Time-resolved photoacoustic measurements of stress waves generated in the interaction show that at a fluence of 1.8 J cm−2 ablation occurs 100–200 ns after the start of the laser pulse, a time which is consistent with the rate of thermal decomposition of PET. The volatile products of ablation are carbon monoxide, carbon dioxide, methane, ethyne, ethene, benzene, ethanal, and small quantities of other products. For fluences close to and appreciably above the threshold the ablated material consists predominantly of involatile species of relatively high molecular weight, whereas at higher fluences substantial fragmentation of the polymer to small molecules occurs.
68 citations
TL;DR: In this article, low-energy As+−ion implantation followed by rapid thermal annealing (RTA) was utilized to modify exciton transition energies of GaAs/AlGaAs quantum wells (QW).
Abstract: Low‐energy As+‐ion implantation followed by rapid thermal annealing (RTA) was utilized to modify exciton transition energies of GaAs/AlGaAs quantum wells (QW). A variety of structures were irradiated at an energy low enough that the disordered region was spatially separated from the QWs. After RTA, exciton energies showed large increases which were dependent on QW widths and the implantation fluence with no significant increases in peak linewidths. The observed energy shifts were interpreted as resulting from the modification of the shapes of the as‐grown QWs due to enhanced Ga and Al interdiffusion at heterointerfaces in irradiated areas. These results are consistent with the model of enhanced intermixing of Al and Ga atoms in depth of the material due to diffusion of vacancies generated near the surface.
59 citations
TL;DR: Improvement in surface quality associated with increasing radiant exposures may result from a more uniform depth of ablation per pulse in the corneal lamellae that absorb laser wavelengths differently.
Abstract: The argon fluoride (193 nm) excimer laser is being used to change the anterior corneal curvature for correction of refractive errors. Uniformity of the surface following laser ablation may play an important role in the rate of epithelial healing and amount and type of stromal scarring. To test the effect of radiant exposure (fluence) on surface smoothness, we ablated rabbit corneas with the 193 nm argon fluoride excimer laser at nine radiant exposures from 50 to 850 mJ/cm2. A total energy of 100 J/cm2 was used for each ablation at a frequency of 1 Hz. Scanning electron microscopy demonstrated progressive improvement of surface smoothness with increasing radiant exposures. Transmission electron microscopy demonstrated no consistent increase in thickness to the surface condensate (pseudomembrane) with increasing radiant exposure. Improvement in surface quality associated with increasing radiant exposures may result from a more uniform depth of ablation per pulse in the corneal lamellae that absorb laser wavelengths differently. Radiant exposures at levels where the depth of ablation is the same regardless of increasing energy densities achieve a more uniform surface because inhomogeneities in the beam and variation in energy from pulse to pulse do not affect the ablation rate.
58 citations
TL;DR: In this article, tensile specimens of 9Cr-1MoVNb and 12Cr 1MoVW steels with up to 2% Ni were tested at room temperature after irradiation at 50 ° C to displacement-damage levels of up to 25 dpa in the High-Flux Isotope Reactor (HFIR).
48 citations
02 Mar 1989-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this article, the ion beam induced modification of polyimide (Pyralyn 2555, mainly BTDA-ODA) by Ar+ at 150 keV has been studied in details by using XPS and REELS techniques.
Abstract: The ion beam induced modification of polyimide (Pyralyn 2555, mainly BTDA-ODA) by Ar+ at 150 keV has been studied in details by using XPS and REELS techniques. Ion fluences ranging from 5 × 1012, to 1 × 1016 ions/cm2 have been used in order to study the chemical evolution of the polyimide in the ranges where chemical and electrical characteristics show the major changes. The modification of the chemical structure and electrical resistivity are shown to occur in relatively narrow fluence ranges. The onset of these successive modification steps are determined and a threshold-like dependence upon the ion fluence is observed.
39 citations
TL;DR: The wavelength, polarization, and fluence dependence of time-resolved reflectivity suggest formation of an initial monovalent metallic phase, which transforms within 30 ps to a low-reflectivity phase, similar to that observed in picosecond reflectivity experiments.
Abstract: We report the first femtosecond time-resolved measurements of melting dynamics of graphite. A high-reflectivity phase, lasting less than 10 ps, appears when and only when the surface is photoexcited above a critical fluence of 0.13 J/${\mathrm{cm}}^{2}$ needed to produce surface damage. The wavelength, polarization, and fluence dependence of time-resolved reflectivity suggest formation of an initial monovalent metallic phase, which transforms within 30 ps to a low-reflectivity phase, similar to that observed in picosecond reflectivity experiments.
TL;DR: In this paper, the authors used high-resolution multiphoton laser-induced fluorescence spectroscopy (LFS) to determine the yield and velocity distributions of Zn atoms ejected from a ZnS single crystal under irradiation by 308-nm photons.
Abstract: Time‐of‐flight (TOF) measurements employing high‐resolution multiphoton laser‐induced fluorescence spectroscopy (LFS) have been used as a probe to determine the yield and velocity distributions of Zn atoms ejected from a ZnS single crystal under irradiation by 308‐nm photons. For fluences between 20 and 80 mJ/cm2 (irradiated area 2 mm2), the velocity distributions could be fitted by Maxwell–Boltzmann distributions where the characteristic temperature increased from 2000 to 9000 K, respectively. The absolute neutral Zn yield also increased from 108 to 1012 atoms per pulse over this same fluence range. Plots of temperature (T) as a function of fluence (F) and yield as a function of 1/T and 1/F suggest thermal evaporation as the mechanism for the Zn emission.The results indicate that with increasing fluence a critical combination of particle density and laser intensity is reached for formation of a plasma which interacts with the surface to cause catastrophic failure. For consecutive laser shots at constant ...
TL;DR: In this paper, the retention of argon in low energy (1 keV) high fluence (1017 cm−2) argon irradiated Mo and Si has been studied by Thermal Argon Desorption Spectrometry and Transmission Electron Microscopy.
Abstract: The retention of argon in low energy (≤ 1 keV) high fluence (≥ 1017 cm−2) argon irradiated Mo and Si has been studied by Thermal Argon Desorption Spectrometry and Transmission Electron Microscopy. It is shown that high fluence argon trapping cannot be described by a linear superposition of low fluence trapping processes. This is partly due to the already known effect of gas-sputtering, but in addition there are important effects due to the development of irradiation-induced substructures. In Mo the creation of self-interstitial loop segments intersecting the surface lead to a significant increase in Ar trapping, while in Si the creation of Ar bubbles via irradiation-induced transport mechanisms is held responsible for extra trapping.
TL;DR: In this paper, the saturation effect in the ablation rate is observed in the laser ablation of polyimide at a wavelength of 355 nm and duration of 30 ps, and the fluence dependence of the reflectivity, emission intensity, and photoacoustic signal have been measured to provide some understanding of the mechanism for the saturation.
Abstract: The saturation effect in the ablation rate is observed in the laser ablation of polyimide at a wavelength of 355 nm and duration of 30 ps. The fluence dependence of the reflectivity, emission intensity, and photoacoustic signal have been measured to provide some understanding of the mechanism for the saturation. A mechanism invoking the blocking of the trailing part of the light pulse by both the plume and the excited polymer generated by the leading part of the pulse explains all the observation satisfactorily. However, the possibility of blocking by only the excited polymer followed by nonstatistical disposal of energy in the ejection process cannot be ruled out.
TL;DR: In this paper, the damage induced at a SiO2 surface by low-energy ion bombardment with He, Ne and Ar ions was investigated by AES and it was shown that, provided the electron fluence is kept very small, the SiOx peak is representative of the true ion beam damage, being initially bigger than the Si peak.
TL;DR: In this paper, the structure changes of graphite due to keV-energy hydrogen ion bombardment are studied by means of high resolution electron microscopy (HREM), where highly graphitized vapor grown graphite fibers of diameters around 700 nm are irradiated side on with 1 keV H + ion beam to various fluences ranging from 1 × 10 14 to 1 ×10 18 H + /cm 2.
TL;DR: In this paper, a tunnel structure called tunnel structure (T.S) was found in the fluence range of about (5-10) × 10 17 H + /cm 2.
TL;DR: In this article, the ionic products from excimer laser photoablation (λ=308 nm) of polyimide (Kapton) film have been studied as a function of fluence.
Abstract: The ionic products from excimer laser photoablation (λ=308 nm) of polyimide (Kapton) film have been studied as a function of fluence. Large ion masses up to about 900a.m.u. are easily observed, the mass distribution depending strongly on the fluence. Velocities of the emitted particles lie between 1400 and 10 000 ms−1, again dependent on the fluence. A mechanism to explain the high velocities is suggested consisting of ionisation of the surface polymer molecules followed by a Coulomb explosion combined with expansion of the high density gas formed by the photoablation.
TL;DR: In this paper, the authors measured the transmission through thin gold films on optical quartz during laser blow-off with 15 ns, 532 nm laser pulses and found that the integrated laser fluence required to blow off the films is roughly independent of incident laser fluences, and is close to that expected to raise the temperature of the film to the boiling point at the film support interface.
Abstract: The transmission through thin gold films on optical quartz during laser blow‐off with 15 ns, 532 nm laser pulses has been measured. Dramatic changes in transmission mark the duration of the blow‐off event with laser fluences above the threshold for removal. The integrated laser fluence required to blow‐off the films is roughly independent of incident laser fluence, and is close to that expected to raise the temperature of the film to the boiling point at the film‐support interface.
TL;DR: The mass spectra obtained at both high and low fluence correspond well to those obtained using electron-impact ionization, and in many instances parallel the magic numbers characteristic of rare-gas clusters as mentioned in this paper.
Abstract: The fluence dependence of lead cluster ion distributions at 222 nm and 308 nm reveal markedly different behavior. Results obtained at 308 nm display a simple uniform increase in intensity with higher laser fluence with little change in relative intensities. At 222 nm, however, a significant transformation is found from a markedly different low fluence distribution to a high fluence pattern, which is essentially indistinguishable from that observed at 308 nm. It is concluded that mass spectra obtained at 308 nm, regardless of fluence, or at 222 nm and high fluence contain appreciable contributions from fragmentation. Hence, under these conditions the mass spectra are found to be dominated by cluster ion stabilities. Magic numbers observed at both high and low fluence correspond well to those obtained using electron-impact ionization, and in many instances parallel the magic numbers characteristic of rare-gas clusters. This suggests the stabilities of both neutral and monovalent cationic lead clusters are largely determined by close-packing considerations, and are not appreciably influenced by electronic structure. Similar preferences for close-packed structures are also found for mixed lead-antimony clusters containing one or two antimony atoms that are ionized using high fluence 308 nm excitation.
TL;DR: The superconducting transport properties of polycrystalline Tl2Ca2Ba2Cu3O10 thin films irradiated with 740 keV oxygen ions were monitored as a function of fluence as mentioned in this paper.
Abstract: The superconducting transport properties of polycrystalline Tl2Ca2Ba2Cu3O10 thin films irradiated with 740 keV oxygen ions were monitored as a function of fluence. Both the transition temperature (Tc ) and the critical current density (Jc ) decreased rapidly with fluence; however, the transition temperature onset remained constant. A fluence of 2×1014 O/cm2 (0.028 dpa) was sufficient to eliminate superconductivity in the films. Jc at 76 K decreased from 25 000 A/cm2 in the unirradiated sample to 2000 A/cm2 after a fluence of 2.1×1013 O/cm2. A room‐temperature anneal caused both Tc and the normal‐state resistivity to recover slightly after low‐fluence irradiations.
02 Mar 1989-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this paper, the damage production and annealing in ion irradiated single crystalline YBa2Cu3O7 thin films with protons, He-ions and Ar-ions at 5 and 77 K was studied.
Abstract: Irradiation of thin YBa2Cu3O7 films with protons, He- and Ar-ions at 5 and 77 K causes distinct changes of the resistivity, the transition temperature to superconductivity, the width of this transition and of the structure of the films. The results depend in a characteristic way on the initial transport properties and the grain structure of the films. About 60% of the irradiation induced changes recover during annealing to 296 K. With increasing fluence first a semiconducting and then an amorphous phase developed. He-ion channeling together with the enhanced scattering cross section for He on oxygen at 3.04 MeV was applied to study the damage production and annealing in ion irradiated single crystalline YBa2Cu3O7 thin films.
TL;DR: In this paper, the effects of ion irradiation on the critical current density of high-Tc superconducting Ba2YCu3O7-δ thin films have been studied.
Abstract: Effects of ion irradiation on the critical current density of high-Tc superconducting Ba2YCu3O7-δ thin films have been studied. For the film subjected to a fluence of 5.02×1011 ions/cm2 of 514 keV O+, the critical current density determined from I-V characteristics was enhanced by about 10% from 53 to 75 K. The temperature dependence of the critical current density was not affected by ion irradiation. Irradiation effects on the critical temperature of these films have also been investigated.
TL;DR: In this article, a decrease of T c in correlation with an extension of the c-axis parameter is observed, and the effects of irradiation with the two projectiles differ dramatically as revealed by the T c reduction and the resistivity increase as function of the irradiation fluence.
Abstract: Epitaxial YBaCuO films with critical current densities up to 5 ∗ 10 6 A/cm 2 at 77 K have been grown in-situ on different single crystalline substrates by excimer laser evaporation. Defects have been generated in these films by a too low oxygen pressure during the evaporation as well as by irradiation with 25 MeV 16 O ions and 173 MeV 129 Xe ions. A decrease of T c in correlation with an extension of the c-axis parameter is observed. The effects of the irradiation with the two projectiles differ dramatically as revealed by the T c reduction and the resistivity increase as function of the irradiation fluence.
TL;DR: In this article, the effects of both structural and substitutional disorder in As+implanted Si at doses from 3×1012 to 1×1016 ions cm−2 on interband optical spectra, particularly on an E1 structure at 3.4 eV, were studied.
Abstract: We have studied the effects of both structural and substitutional disorder in As+‐implanted Si at doses from 3×1012 to 1×1016 ions cm−2 on interband optical spectra, particularly on an E1 structure at 3.4 eV. Increasing the implant dose results in a structural order‐disorder transition, found to be well described by the peak‐to‐peak height of the E1 derivative spectra. The damage rate is defined as being based on the E1 spectral structure and is related to the fractional strength of interband optical response, in good agreement with that in effective‐medium theory. The relationships between interband optical properties and electronic structure in the implanted region are discussed. After annealing, there is a red shift of the E1 threshold together with a decrease in optical response. This is attributed to substitutional impurities and screening of free carriers, which was observed and compared with previous results.
TL;DR: In this article, a CW supersonic molecular beam under 355- and 560-nm pulsed laser irradiation was used for chemical etching of Si(111) surface with molecular chlorine.
Abstract: Laser-induced chemical etching of Si(111) surface with molecular chlorine is investigated by use of a CW supersonic molecular beam under 355- and 560-nm pulsed laser irradiation. Two major desorbed reaction products, SiCl and SiCl{sub 2}, are observed by time-resolved mass spectrometry. The time-of-flight (TOF) spectra of the desorbed species are measured as a function of laser fluence and the translational energy of the incident chlorine molecules. The measured TOF spectra can be fitted to a Maxwell-Boltzmann distribution at laser fluences lower than 120 mJ/cm{sup 2}. The enhancement of the laser-induced gas-surface reaction on raising the normal component of the translational energy of the chlorine molecules is obvious for both laser wavelength cases. A reaction mechanism that mainly involves the dissociative chemisorption of Cl{sub 2}, the reaction of the adsorbed species, and the laser-induced desorption on the surface is proposed.
TL;DR: In this article, visible-UV spectroscopic and ion probe measurements on the ablation products from bulk Y-Ba-Cu-O targets irradiated using a TEA CO2 laser are reported.
TL;DR: In this paper, the rotational and vibrational energy distributions of carbon monoxide, which is liberated during the 193 nm excimer laser ablation of polyimide, have been measured using mass-resolved resonant multiphoton ionization spectroscopy.
Abstract: Rotational and vibrational energy distributions of carbon monoxide, which is liberated during the 193 nm excimer laser ablation of polyimide, have been measured using mass‐resolved resonant multiphoton ionization spectroscopy. Using 150 mJ/cm2 fluence (1000 A removed per pulse), the rotational temperature is seen to decrease monotonically from 1150 to 650 K from beginning to end of the ablation plume. Additionally, the vibrational temperature is measured to be 3400 K and is constant over time. When using 6 mJ/cm2 to ablate (0.1 A removed per pulse), the rotational and vibrational temperatures are measured to be 700 and 900 K, respectively. The influence of collisional perturbations and high ablation fluences on the measured rotational and vibrational temperatures is discussed.
Patent•
17 Mar 1989
TL;DR: In this article, the authors showed that PTFE can be ablated using laser pulses with a duration of 300 fs, a wavelength of 248 nm and a fluence between approximately 0.5 and 1 J/cm2.
Abstract: Fluoroplastics, for example PTFE, are ablated using laser pulses with a duration of 300 fs, a wavelength of 248 nm and a fluence between approximately 0.5 and 1 J/cm2. Under these conditions, the ablation rate is of the order of 1 micrometre per laser pulse.