Subthreshold Fluences Research Articles

Transition to efficient, unsuppressed bulk-target ion acceleration via high-fluence laser irradiation

A high-intensity laser irradiating a few-$\ensuremath{\mu}\mathrm{m}$ solid foil will accelerate ions from the bulk of the target as well as protons from a surface contaminant layer. Experimental measurements of ion spectra using the OMEGA EP laser (0.25--1 kJ, 10 ps) show, as suggested previously [Petrov et al., Phys. Plasmas 17, 103111 (2010)], that at a laser fluence exceeding 1 $\mathrm{J}/\ensuremath{\mu}{\mathrm{m}}^{2}$, the contaminant layer is accelerated enough that ions from the bulk of the target are more effectively accelerated. When using ${\mathrm{CD}}_{2}$ as a target, the high fluence results in a 100-fold increase in deuteron acceleration efficiency (near 1% of laser energy) compared to subthreshold fluence. This is found to be due to the fact that the deuterons have a higher density at many locations during acceleration, allowing a larger electric field to develop, leading to improved efficiency. Using a pitcher-catcher setup, these deuterons, as well as protons from the contaminant layer, strike a LiF target and generate neutrons via (d,n) and (p,n) nuclear reactions. CR39 plastic and nuclear activation detectors measured broadband neutron yields of $4\ifmmode\times\else\texttimes\fi{}{10}^{9}\phantom{\rule{4pt}{0ex}}{\mathrm{sr}}^{\ensuremath{-}1}$ and yields of ${10}^{8}\phantom{\rule{4pt}{0ex}}{\mathrm{sr}}^{\ensuremath{-}1}$ for neutrons above 11 MeV.

Femtosecond laser-induced ultrafast transient snapshots and crystallization dynamics in phase change material.

We report here femtosecond laser-driven transient snapshots of ultrafast crystallization of Ge2Sb2Te5 films from its as-deposited amorphous phase, and the local structural change is validated by micro-Raman spectroscopy and x-ray diffraction. The decay time constant of ∼5 ps in transient spectra with a precise temporal resolution using 400nm (pump) reveals about 68 volumetric percentage crystallization at a remarkably low fluence of 4.78 mJ·cm-2. This is attributed to reiterated excitation after a complete carrier relaxation and formation of a long-lasting transient phase at sub-threshold fluences. Furthermore, Raman spectra of irradiated spots confirm defective-octahedral modes at 110 and 160 cm-1 validating crystallization.

Single-shot front-side nanoscale femtosecond laser ablation of a thin silver film

Nano- and microscale holes, as well as related sub-ablative nanospikes and sub-micron bumps, were produced in a 30-nm thick silver film on a silica substrate by single femtosecond laser pulses with variable pulse energies, focused by different strong focusing optics. Characteristic laser energy deposition dimensions exceed the expected focal spots by nearly 2 microns, indicating the considerable lateral thermal transport in the film, while the effective hole formation thresholds decrease versus increasing numerical aperture of focusing optics. Morphologies of the sub-ablative solidified surface nanostructures and numerical estimates of deposited volume energy density undermine blowing-off the molten film due to subsurface boiling and near-critical phase explosion at lower and higher sub-threshold fluences, respectively.

Enhanced cytotoxic effects of 5-aminolevulinic acid-mediated photodynamic therapy by concurrent hyperthermia in glioma spheroids

During photodynamic therapy (PDT) both normal and pathological brain tissue, in close proximity to the light source, can experience significant temperature increases. The purpose of this study was to investigate the anti-tumor effects of concurrent 5-aminolevulinic acid (ALA)-mediated PDT and hyperthermia (HT) in human and rat glioma spheroids. Human or rat glioma spheroids were subjected to PDT, HT, or a combination of the two treatments. Therapies were given concurrently to simulate the conditions that will occur during patient PDT. Predictions of diffusion theory suggest that brain tissue immediately adjacent to a spherical light applicator may experience temperature increases approaching 8 degrees C for laser input powers of 2 W. In the in vitro model employed here, HT had no effect on spheroid survival at temperatures below 49 degrees C, while sub-threshold fluence PDT results in only modest decrease in survival. HT (40-46 degrees C) and PDT interact in a synergistic manner if the two treatments are given concurrently. The degree of synergism increases with increasing temperature and light fluence. Apoptosis is the primary mode of cell death following both low-fluence rate PDT and combined HT + PDT.

Laser conditioning of ZrO 2:Y 2O 3/SiO 2 mirror coatings prepared by E-beam evaporation

Laser conditioning effects of the dielectric mirror coatings with different designs were investigated. Simple quarter-wave ZrO 2:Y 2O 3/SiO 2 mirrors and half-wave SiO 2 over-coated ZrO 2:Y 2O 3/SiO 2 mirror coatings were fabricated by E-beam evaporation (EBE). The absorbance of the samples before and after laser conditioning was measured by surface thermal lensing (STL) technology and the defects density was detected under Nomarski microscope. The enhancement of the laser damage resistance was found after laser conditioning. The dependence of the laser conditioning on the coating design was also observed and the over-coated sample obtained greatest enhancement, whereas the absorbance of the samples did not change obviously. During the sub-threshold fluence raster scanning, the minor damage about defects size was found and the assumption of pre-damage mechanism, based on the functional damage concept, was put forward. The improvement of the laser induced damage threshold (LIDT) was attributed to the benign damage of the defects and the dependence on the coating design owed to the damage growth behavior of different coating designs.

Development of a novel indwelling balloon applicator for optimizing light delivery in photodynamic therapy

Background and Objective A human glioma spheroid model is used to investigate the efficacy of different light delivery schemes in 5-aminolevulinic acid (ALA)—mediated photodynamic therapy (PDT). The results provide the rationale for the development of an indwelling balloon applicator for optimizing light delivery. Study Design/Materials and Methods Human glioma spheroids were incubated in ALA (100 or 1000 μg ml−1) for 4 hours and subjected to various light irradiation schemes. In one set of experiments, spheroid survival was monitored as a function of light fluence rate (5–200 mW cm−2). In all cases, spheroids were exposed to fluences of either 25 or 50 J cm−2. In a second study, the effects of repeated weekly PDT treatments, using sub-threshold fluences, were investigated. One group of spheroids was subjected to three treatments using fluences of 12, 12, and 25 J cm−2. Results were compared to spheroids receiving single treatments of either 12 or 25 J cm−2. A fluence rate of 25 mW cm−2 was used for all three groups of spheroids. In all cases, the effect of a given irradiation scheme was evaluated by monitoring spheroid growth. Results Low fluence rates produce greater cell kill than high fluence rates. The minimum effective fluence rate in human glioma spheroids is approximately 10 mW cm−2. Repeated weekly PDT treatments with sub-threshold fluences result in significant cell kill. In spheroids surviving the PDT treatments, growth is suppressed for the duration of the treatment period. Conclusion The results of the in vitro studies support the development of an indwelling balloon applicator for the delivery of light doses in long term multi-fractionated PDT regimens. Lasers Surg. Med. 29:406–412, 2001. © 2001 Wiley-Liss, Inc.

Development of a novel indwelling balloon applicator for optimizing light delivery in photodynamic therapy.

A human glioma spheroid model is used to investigate the efficacy of different light delivery schemes in 5-aminolevulinic acid (ALA)--mediated photodynamic therapy (PDT). The results provide the rationale for the development of an indwelling balloon applicator for optimizing light delivery. Human glioma spheroids were incubated in ALA (100 or 1000 microg /ml-1) for 4 hours and subjected to various light irradiation schemes. In one set of experiments, spheroid survival was monitored as a function of light fluence rate (5-200 mW cm-2). In all cases, spheroids were exposed to fluences of either 25 or 50 J cm-2. In a second study, the effects of repeated weekly PDT treatments, using sub-threshold fluences, were investigated. One group of spheroids was subjected to three treatments using fluences of 12, 12, and 25 J cm-2. Results were compared to spheroids receiving single treatments of either 12 or 25 J cm-2. A fluence rate of 25 mW cm-2 was used for all three groups of spheroids. In all cases, the effect of a given irradiation scheme was evaluated by monitoring spheroid growth. Low fluence rates produce greater cell kill than high fluence rates. The minimum effective fluence rate in human glioma spheroids is approximately 10 mW cm-2. Repeated weekly PDT treatments with sub-threshold fluences result in significant cell kill. In spheroids surviving the PDT treatments, growth is suppressed for the duration of the treatment period. The results of the in vitro studies support the development of an indwelling balloon applicator for the delivery of light doses in long term multi-fractionated PDT regimens.

Incubation: Subthreshold ablation of poly-(methyl methacrylate) and the nature of the decomposition pathways

A study of the decomposition of poly-(methyl methacrylate) (PMMA) by pulsed UV laser ablation in the incubation regime is presented here. We propose that incubation proceeds via the photoinduced formation of defects centers; while ablation following incubation, at subthreshold fluences, is a thermally driven phenomena. The light scattering data, in the incubation regime, is consistent with molecular weight reduction via backbone cleavage. The defect centers, C=C at chain ends following backbone cleavage, increase the UV absorption coefficient thereby lowering the ablation threshold. Similarities between the mass spectra of ablative PMMA following incubation and pyrolysis suggest that ablation following incubation is thermally driven. The implications of these results in an extensive and contradictory body of available data are discussed.

Wavelength and Fluence Effect on Vascular Damage with Photodynamic Therapy on Skin

Normal skin phototoxicity is clinically predictable during photodynamic therapy with light at 690 and 458 nm wavelengths, in the first 5 h after intravenous bolus infusion of benzoporphyrin derivative mono-acid ring A. This study goal was to determine histologic milestones that lead to tissue necrosis with exposure to red (690 nm) and blue (458 nm) light. The threshold doses for skin necrosis on rabbits were equal at both wavelengths. Lower, equal to, and higher than threshold fluences were delivered in duplicates at hourly intervals, with 40% increments, at constant irradiance. Pathology specimens from irradiated and control sites, were collected at 0, 2, 7, 24, 48 h, and 2 wk after treatment and were paired to equivalent treated sites for clinical evaluation. Immediately after irradiation, at 690 and 458 nm thresholds, light microscopy showed stasis and inflammatory infiltrate in the papillary dermis, respectively; electron microscopy demonstrated pericyte and endothelial cell damage - greater at 690 than 458 nm. At day 1, vascular stasis in the dermis showed a steeper dose-response with red than blue light, and led to necrosis of skin appendages (day 1) and epidermis (days 1-2) at both wavelengths. Sub-threshold fluences induced similar, but significantly milder (p < 0.05) changes and epidermis recovered. Skin necrosis, at threshold fluences in photodynamic therapy with benzoporphyrin derivative mono-acid ring A, was primarily due to vascular compromise to a depth potentially reaching the subcutaneous muscle at 690 nm, whereas at 458 nm vascular damage was confined to upper dermis. This system facilitates selective destruction of skin vasculature, sparing normal epidermis.

Structural-modification mechanism for polyimide-doped poly(tetrafluoroethylene) at subthreshold fluences using 248 nm radiation

Single-photon excimer laser ablation of neat poly(tetrafluoroethylene) (PTFE) is not observed at emissions in the ‘‘quartz’’ UV, i.e., from about 190–380 nm. However, it has been successfully demonstrated that, when the fluoropolymer is doped with small quantities of polyimide (PI), ablation in the quartz UV, e.g., at 248 and 308 nm and pulse widths of about 25 ns, is readily achieved. When PI-PTFE blends are exposed to subthreshold fluences, considerable changes in surface topography occur although clearly defined structures, e.g., pits, are not formed. Using photoacoustic infrared spectroscopy to evaluate surface and bulk chemical changes to blends exposed to subthreshold excimer laser fluences, &amp;lt;100 mJ/cm2, it is shown that PI (1) is distributed throughout the bulk and resides at the surface and (2) is selectively absorbing the high-energy photons and as a result being preferentially removed from the surface.

Excimer laser ablation of polyimide-doped poly(tetrafluoroethylene) at 248 and 308 nm

Experimental data on the 248 and 308 nm wavelength excimer laser ablation at poly(tetrafluoroethylene) doped with polymide (PI) are reported for a range of fluences and dopant concentrations. Threshold fluences were determined and a correlation was obtained between the dopant concentration and the threshold fluence. The threshold fluences and the limiting etch rates at high fluences decreased with increasing dopant concentration, and there is a minimum dopant concentration below which there is no ablation at both of the wavelengths. The side wall taper of the ablated holes increased with increasing dopant concentration. At subthreshold fluences, the polymer surface was modified with selective removal of PI from the polymer blend. The etch rates have been modeled using a two parameter thermal model to describe the etching process. The parameters obtained by fitting the data are qualitatively correlated to the dopant concentration and the measured limiting etch rates.

Study of surface reflectivity and etch rates of polyimide (Kapton H) utilizing double-pulsed 308 nm laser radiation

Studies of the ablation of polyimide accompanying two rapid sequential pulses of 308 nm laser radiation are presented. Measurements of the change in surface reflectivity revealed that both transient and permanent changes in the surface reflectivity resulted when pulses above the ablation threshold irradiated the surface. The permanent change was due to physical modification of the polymer surface and redeposition of ablated material. The transient change was likely due to increased surface absorption of the incident radiation at subthreshold fluences. Above threshold, scattering and absorption of the incident probe radiation by particles in the plume is likely the dominant process. Double pulsed etch rate measurements are also presented.

Consequences of simultaneous exposure of inorganic solids to excimer laser light and an electron beam

In previous work we examined the changes in surface topography of sodium trisilicate glass (Na2O⋅3SiO2) with exposure to pulsed 248-nm excimer laser light at fluences of 2.6–5 J/cm2 (per laser pulse), as well as the character of the products emitted from the glass surface (e.g., ± ions, electrons, ground state and excited neutral atoms and molecules). At fluences above a threshold, ablative etching occurs only after a number of preliminary laser pulses. In this work, we show a dramatic synergism in the ablation process by simultaneous bombardment of the glass surface with 0.5–2 keV electrons and laser pulses. Extensive etching can be initiated immediately and sustained indefinitely at subthreshold fluences. Reflection electron energy loss spectroscopy performed on electron-bombarded surfaces shows band gap states growing with exposure. We propose that surface and near-surface defects produced by inelastic scattering of the electron beam provide single-photon absorption centers, facilitating etching at subthreshold fluences. The potential for single-photon driven etching/ablation of other wide band gap dielectric materials is also discussed. These studies also support the hypothesis that etching and damage of such surfaces can occur after prolonged exposure to laser irradiation alone due to an accumulation of absorption centers.