Fluence Region Research Articles

Nanostructuring of silicon surface by femtosecond laser pulse mediated with enhanced near-field of gold nanoparticles

Nanostructure fabrication on a silicon substrate using a near-field electromagnetic enhancement in the vicinity of gold nanoparticles is studied. Experimental and theoretical investigations are made in order to clarify the properties of the nanoholes produced. The obtained results show that this nanofabrication technique is a flexible method for precise surface nanostructuring. Gold particles with diameters of 40, 80, or 200 nm are placed on the silicon substrate by a spin-coating method, and a 100 fs laser pulse (λ=820 nm) is used to irradiate the samples. A theoretical treatment of the results obtained is made by FDTD (finite difference time domain) simulation and calculations of the near field scattering cross section based on Mie’s theory. The effect of the laser fluence, polarization, and particle size and arrangement on the nanohole properties is studied. The nanohole profiles correspond to the field distributions on the substrate in the low laser fluence region. By using a lower laser fluence than the ablation threshold of bulk silicon, a hole with a diameter of about 40 nm can be produced using a 200 nm gold particle. Furthermore, the gold particles exhibit a considerable field enchantment even at gold particle sizes of a few tens of nanometers, which offers a good opportunity for further decreases in the size of the produced structures. A highest electric field enhancement factor of about 26 is obtained for particles with a diameter of 200 nm. With the decrease of the particle size the field enhancement also decreases. Particle arrangement on the substrate surface strongly affects both the electric field distribution and the enhancement effect of the particles. The highest enhancement for a particle pair is obtained when the polarization of the incident radiation is parallel to the pair axis (longitudinal mode). A good correlation is obtained between the near-field distribution predicted by FDTD simulation and the parameters of the experimentally produced nanoholes.

Effect of Transverse Neutron Leakage on the Attenuation of the Displacements per Atom in the Reactor Pressure Vessel

The changes in the energy dependence of the neutron removal cross section at the vessel inner wall water/steel interface produce a substantial shift in the neutron spectrum as the fluence propagates into the pressure vessel. To account for this spectral shift, Regulatory Guide 1.99, Revision 2 requires that the fluence used in determining the reference temperature for nil-ductility transition RTNDT be extrapolated from the pressure vessel inner surface using the displacements per atom (dpa).The strong azimuthal and axial variation of the fluence at the vessel inner wall results in a substantial redistribution of the fluence as it propagates through the vessel due to transverse neutron leakage (i.e., perpendicular to the radial direction through the vessel). This transverse leakage tends to increase the dpa radial attenuation in regions of high fluence and reduce the attenuation in regions of low fluence.A series of pressure vessel fluence calculations has been carried out to determine the effect of (a) the transverse neutron leakage and (b) the plant-specific reactor design configuration on the radial attenuation of the dpa through the vessel. The calculations were performed for four operating pressurized water reactors and were carried out using the methods described in U.S. Nuclear Regulatory Commission Regulatory Guide 1.190. The calculations were performed with the DORT discrete ordinates transport code using ENDF/B-VI neutron transport and dpa cross sections.The transverse leakage is found to introduce a substantial variation of the dpa attenuation rate over the inner surface of the vessel. In the belt-line region opposite the core, the transverse leakage results in an ~6 to 14% azimuthal variation and an ~3 to 11% axial variation in the dpa at a 15-cm depth into the vessel, depending on the plant configuration.In order to simplify the determination of ΔRTNDT in probabilistic fracture mechanics analyses, conservative belt-line and reflector region dpa attenuation rates have been determined. Plant-specific analytic expressions for the radial dependence of the dpa through the vessel have also been determined.

Spectroscopic Study of Laser-Induced Phase Transition of Gold Nanoparticles on Nanosecond Time Scales and Longer

The pulsed laser induced phase transition of gold nanoparticles in aqueous solution was observed via a transient absorption on nanosecond time scales and longer. Gold nanoparticles were excited with an intense picosecond laser pulse (355 nm, 30 ps), and the subsequent changes were monitored using two continuous wave laser wavelengths (488 and 635 nm). On the nanosecond time scale, below 6.3 mJ cm(-2), no change was observed; however, in the low fluence region between 6.3 and 17 mJ cm(-2), gold nanoparticles produced a bleach signal (488 nm) attributed to the melting of the gold nanoparticles, which decreased linearly with increasing laser fluence. Laser fluences above 17 mJ cm(-2) resulted in a strong absorption at both wavelengths, which is ascribed to vaporization of gold nanoparticles rather than solvated electrons (ejected from gold nanoparticles) or light scattering. The decay of both signals was faster than the 5 ns time resolution used in our experimental system. On the microsecond time scale, increase in absorbance at 635 nm was observed with a time constant of 1.0 micros, while no change was observed at 488 nm. It is considered that this increase is attributed to the formation of smaller gold nanoparticles resulting from pulsed laser induced size reduction of initial gold nanoparticles.

Deuterium and helium retentions of V–4Cr–4Ti alloy used as first wall of breeding blanket in a fusion reactor

The deuterium and helium retention properties of V–4Cr–4Ti alloy were investigated by thermal desorption spectroscopy (TDS). Ion energies of deuterium and helium were taken at 1.7 and 5 keV, respectively. The retained amount of deuterium in the sample irradiated at 380 K increased with the ion fluence and was not saturated to fluence of up to 1 × 10 23 D/m 2. For the irradiation at 773 K, 0.1% of implanted deuterium was retained at the highest fluence. For the helium ion irradiation at room temperature, three groups of desorption peaks appeared at around 500, 850, and 1200 K in the TDS spectrum. In the lower fluence region (<1 × 10 21 He/m 2), the retained helium desorbed mainly at around 1200 K. With increasing fluence, the amount desorbed at 500 K increased. Total amount of retained helium in the samples saturated at fluence up to 5 × 10 21 He/m 2 and saturation level was 2.7 × 10 21 He/m 2.

Effect of heat treatment on helium trapping in vanadium alloy at high ion implantation fluence

Helium was implanted into a V–4Ti alloy by using a 5keV helium ion beam with a flux of 1014He/cm2s, and then helium retention was analyzed by a technique of thermal desorption spectroscopy (TDS). The influence of heat treatment on helium trapping was studied by pre-annealing of the samples before ion irradiation. The annealing temperature was 973, 1223 and 1373K, respectively. Results indicated that pre-annealing treatments did not influence significantly the helium retention and release behavior in vanadium alloy on the present experimental conditions where the helium ion implantation fluence is in the region of 1017–1018 He/cm2, which shows a different tendency from the situation at a low fluence region (1013–1014 He/cm2). The reason could be due to different trap mechanisms of helium at different ion fluences.

Alternative Approaches to Modeling Fluence Distribution and Microbial Inactivation in Ultraviolet Reactors: Lagrangian versus Eulerian

A study was performed to evaluate alternative methods for predicting the ultraviolet (UV) reactor performance using computational fluid dynamics. The study consists of modeling the UV fluence distribution and microbial inactivation using either Lagrangian or Eulerian methods for both low- and medium-pressure UV reactors. In the Eulerian method, fluence distributions were calculated using a flow-weighted and a mass-weighted fraction technique. The results show that the Eulerian flow-weighted fraction fluence distribution agreed well with the Lagrangian particle tracking fluence distribution when applied to the UV reactor outlet plane. However, when applied to planes downstream from effluent hydraulic structures, the Eulerian fluence distribution method was influenced by the additional convective mixing from these hydraulic structures and predicts a tighter fluence distribution range than the Lagrangian method. The Eulerian approach to modeling microbial inactivation seems comparable to the Lagrangian particle tracking approach and can be viewed as a suitable alternative to the Lagrangian approach. The results also show that the Eulerian mass-weighted fraction distribution is comparable to the microbial kinetic weighted Lagrangian particle tracking approach, which can provide greater sensitivity to the low fluence regions in the UV reactor.

Low roughness laser etching of fused silica using an adsorbed layer

High quality etching of transparent materials for applications in microstructuring and precision engineering is still a challenge for current laser processing techniques. Laser etching at a surface adsorbed layer called LESAL allows precision etching of fused silica at low laser fluences with small etch rates. The etch rate and the surface quality depend on the applied laser fluence and can be divided into three regions with different etch behavior. The poor surface quality in the low and the high fluence regions are caused by incubation processes and thermal ablation processes, respectively. In the fluence range from 2.0 to 5.0 J/cm 2 a constant etch rate was observed that increases from 1 to 2 nm/pulse with raising the temperature from 60 to 105 °C. In this region of etch rate saturation a very low surface roughness of less than 1 nm rms was measured. Both the etch rate saturation and the low roughness of the LESAL process enable new applications of laser processing transparent materials.

Deuterium retention in V–4Cr–4Ti alloy after deuterium ion irradiation

The deuterium retention properties of vanadium alloy, V–4Cr–4Ti, were investigated by thermal desorption spectroscopy after deuterium ion irradiation. The deuterium ion irradiation was carried out at 380, 573 and 773 K with ion energy of 1.7 keV. Deuterium retained in the sample was desorbed in the forms of D 2, HD, HDO and D 2O. The amount of retained deuterium at 380 K increased with the ion fluence, and did not saturate to a fluence up to 1 × 10 19 D/cm 2. In addition, more than 100% of implanted D was retained at high deuterium fluence. The retained amount at 380 K was one and two orders of magnitude larger than graphite and tungsten, respectively. For the irradiation at 773 K, the amount of retained deuterium decreased with increase in ion fluence in the high fluence region, and the retained amount was almost the same as that for graphite or tungsten.

Formation and migration of helium bubbles in Fe and Fe–9Cr ferritic alloy

Formation and migration of helium bubbles in high purity Fe and an Fe–9Cr ferritic alloy have been studied by in situ electron microscopy and scanning transmission electron microscope–electron energy loss spectrometry (STEM–EELS) analysis. The swelling under irradiation with 10 keV He + ions in the ferritic alloy was retarded in the low fluence region at 400 °C and was reduced at 600 °C even under heavy irradiation. These results suggest trapping effects of vacancies by Cr and its modulation with helium atoms in the bubble formation. Furthermore, it is found that the mean square of the bubble migration distance in both materials is proportional to time, which is a quantitative evidence of Brownian type motion and yields lower diffusivities of the bubbles in Fe–9Cr than in Fe. The retardation of the bubble mobility in Fe–9Cr should be caused by Cr segregation on the bubble surface which was revealed by STEM–EELS analysis.

Influence of high-energy electron irradiation on the formation and annihilation of the photoluminescenceWcenter and the center’s origin in a proton-implanted silicon crystal

The population change of the photoluminescence (PL) W center (or I l center) with 2-MeV electron irradiation in proton-implanted silicon crystal was observed to investigate the origin of the center. While a straightforward annihilation of the W centers formed by proton implantation with an increase of the electron fluence was observed in the low fluence region, the number of centers increased in the high fluence region (>2 X 10 1 8 electron/cm 2 ) due to the predominance of formation over annihilation. The annihilation and formation of the W centers were analyzed as first and second order with respect to the numbers of vacancies and self-interstitials, respectively. The efficiency of the W-center formation from element pairs produced by electron irradiation was much smaller than that for ion implantation. Considering the findings obtained in the present study and those given by other studies, the split monointerstitial and the ST di-interstitial (ST: split triple) were chosen as the probable candidates for the W center.

Monte Carlo transport model comparison with 1A GeV accelerated iron experiment: heavy-ion shielding evaluation of NASA space flight-crew foodstuff

Deep-space manned flight as a reality depends on a viable solution to the radiation problem. Both acute and chronic radiation health threats are known to exist, with solar particle events as an example of the former and galactic cosmic rays (GCR) of the latter. In this experiment Iron ions of 1A GeV are used to simulate GCR and to determine the secondary radiation field created as the GCR-like particles interact with a thick target. A NASA prepared food pantry locker was subjected to the iron beam and the secondary fluence recorded. A modified version of the Monte Carlo heavy ion transport code developed by Zeitlin at LBNL is compared with experimental fluence. The foodstuff is modeled as mixed nuts as defined by the 71 st edition of the Chemical Rubber Company (CRC) Handbook of Physics and Chemistry. The results indicate a good agreement between the experimental data and the model. The agreement between model and experiment is determined using a linear fit to ordered pairs of data. The intercept is forced to zero. The slope fit is 0.825 and the R 2 value is 0.429 over the resolved fluence region. The removal of an outlier, Z=14, gives values of 0.888 and 0.705 for slope and R 2 respectively.

Backside etching of UV-transparent materials at the interface to liquids

KrF-excimer laser etching at the interface to liquids has been studied for the fabrication of well-defined micro-structures in UV-transparent materials. The etch rates and the surface morphologies of the investigated materials including oxides (fused silica, sapphire) and fluorides (CaF 2, MgF 2) depend on the laser parameters, the used solution, and the material itself. The etch threshold fluence and the etch rates show a dependency on the liquid solvent (acetone, cyclo-hexane, tetrachloroethylene) and the concentration of the dissolved pyrene absorber molecule (0–0.4 mol/l). For quartz and sapphire two fluence regions could be distinguished. In the low fluence region, up to 1.5 J/cm 2, low etch rates up to 25 and 20 nm/pulse were found, whereas in the high fluence region, up to 3.5 J/cm 2, the rates increase to 200 and 300 nm/pulse for fused silica and sapphire, respectively. The best machining results with low surface roughness and steep walls were achieved in fused silica in the low fluence range. The examination of etched crystalline samples by RBS spectroscopy shows an amorphization of the surface after the laser interaction. Mask projection and scanning contour mask techniques were applied for the fabrication of three-dimensional micro-structures.

Leaf sequencing with secondary beam blocking under leaf positioning constraints for continuously modulated radiotherapy beams

The creation of arbitrary photon fluence patterns for intensity modulated radiotherapy is addressed. The proposed method is intended for a class of multileaf collimators with a requirement for minimum leaf separation. Unlike the solution of Convery and Webb in which discrete beam intensity modulation was assumed, the present method deals with continuous modulation or that consisting of infinitely small bixels. The method begins with the time-optimal solution of Spirou-Stein-Svensson disregarding the minimum gap requirement. Subsequently, the gaps are restored by mobilizing the secondary beam blocking devices to prevent overexposure resulting from the leaf separation process. The secondary beam blocking is provided by means of two orthogonal backup diaphragms that are computer controlled. The results indicate that the method can be used to accurately deliver the desired modulation while satisfying the leaf positioning constraints. Furthermore, an example is presented which illustrates the efficacy of using the horizontal backup diaphragms (moving in perpendicular direction of the leaves) in addition to the vertical backup diaphragms (moving in the parallel direction of the leaves) to generate zero fluence regions.

Optical nonlinearities of inorganic metal cluster μ3-MoSe4Ag3(PPh3)3Cl

Optical nonlinearity and dynamic response of a novel inorganic metal cluster (μ3-MoSe4)Ag3(PPh3)3Cl were studied. Large optical limiting effect was observed with the inorganic metal cluster (μ3-MoSe4)Ag3(PPh3)3Cl dissolved in CH2Cl2. We also proved that nonlinear scattering contributes much to its optical limiting performance, which counts for nearly 75% of the total nonlinear energy loss used. The dynamic response of the cluster when pumped at high fluence was tested by a standard time-resolved pump-probe technique. The effective third-order susceptibility χ(3) for this sample was determined in a standard backward degenerate four wave mixing experiment with pump fluence lower than limiting threshold Fth. The conjugate signal intensity tends to saturate when the incident fluence rolls over this value (Fth) in spite of the fact that large optical limiting effect was observed in this fluence region, which implies that the nonlinearity mechanism is different for these two regions.

Experimental and numerical investigation of heat transfer and phase change phenomena during excimer laser interaction with nickel

This work investigates heat transfer and phase change phenomena during excimer laser interaction with nickel specimens. Based on time-resolved measurements in the laser fluence range between 2.5 J cm −2 and 10.5 J cm −2, it is found that surface evaporation occurs when the laser fluence is below 5.2 J cm −2. At a laser fluence of 5.2 J cm −2 or higher, explosive-type vaporization takes place. Numerical calculations show the maximum surface temperature reaches 0.84 T c at a laser fluence of 5.2 J cm −2, and 0.9 T c at a laser fluence of 5.9 J cm −2. The numerical results agree with the experiments on the mechanisms of materials removal in different laser fluence regions.

High Resolution Measurement of Excimer Laser Beam Profile by Using Resist Film.

The correlation between the light intensity fluctuation and the grain size distribution of excimer laser crystallization (ELC) is investigated using the negative type resist. The sensitivity is dependent on the concentration of the chromophore 3, 3′-diazidodiphenyl sulfone and molecular weight of the resin poly(p-vinylphenol). The γ -characteristics is linear in the fluence region less than 4OmJ/cm2 on 1 shot irradiation. The vertical resolution, or dynamic range, is 50:1 less than one fourth of CCD camera. The lateral resolution of resist is dependent on the resolution of measurement equipments, and can be guaranteed up to nanometer order by AFM.

A fast quantitative modelling of ns laser ablation based on non-stationary averaging technique

A semi-analytical approach to a quantitative analysis of thermal ns laser ablation is presented. The nonlinear heat equation is reduced to three ordinary differential equations for the surface temperature, spatial width of the enthalpy distribution, and the ablated depth. Due to its speed and flexibility, the method provides a convenient tool for the fast analysis of experimental data. The influence of different factors on ablation curves (ablated depth h vs. fluence φ) is studied. Analytical formulas for threshold fluence φ th and h( φ) dependences are discussed. The ablation curves reveal three regions of fluence: Arrhenius, linear, and the screening region. Small vaporization enthalpy results in a sub-linear but faster than logarithmic h( φ) dependence. Weakly-absorbing materials may exhibit two different ablation regimes – without or with ablation of the heated subsurface layer. The method is applied to the analysis of the single pulse ablation of polyimide. The single thermal evaporation energy ≈1.51 eV describes satisfactory the ablation at 248, 308 and 351 nm wavelengths.

Laser-induced periodic surface structure formation on polyethylene-terephthalate

Periodic surface structures generated by linearly polarized ArF excimer laser (193 nm) were studied on polyethylene–terephthalate (PET), in a narrow fluence region (3–5 mJ/cm 2). Atomic force microscopy was used to follow the morphological changes caused by subsequent laser pulses on the same part of the surface. A homemade kinematic base plate ensured the reproducible positioning of the sample with respect to the AFM and the laser beam. The main subsequent phases of the Laser-Induced Periodic Surface Structure (LIPSS) formation were identified as: laser light scattering on the originally existing granules, formation of elliptical structures around the granules, and the transformation of ellipses into ripple-shape modulation. The LIPSS generating feedback mechanism was described: the incoming beam and the beams scattered on the granules and already existing LIPSS interfere and cause spatially modulated melting and crystallisation, which develops the structure. The dependence of the LIPSS period on the wavelength and on the angle of incidence was studied. The structure's height and surface-roughness vs. the number of laser shots was measured. The experimental results are in good agreement with the theory of the self-organized diffraction gratings. It was proved experimentally that the LIPSS are formed by redistribution of the material on the surface, without significant material removal.

Photoluminescence study of silicon solar cells irradiated with large fluence electrons or protons

In order to investigate the anomalous degradation of space silicon solar cells which was found in large fluence region, photoluminescence measurements are carried out for the cells irradiated with 1 MeV electrons with a fluence exceeding 1×1016 e/cm2 and 10 MeV protons with a fluence exceeding 1×1013 p/cm2. For both irradiation, the intensity of boron-related bound exiton line decreases with fluence and it disappears at the fluences where the anomalous degradation occurs. The dominant defect is a complex of an interstitial carbon and an interstitial oxygen (CI–OI). The generation of five-vacancy-defects was also observed for the proton irradiation. Variations of photoluminescence line intensity are discussed in terms of displacement damage dose calculated based on non-ionizing energy loss (NIEL).

Lattice location and electrical conductivity in ion implanted TiO 2 single crystals

Single crystals of TiO 2 (rutile) were implanted at room temperature with Ar, Sn and W ions applying fluences of 10 15/cm 2 to 10 16/cm 2 at 300 keV. The lattice location, together with ion range and damage distribution was measured with Rutherford Backscattering and Channeling (RBS-C). The conductivity, σ, was measured as a function of temperature. The implanted Sn and W atoms were entirely substitutional on Ti sites in the applied fluence region, where the radiation damage did not yet reach the random level. A large σ increase was observed for all implants at displacement per atom values (dpa) below 1. Above dpa = 1, σ reveals a saturation value of 0.3 Ω −1 cm −1 for Ar implants, while for W and Sn implants a further increase of σ up to 30 Ω −1 cm −1 was measured. Between 70 K and 293 K ln σ was proportional to T −1 2 , (Ar,W) and T −1 4 (Sn), indicating that the transport mechanism is due to variable range hopping.