Gaussian Energy Distribution Research Articles

A Comparative Study of High-Power Diode Laser and CO2 Laser Surface Hardening of AISI 1045 Steel

The study investigates laser surface hardening in the AISI 1045 steel using two different types of industrial laser: a high-power diode laser (HPDL) and a CO2 laser, respectively. The effect of process parameters such as beam power, travel speed on structure, case depth, and microhardness was examined. In most cases, a heat-affected zone (HAZ) formed below the surface; a substantial increase in surface hardness was achieved. In addition, big differences were found between the hardened specimens after HPDL surface hardening and CO2 laser surface hardening. For HPDL, depths of the HAZ were almost equal in total HAZ o, without surface melting. For CO2 laser, the depths changed a lot in the HAZ, with surface melting in the center. To better understand the difference of laser hardening results when use these two types of laser, numerical (ANSYS) analysis of the heat conduction involved in the process was also studied. For HPDL method, a rectangular beam spot and uniform energy distribution across the spot were assumed, while for CO2 laser, a circular beam spot and Gaussian energy distribution were assumed. The results showed that the energy distribution variety altered the thermal cycles of the HAZ dramatically. The rectangular HPDL laser beam spot with uniform energy distribution is much more feasible for laser surface hardening.

DESIGN OF GAUSSIAN SPATIAL FILTER TO DETERMINE THE AMOUNT OF REFRACTION ERROR IN HUMAN EYE

The subjective method of eye testing (Myopia/Hyperopia) involves the communication between the optometrist and the Patient. Communication about the vision/clarity of an eye chart, which the Patient experiences, to the Optometrist and the communication which the Optometrist does to Patient would decide the final prescription. Though the Optometrist is expert in communication, if the Patient gives incorrect responses, it leads to inaccurate prescription. To overcome the above problem, the idea is to develop a novel eye testing method called “Patient Subjective Eye Testing (PSET)”, in which a Myopia/Hyperopia Patient interacts with a software application installed on a tablet. A standard eye chart is projected on to a screen. A similar eye chart is also displayed on the tablet. Now the Patient blurs the eye chart that is displayed on the tablet device by applying power in the units of 0.0625 diopters until he/she experiences no difference in the blurness of the optotypes in the projected chart and the displayed chart. The power is recorded and written in to prescription. For the realization of the above method, a Gaussian spatial filter is designed, which blurs the optotypes of the displayed eye chart similar to the blurness experienced in human eye. To design the filter, the amount of Gaussian blur per unit refraction error in human eye is determined by designing a human eye model using ZEMAX tool. For every unit of 0.0625 diopters ( in the range -2 diopters to +2 diopters) change in refraction power of the eye the amount of deviation in Gaussian energy distribution, on the retina, is determined. With these deviation values Gaussian masks are generated which are used by Gaussian filter to blur the eye chart images that are displayed on a tablet device.

Homogenization Optics to Improve Detectability of Fluorescence Response to a Single Laser Pulse: Detection of Feces on Apples

<abstract> <bold><sc>Abstract.</sc></bold> Fecal contamination of produce in fields is a known food safety risk. It is theoretically possible to enhance the current practice of visually inspecting fields for fecal material prior to harvest by using imaging to detect the fluorescence response of fecal material to UV excitation. For field application, an expanded-beam pulsed laser and a gated-intensified camera can be used for imaging to negate problems due to reflected ambient light masking fluorescent responses. A particular problem when using an expanded laser beam for illumination is that temporally inconsistent and spatially non-uniform energy distributions can produce false positives. This technical note describes an optical system that expands and homogenizes a high-energy laser beam using a lightpipe. To validate the homogenizing optics, images were acquired using both homogenizing optics and a simple optics system that did not incorporate a lightpipe. Sequential images acquired using uniform fluorescent targets demonstrated that temporal variability in illumination intensity for individual pixels was reduced when using the homogenizing optics. For single images, the Gaussian energy distribution seen when using simple optics was eliminated when using the homogenizing optics. To test how these technical advantages might affect the quality of acquired images in practice, apples artificially contaminated with cow feces were imaged using a multispectral adapter with 678 and 500 nm filters. Threshold detection rates for 1:1000 dilutions were 50% and 15% when using the homogenizing and simple optics, respectively. In addition, for 1:2 and 1:20 dilutions using 678/500 nm ratio images, the relative magnitude of the measured response was greater when using the homogenizing optics. It is recommended that homogenizing optics be used to expand pulsed laser beams that are used to excite fluorescence responses.

Selective laser melting of an Al86Ni6Y4.5Co2La1.5 metallic glass: Processing, microstructure evolution and mechanical properties

In this study, single line scans at different laser powers were carried out using selective laser meting (SLM) equipment on a pre-fabricated porous Al86Ni6Y4.5Co2La1.5 metallic glass (MG) preform. The densification, microstructural evolution, phase transformation and mechanical properties of the scan tracks were systematically investigated. It was found that the morphology of the scan track was influenced by the energy distribution of the laser beam and the heat transfer competition between convection and conduction in the melt pool. Due to the Gaussian distribution of laser energy and heat transfer process, different regions of the scan track experienced different thermal histories, resulting in a gradient microstructure and mechanical properties. Higher laser powers caused higher thermal stresses, which led to the formation of cracks; while low power reduced the strength of the laser track, also inducing cracking. The thermal fluctuation at high laser power produced an inhomogeneous chemical distribution which gave rise to severe crystallization of the MG, despite the high cooling rate. The crystallization occurred both within the heat affected zone (HAZ) and at the edge of melt pool. However, by choosing an appropriate laser power crack-free scan tracks could be produced with no crystallization. This work provides the necessary fundamental understanding that will lead to the fabrication of large-size, crack-free MG with high density, controllable microstructure and mechanical properties using SLM.

Influence of Blocking Effect and Energetic Disorder on Diffusion in One-dimensional Lattice

The diffusion in one-dimensional disordered lattice with Gaussian distribution of site and transition energies has been studied by mean of kinetic Monte-Carlo simulation. We focus on investigating the influence of energetic disorders and diffusive particle density on diffusivity. In single-particle case, we used both analytical method and kinetic Monte-Carlo simulation to calculate the quantities that relate to diffusive behavior in disordered systems such as the mean time between two consecutive jumps, correlation factor and diffusion coefficient. The calculation shows a good agreement between analytical and simulation results for all disordered lattice types. In many-particle case, the blocking effect results in decreasing correlation factor F and average time \(\tau _{jump}\) between two consecutive jumps. With increasing the number of particles, the diffusion coefficient \(D_{M}\) decreases for site-energy and transition-energy disordered lattices due to the F-effect affects stronger than \(\tau\)-effect. Furthermore, the blocking effect almost is temperature independent for both lattices.

Near-threshold 7Li(p,n)7Be neutrons on the practical conditions using thick Li-target and Gaussian proton energies for BNCT

The near threshold 7Li(p,n)7Be neutrons generated by incident proton energy having Gaussian distribution with mean energies from 1.85 to 1.95MeV, were studied as a practical neutron source for BNCT wherein an RFQ accelerator and a thick Li-target are used. Gaussian energy distributions with the standard deviation of 0, 10, 20 and 40keV for mean proton energies from 1.85 to 1.95MeV were surveyed in 0.01MeV increments. A thick liquid Li-target whose dimensions were established in our previous experiments (i.e., 1mm-thick with 50mm width and 50mm length) was considered in this study. The suitable incident proton energy and physical dimensions of Pb layer which serves as a gamma absorber and a Polyethylene layer which is used as a BDE were surveyed by means of the concepts of TPD. Dose distribution were calculated by using MCNP5. A proton beam with mean energy of 1.92MeV and a Gaussian energy distribution with a standard deviation of 20keV at a current of 10mA was selected from the viewpoint of irradiation time and practically achievable proton current. The suitable thicknesses of Pb gamma absorber was estimated to be about 3cm. The estimated thickness of the polyethylene BDE was about 24mm for an ideal proton current of 13mA, and was 18mm for a practical proton current of 10mA.

Gaussian Based Image Segmentation In The Presence Of Intensity Inhomogeneity

In this project a variational level set approach for bias correction and segmentation in the analysis of magnetic resonance (MR) images with intensity inhomogeneities is proposed. Local intensity variations in relatively smaller regions are separable, despite of the inseparability of the whole image. Local intensity variations are described by the Gaussian distributions with different mean and variance. In this work the objective functions are integrated over the entire domain with local Gaussian distribution of fitting energy, ultimately analyzing the data with a level set framework.

Thermal behavior and densification mechanism during selective laser melting of copper matrix composites: Simulation and experiments

Simulation of temperature distribution and densification process of selective laser melting (SLM) WC/Cu composite powder system has been performed, using a finite volume method (FVM). The transition from powder to solid, the surface tension induced by temperature gradient, and the movement of laser beam power with a Gaussian energy distribution are taken into account in the physical model. The effect of the applied linear energy density (LED) on the temperature distribution, melt pool dimensions, behaviors of gaseous bubbles and resultant densification activity has been investigated. It shows that the temperature distribution is asymmetric with respect to the laser beam scanning area. The center of the melt pool does not locate at the center of the laser beam but slightly shifts towards the side of the decreasing X-axis. The dimensions of the melt pool are in sizes of hundreds of micrometers and increase with the applied LED. For an optimized LED of 17.5kJ/m, an enhanced efficiency of gas removal from the melt pool is realized, and the maximum relative density of laser processed powder reaches 96%. As the applied LED surpasses 20kJ/m, Marangoni flow tends to retain the entrapped gas bubbles. The flow pattern has a tendency to deposit the gas bubbles at the melt pool bottom or to agglomerate gas bubbles by the rotating flow in the melt pool, resulting in a higher porosity in laser processed powder. The relative density and corresponding pore size and morphology are experimentally acquired, which are in a good agreement with the results predicted by simulation.

Investigation of positioning algorithm and method for increasing the linear measurement range for four-quadrant detector

The paper studied the relationship between the light spot position and the output voltage in the case of laser spot which is in the four-quadrant (4Q) detector whose shape is circle or elliptic, energy distribution uniform or Gaussian. The study is based on add–subtract algorithm, diagonal algorithm and difference over sum (Δ/∑) algorithm. By using methods of polynomial fitting it can get a simple polynomial expression to enhance measuring speed and the simulation error is given. A method of using Binary Optics Elements (BOE) to shape the laser beam in the 4Q detector to square light spot with uniform intensity distribution is proposed to expand linear measurement range. The simulation result is verified for a circular spot with Gaussian energy distribution.

Subdiffusive exciton motion in systems with heavy-tailed disorder

We study the transport of collective excitations (Frenkel excitons) in systems with static disorder in the transition energies, not limiting ourselves to Gaussian transition energy distributions. Instead, we generalize this model to the wider class of Lévy stable distributions, characterized by heavy tails. Phonon-assisted scattering of excitons, localized by the disorder, leads to thermally activated exciton motion. The time evolution of the second moment of the exciton distribution is shown to be sublinear, thus indicating that the exciton dynamics in such systems is not diffusive, but rather subdiffusive instead. The heavier the tail in the transition energy distribution is, the larger are the deviations from the diffusive regime. This from fluctuations of site energies larger than the exciton band width (outliers). We show that the occurrence of subdiffusive transport for heavy-tailed disorder distributions can be understood from the scattering rate distributions, which possess a (second) peak at zero scattering rate.

THE ABSORPTION AND CONDUCTIVITY FOR MICROSTRUCTURED SILICON IN THE IMPURITY-DEFECT MODEL

In this paper, a model for the optical absorption and carrier concentration in microstructured silicon is presented based on Gaussian energy distribution of defect and impurity, as well as the Boltzmann distribution of carriers. The parameters of the impurity and defect model contributing to the absorption and carrier concentration are investigated. The simulation results show that the absorption in the single level impurity-defect model ranges from 64% to 68%; larger than that in the single level impurity model (55%–65%); as well as that in the defect model (50%–66%). It also indicates that the absorption increases with bigger correlation energy (U) and the best bandwidth (d) is 0.92 eV. However, the carrier concentration gets stronger with bigger bandwidth, smaller correlation energy and higher temperature. Thus, these calculations would play an important role in the applications of microstructured silicon.

The role of an interface on Ni film removal and surface roughness after irradiation by femtosecond laser pulses

We have observed thin film removal from glass substrates after the irradiation of Ni films with femtosecond laser pulses in air. It was found that the material removal threshold and laser-induced morphology are dependent on film thickness. With decreasing thickness, material removal transitions from intra-film separation to removal at the Ni-glass interface. The Gaussian energy distribution of the laser pulse allows for intra-film separation in the annular region of the crater and interface separation in the center. We propose a model to explain these data as well as the observed increased surface roughness in the interfacial removal regions.

Beam characterization of a lab bench cold cathode ultra-soft X-ray generator

The aim of this work is to characterize the Ultra Soft X-ray (USX, 1.5keV, Al Kα) photon beam of a customized lab bench cold cathode generator. Within this generator, the electron beam is slowed down in a thin aluminium foil (16μm) supported by an easily exchangeable anode. It is shown that the thickness of the foil and the anode configuration determine the spatial distribution and the fluence rate of the photon beam, whereas accelerating voltage determines both fluence rate and energy spectrum feature. It is shown also that under specific operation parameters (i.e. accelerating voltage), a Gaussian energy distribution of the beam can be generated which is centred on the energy of the Al Kα line (1.5keV). Dosimetric films of GAFCHROMIC® HD-810 were used to estimate the photon fluence rate distribution of the beam. Its variation, when the generator acts as a monoenergetic source, was characterized with the two different configurations of the anode assembly. Finally, it is verified that the anode assembly consisting in a flat washer, on which the aluminium foil is set, acts as a simple point-source.

MODELING THE FREQUENCY DEPENDENCE OF RADIO BEAMS FOR CONE-DOMINANT PULSARS

Beam radii for cone-dominant pulsars follow a power-law relation with frequency, $\vartheta = (\nu/\nu_0)^k+\vartheta_0$, which has not yet well explained in previous works. We study this frequency dependence of beam radius (FDB) for cone-dominant pulsars by using the curvature radiation mechanism. Considering various density and energy distributions of particles in the pulsar open field line region, we numerically simulate the emission intensity distribution across emission height and rotation phase, and get integrated profiles at different frequencies and obtain the FDB curves. For the density model of a conal-like distribution, the simulated profiles always shrink to one component at high frequencies. In the density model with two separated density patches, the profiles always have two distinct components, and the power-law indices $k$ are found to be in the range from -0.1 to -2.5, consistent with observational results. Energy distributions of streaming particles have significant influence on the frequency-dependence behavior. Radial energy decay of particles are necessary to get proper $\vartheta_0$ in models. We conclude that by using the curvature radiation mechanism, the observed frequency dependence of beam radius for the cone-dominant pulsars can only be explained by the emission model of particles in two density patches with a Gaussian energy distribution and a radial energy loss.

Laser flash photolysis of benzophenone in thin silicone films

The decay kinetics of benzophenone (BP) triplet state in the thin (∼200nm) films of poly(dimethylsiloxane) have been studied with a nanosecond laser flash photolysis technique. Decay kinetics of a triplet state of BP (3BP) is dispersive kinetics, and it can be well described by the kinetic law with Gaussian distribution of the activation free energy. The average rate constants of 3BP decay kav are 1500 and 70s−1 for air saturated and deoxygenated films, respectively. Width of distribution are 1.2 and 4.3 (dimensionless) and correspond to the first and second kav, respectively presented above. The rate of hydrogen abstraction of H-atom from Si–CH3 groups by 3BP is low which results in long-lived 3BP in silicone films.

Formation of a plateau on time-of-flight curves of molecularly doped polymers

A program for numerical computation of the curves of the transient current in samples of molecularly doped polymers has been developed with allowance for the presence of a defective surface layer in them. Time-of-flight curves are calculated with the multiple-trapping model with a Gaussian energy distribution of traps. The model parameters are determined from the results of independent measurements. Numerical computations are in qualitative agreement with experimental data for a typical molecularly doped polymer. The features of the formation of a flat plateau in samples of different thickness are discussed in terms of the proposed model.

Trap states in ZnPc:C60 small-molecule organic solar cells

Trap states are known to be one of the key parameters limiting charge transport in organic semiconductors and hence the performance of organic solar cells. Here, small-molecule organic solar cells based on a bulk heterojunction between zinc-phtalocyanine (ZnPc) and the fullerene C60 are characterized according to their trapping nature by noninvasive methods and under ambient conditions. We show how impedance spectroscopy, applied to systematically varied device structures, reveals the trap localization as well as its occupation mechanisms. Further insight is given from investigations of different device working points and illumination intensities. Thus, we find the traps to be bulk states in the active layer with an electron-trapping nature. They can be described by a Gaussian energy distribution of 55 meV width, centered at 0.46 eV below the electron transport level and with a concentration of 3.5 $\ifmmode\times\else\texttimes\fi{}$ 10${}^{16}$ cm${}^{\ensuremath{-}3}$. Moreover, the trap states act as recombination centers in the presence of injected or photogenerated charge carriers. The results are confirmed by electrical simulations.

Tunneling dynamics of excitons in random semiconductor alloys

An analytical model describing the low-temperature tunneling dynamics of excitons in disordered systems is proposed, reproducing the time-resolved photoluminescence (TRPL) line shape and the temporal red-shift of the TRPL maximum of localized excitons in detail. Assuming a Gaussian energy distribution of the localized states, the observed asymmetric spectral line shape can be interpreted as the distribution of the lowest states within a certain tunneling volume. Using (Mg,Zn)O and Cd(S,Se) as model systems, the number of reachable states is determined from the time dependence of the photoluminescence signal and the density of localized states is estimated. For (Mg,Zn)O, we also reveal the exciton capture at donors and its influence on the TRPL transients.

Characterization of TC17 Titanium Alloy Treated by Square-Spot Laser Shock Peening

Laser shock peening (LSP) is widely known as a cold-worked surface treatment, and this technology has been to greatly improve the fatigue life of many metallic components. Our works focused on laser shock peening with Nd: glass laser system (pulse duration 30ns) and square laser spot size of 4mm×4mm for TC17 titanium alloy. Surface morphology, residual stresses and fatigue performance had been studied for TC17 alloy specimens and blades processed by LSP treatment. The results show that plastic strains in shocked dents become more homogeneous than ones produced by original circle spot with gaussian energy distribution. Surface residual stresses which measured using x-ray diffraction method showed different characteristic as varying specimen thickness, and LSP with overlapping ratio of 8% provided uniform residual stresses on peened surface. Low fluence peening which was implemented at borderline of peened surface was effective to diminish the stress gradient. Compared with mechanical shot peening, LSP attained smoother surface, lower microhardness and better fatigue performance. In a word, Square-spot LSP is an excellent way to improve fatigue life of titanium blade.

Deuteron spin–lattice relaxation in the presence of an activation energy distribution: Application to methanols in zeolite NaX

A new method is introduced for analyzing deuteron spin–lattice relaxation in molecular systems with a broad distribution of activation energies and correlation times. In such samples the magnetization recovery is strongly non-exponential but can be fitted quite accurately by three exponentials. The considered system may consist of molecular groups with different mobility. For each group a Gaussian distribution of the activation energy is introduced. By assuming for every subsystem three parameters: the mean activation energy E0, the distribution width σ and the pre-exponential factor τ0 for the Arrhenius equation defining the correlation time, the relaxation rate is calculated for every part of the distribution. Experiment-based limiting values allow the grouping of the rates into three classes. For each class the relaxation rate and weight is calculated and compared with experiment. The parameters E0, σ and τ0 are determined iteratively by repeating the whole cycle many times. The temperature dependence of the deuteron relaxation was observed in three samples containing CD3OH (200% and 100% loading) and CD3OD (200%) in NaX zeolite and analyzed by the described method between 20K and 170K. The obtained parameters, equal for all the three samples, characterize the methyl and hydroxyl mobilities of the methanol molecules at two different locations.