In this work, we performed an experimental investigation supported by a theoretical analysis, to improve knowledge on the laser ablation of silicon with THz bursts of femtosecond laser pulses. Laser ...ablated craters have been created using 200 fs pulses at a wavelength of 1030 nm on silicon samples systematically varying the burst features and comparing to the normal pulse mode (NPM). Using bursts in general allowed reducing the thermal load to the material, however, at the expense of the ablation rate. The higher the number of pulses in the bursts and the lower the intra-burst frequency, the lower is the specific ablation rate. However, bursts at 2 THz led to a higher specific ablation rate compared to NPM, in a narrow window of parameters. Theoretical investigations based on the numerical solution of the density-dependent two temperature model revealed that lower lattice temperatures are reached with more pulses and lower intra-burst frequencies, thus supporting the experimental evidence of the lower thermal load in burst mode (BM). This is ascribed to the weaker transient drop of reflectivity, which suggests that with bursts less energy is transferred from the laser to the material. This also explains the trends of the specific ablation rates. Moreover, we found that two-photon absorption plays a fundamental role during BM processing in the THz frequency range.
Understanding the differences between photon-induced and plasmon-induced hot electrons is essential for the construction of devices for plasmonic energy conversion. The mechanism of the plasmonic ...enhancement in photochemistry, photocatalysis, and light-harvesting and especially the role of hot carriers is still heavily discussed. The question remains, if plasmon-induced and photon-induced hot carriers are fundamentally different or if plasmonic enhancement is only an effect of field concentration producing these carriers in greater numbers. For the bulk plasmon resonance, a fundamental difference is known, yet for the technologically important surface plasmons, this is far from being settled. The direct imaging of surface plasmon-induced hot carriers could provide essential insight, but the separation of the influence of driving laser, field-enhancement, and fundamental plasmon decay has proven to be difficult. Here, we present an approach using a two-color femtosecond pump–probe scheme in time-resolved 2-photon-photoemission (tr-2PPE), supported by a theoretical analysis of the light and plasmon energy flow. We separate the energy and momentum distribution of the plasmon-induced hot electrons from that of photoexcited electrons by following the spatial evolution of photoemitted electrons with energy-resolved photoemission electron microscopy (PEEM) and momentum microscopy during the propagation of a surface plasmon polariton (SPP) pulse along a gold surface. With this scheme, we realize a direct experimental access to plasmon-induced hot electrons. We find a plasmonic enhancement toward high excitation energies and small in-plane momenta, which suggests a fundamentally different mechanism of hot electron generation, as previously unknown for surface plasmons.
Abstract
The meteorological conditions in which organogenesis stages take place and hydrothermal regime can determine the phenotypic manifestation of quantitative signs, such as, for example, the ...activity of alpha-amylase. The purpose of our work was to determine what factors (temperature, precipitation, heterothermal coefficient) affect the enzyme activity, at what time of the vegetation, and in what way this influence is evident. The analysis of weather conditions and falling number (FN) for the period from 2011 to 2020 was carried out. Analysis of the FN value over 10 years showed that it can vary from 90 s to 429 s, and at that, the nature of its change is the same, regardless of whether we are considering one variety or the average value for a group of varieties. The correlation coefficient between the FN of a group of varieties and individual FN is 0.94-0.98. Generally, during the vegetation season, the alpha-amylase activity was influenced to a greater extent by the amount of precipitation than by temperature. This dependence is negative - an increase in precipitation leads to a decrease in the FN, and, consequently, to an increase in the enzyme activity. However, it was found that in different phases of ontogeny the influence of temperature and precipitation can be diametrically opposite.
Understanding the mechanisms and controlling the possibilities of surface nanostructuring is of crucial interest for both fundamental science and application perspectives. Here, we report a direct ...experimental observation of laser-induced periodic surface structures (LIPSS) formed near a predesigned gold step edge following single-pulse femtosecond laser irradiation. Simulation results based on a hybrid atomistic-continuum model fully support the experimental observations. We experimentally detect nanosized surface features with a periodicity of ∼300 nm and heights of a few tens of nanometers. We identify two key components of single-pulse LIPSS formation: excitation of surface plasmon polaritons and material reorganization. Our results lay a solid foundation toward simple and efficient usage of light for innovative material processing technologies.
Industrial use of ultrashort pulse surface structuring would significantly increase by effective utilization of the average laser powers available currently. However, the unexplained degradation of ...surfaces processed with numerous pulses at high average laser power makes this difficult. Based on a systematic experimental study, the structure formation underlying such surface degradation is investigated. Furthermore, a hierarchical structural formation model that bridges the gap between laser‐induced periodic surface structures and surface degradation is presented. Contrary to expectations based on previous research, less structure formation on titanium was observed for higher laser fluences. As a possible reason, enhanced electron diffusion with increasing intensity is investigated within the framework of the two‐temperature model. Our findings provide a deeper understanding of the microscopic mechanisms involved in surface structuring with ultrashort pulses.
The authors explain the microscopic origin of bumpy surfaces limiting the ablation rates in ultrashort pulse laser micromachining. Experimental observations supported by theoretical modeling reveal the origin of these bumps to be in an inhomogeneous energy deposition inducing periodic surface structures. They gradually rise with increasing pulse number. Convective growth of surface grooves leads to the final structures.
• The developed method for predicting X-ray properties of the polymer. • Higher content of the fillers results in an increase of mechanical properties. • X-ray defensive properties of the samples ...were investigated experimentally.
Polymers are a base for creating of composite materials with high mechanical and chemical properties. Using the heavy metals as filler in these composites can give them X-ray protective properties. These materials have high deactivation rates and can be used to create Personal Protective Equipment (PPE) used in aggressive environments. It was proposed a model for calculation of X-ray protection properties of the polymer-based nanocomposite materials with ultra-high molecular weight polyethylene (UHMWPE) matrix, filled with tungsten and boron carbide particles. X-ray protective properties were calculated in a wide range of filler content using the developed model. Results of calculations allow selecting most effective compounds of X-ray protective UHMWPE based composites.
The uniqueness of the nuclear fuel cycle facilities and their equipment, which must be adjusted to the specific conditions of manufacturing and (or) reprocessing the fuel of a particular reactor type ...in each case, complicates the application of traditional methods for assessing reliability and safety, thus relying on the availability of a significant operating experience. As a result, at the design stage, the reliability level of such equipment is determined by the available data on the reliability of analog elements. Following completion of commissioning of a nuclear fuel cycle facility, the regulatory requirements demand the updating of reliability and safety calculations to bring them in line with the actual state of the facility based on the results of the construction, commissioning, as well as pilot- and pilot-industrial operation. The article presents a universal method for accounting for the statistics of equipment operation at a high level of reliability in the absence of representative samples during the first years of its operation, which is compensated by the use of prior information within the framework of the Bayesian approach. The simulation of the time between failures is demonstrated using the synthesis of the prior information and the results of prototype bench tests for 5 years.
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