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Polarized Positron Beams via Intense Two-Color Laser Pulses
摘要： The generation of ultrarelativistic polarized positrons during the interaction of an ultrarelativistic electron beam with a counterpropagating two-color petawatt laser pulse is investigated theoretically. Our Monte Carlo simulation, based on a semiclassical model, incorporates photon emissions and pair productions, using spin-resolved quantum probabilities in the local constant field approximation, and describes the polarization of electrons and positrons for the pair production and photon emission processes, as well as the classical spin precession in between. The main reason for the polarization is shown to be the spin asymmetry of the pair production process in strong external fields, combined with the asymmetry of the two-color laser field. Employing a feasible scenario, we show that highly polarized positron beams, with a polarization degree of ζ ≈ 60%, can be produced in a femtosecond timescale, with a small angular divergence, ～74 mrad, and high density, ～1014 cm?3. The laser-driven polarized positron source raises hope for providing an alternative for high-energy physics studies.
关键词： polarized positron beams,ultrarelativistic electron beam,local constant field approximation,spin-resolved quantum probabilities,high-energy physics,Monte Carlo simulation,two-color laser pulses
更新于2025-09-11 14:15:04
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Microwave-Assisted Synthesis of High-Energy Faceted TiO2 Nanocrystals Derived from Exfoliated Porous Metatitanic Acid Nanosheets with Improved Photocatalytic and Photovoltaic Performance
摘要： A facile one-pot microwave-assisted hydrothermal synthesis of rutile TiO2 quadrangular prisms with dominant {110} facets, anatase TiO2 nanorods and square nanoprisms with co-exposed {101}/[111] facets, anatase TiO2 nanorhombuses with co-exposed {101}/{010} facets, and anatase TiO2 nanospindles with dominant {010} facets were reported through the use of exfoliated porous metatitanic acid nanosheets as a precursor. The nanostructures and the formation reaction mechanism of the obtained rutile and anatase TiO2 nanocrystals from the delaminated nanosheets were investigated. The transformation from the exfoliated metatitanic nanosheets with distorted hexagonal cavities to TiO2 nanocrystals involved a dissolution reaction of the nanosheets, nucleation of the primary [TiO6]8? monomers, and the growth of rutile-type and anatase-type TiO2 nuclei during the microwave-assisted hydrothermal reaction. In addition, the photocatalytic activities of the as-prepared anatase nanocrystals were evaluated through the photocatalytic degradation of typical carcinogenic and mutagenic methyl orange (MO) under UV-light irradiation at a normal temperature and pressure. Furthermore, the dye-sensitized solar cell (DSSC) performance of the synthesized anatase TiO2 nanocrystals with various morphologies and crystal facets was also characterized. The {101}/[111]-faceted pH2.5-T175 nanocrystal showed the highest photocatalytic and photovoltaic performance compared to the other TiO2 samples, which could be attributed mainly to its minimum particle size and maximum specific surface area.
关键词： high-energy facets,photocatalytic activity,photovoltaic performance,anatase TiO2 nanocrystals
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Efficient Operation of a High Energy Yb:YAG DPSSL Amplifier
摘要： Improving the efficiency and scaling the pulse rate of high energy Yb:YAG nanosecond pulsed diode pumped solid state laser (DPSSL) amplifiers is vital to unlock their potential for practical applications. Applications include industrial materials processing, sources for high density matter compression experiments, and as pump sources for PW-class amplifiers, which can themselves generate high- brightness secondary radiation and ion sources for remote imaging and medical applications.
关键词： efficiency,DPSSL,high energy,pulse rate,Yb:YAG
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Multipass Spectral Broadening with Tens of Millijoule Pulse Energy
摘要： Nonlinear spectral broadening is widely used for applications like attosecond pulse generation, XUV sources, supercontinuum generation or ultrafast spectroscopy. In these cases the laser is typically spectrally broadened and compressed to reach pulse durations much shorter than originally available. For pulse energies in the millijoule regime, spectral broadening is routinely implemented using capillary optical fibers with remarkable results in terms of output pulse durations and beam quality. In addition to fiber-based approaches, the application of multipass cells for spectral broadening recently attracted increasing interest. These cells are capable of preserving a nearly Gaussian mode in the presence of nonlinear effects without a spatial filter or optical fiber, they are robust to input pointing, have a high throughput of more than 90% and can be used with high average powers and energies. In a multipass cell the nonlinear interaction with either gas or a thin solid provides a small amount of spectral broadening within each pass. Based on this concept we recently demonstrated output energies of almost 18 mJ by guiding near infrared pulses with a pulse duration of 1.3 ps and a repetition rate of 5 kHz through an argon filled multipass cell. We obtained a broadening factor of over 33 and could show compressibility down to 41 fs. The energy was limited by the breakdown intensity of argon and the damage threshold of the used optics. To increase the throughput energy, we developed a new mirror coating with a damage threshold measured to be about 50% higher than the coating used in [4]. The setup length was increased from 3m to 8m leading to about 170% bigger beam areas. Output energies of 39.5 mJ with a broadening factor over 38 (output bandwidth 85nm at 20 dB, output fourier limit 34.2 fs) are reached in first experiments by focusing a 1.3 ps pulse 43 times in 250 mBar of argon (cf. Fig. 1 and Fig. 2). The beam profile after propagation depicted as inset in Fig. 2 keeps its nearly Gaussian shape indicating a preserved beam quality. The throughput is well above 95% with an output spectrum typical for self-phase modulation. Ongoing optimization is expected to lead to even higher throughput energies. This laser source will combine pulse durations below 40 fs with high energies and kilohertz repetition rate and can be used as driver laser for an all-laser-driven Thomson X-ray source yielding a significant increase of photon flux. Moreover this technique is widely applicable to different wavelength ranges, input energies and input powers turning it into a promising scalable alternative to fiber based spectral broadening.
关键词： multipass cells,Nonlinear spectral broadening,pulse compression,high-energy lasers
更新于2025-09-11 14:15:04
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Performance of active edge pixel sensors
摘要： To cope with the High Luminosity LHC harsh conditions, the ATLAS inner tracker has to be upgraded to meet requirements in terms of radiation hardness, pile up and geometrical acceptance. The active edge technology allows to reduce the insensitive area at the border of the sensor thanks to an ion etched trench which avoids the crystal damage produced by the standard mechanical dicing process. Thin planar n-on-p pixel sensors with active edge have been designed and produced by LPNHE and FBK foundry. Two detector module prototypes, consisting of pixel sensors connected to FE-I4B readout chips, have been tested with beams at CERN and DESY. In this paper the performance of these modules are reported. In particular the lateral extension of the detection volume, beyond the pixel region, is investigated and the results show high hit e?ciency also at the detector edge, even in presence of guard rings.
关键词： Particle tracking detectors,Large detector systems for particle and astroparticle physics,Performance of High Energy Physics Detectors
更新于2025-09-10 09:29:36
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Monte Carlo methods for TMD analyses
摘要： Monte Carlo simulations are an indispensable tool in experimental high-energy physics. Indeed, many discoveries rely on realistic modeling of background processes. In the field of transverse-momentum-dependent parton distribution and fragmentation functions there is a clear lack of a reliable Monte Carlo physics generator that can be used in experimental and phenomenological analyses. The need for such Monte Carlo generators, the status of some solutions and prospects are discussed.
关键词： Monte Carlo simulations,fragmentation functions,transverse-momentum-dependent parton distribution,high-energy physics
更新于2025-09-10 09:29:36
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Effect of Milling on the Densification of SiC-BasedComposites from Polysilazane
摘要： High density SiC-based composites from polysilazane were fabricated by high energy milling and hot pressing. After cross-linking at 200 °C, the polysilazane was pyrolysed at 1000 °C in N2 for 2 h to form amorphous Si-C-N-O followed by high energy milling for 1 h. Milled amorphous Si-C-N-O was sintered in hot pressing at 1600 and 1700 °C for 1 h in vacuum under the applied pressure of 50 MPa. Although no sintering additives were used, dense SiC-based composites were obtained with this method. Sintered density was 3.04 g/cm3, while only 2.57 g/cm3 for amorphous Si-C-N-O without milling. It is suggested that the liquid phase generated during the formation of Si2N2O facilitated the densification of the composite.
关键词： polysilazane,Si2N2O,high energy milling,SiC-based composites
更新于2025-09-10 09:29:36
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High-Energy Charge-Separated States by Reductive Electron Transfer Followed by Electron Shift in Tetraphenylethylene – Aluminum(III) Porphyrin – Fullerene Triad
摘要： A high potential supramolecular triad (TPE-AlPor?Im-C60) composed of aluminum(III) porphyrin (AlPor), fullerene (C60), and tetraphenylethylene (TPE) has been constructed. The fullerene and tetraphenylethylene units are bound axially to opposite faces of the porphyrin plane via coordination and covalent bonds, respectively. The ground and excited-state properties of the triad and reference dyads are studied using steady-state and time-resolved spectroscopic techniques. The transient data show that photoexcitation results in charge separation (CS) from tetraphenylethylene to the excited singlet state of the porphyrin (1AlPor*), generating high-energy (2.14 eV) charge-separated state, (TPE)?+-(AlPor)?? in toluene. A subsequent electron migration from the AlPor?? to fullerene generates a second high-energy (1.78 eV) charge-separated state (TPE)?+-AlPor?Im-(C60)??. The lifetime of the charge separation persists about 25 ns. The high energy stored in the form of charge-separated states along with its reasonable lifetimes makes these donor-acceptor systems potential electron-transporting catalysts to carry out energy-demanding photochemical reactions, especially in artificial photosynthesis for conversion of solar energy into chemical energy.
关键词： high-energy charge-separated states,aluminum(III) porphyrin,tetraphenylethylene,fullerene,reductive electron transfer,electron shift,triad
更新于2025-09-09 09:28:46
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Optimization and Characterization of High-Harmonic Generation for Probing Solid Density Plasmas
摘要： The creation of high energy density plasma states produced during laser–solid interaction on a sub-picosecond timescale opens a way to create astrophysical plasmas in the lab to investigate their properties, such as the frequency-dependent refractive index. Available probes to measure absorption and phase-changes given by the complex refractive index of the plasma state are extreme-UV (EUV) and soft X-ray (XUV) ultra-short pulses from high harmonic generation (HHG). For demanding imaging applications such as single-shot measurements of solid density plasmas, the HHG probe has to be optimized in photon number and characterized in intensity and wavefront stability from shot-to-shot. In an experiment, a coherent EUV source based on HHG driven by a compact diode-pumped laser is optimized in photons per pulse for argon and xenon, and the shot-to-shot intensity stability and wavefront changes are characterized. The experimental results are compared to an analytical model estimating the HHG yield, showing good agreement. The obtained values are compared to available data for solid density plasmas to con?rm the feasibility of HHG as a probe.
关键词： pump-probe,XUV,diode-pumped lasers,high energy density physics,high-harmonic generation
更新于2025-09-04 15:30:14
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Encyclopedia of Spectroscopy and Spectrometry || Ion Collision, Theory ☆
摘要： Collisions between an ion and neutral species result in a number of possible outcomes depending upon the chemical and physical properties of the two reactants, their relative velocities, and the impact parameter of their trajectories. These include elastic and inelastic scattering of the colliding particles, charge transfer (including dissociative charge transfer), atom abstraction, complex formation and dissociation of the colliding ion. Each of these reactions may be characterized in terms of their energy-dependent rate coefficients, cross sections and reaction kinetics. This article outlines a theoretical framework for discussing these processes that emphasizes simple models and classical mechanics. The discussion of collision processes has been divided into two categories: low-energy and high-energy thermal or quasi-thermal collisions. Experiments under conditions – swarms, drift tubes, chemical ionization and ion cyclotron resonance – are strongly influenced by long-range forces and often involve ‘capture collisions’ in which atom exchange and extensive energy exchange are common characteristics. High-energy collisions are typically impulsive, involve short-range intermolecular forces and are direct, fast processes.
关键词： Low-energy Collisions,Elastic and Inelastic Scattering,Reaction Kinetics,Theory,Complex Formation,Rate Coefficients,Charge Transfer,Atom Abstraction,High-energy Collisions,Ion Collision,Dissociation,Cross Sections
更新于2025-09-04 15:30:14