- 标题
- 摘要
- 关键词
- 实验方案
- 产品
-
Path-integral Monte Carlo study of electronic states in quantum dots in an external magnetic field
摘要： We explore the correlated electron states in harmonically confined few-electron quantum dots in an external magnetic field by the path-integral Monte Carlo method for a wide range of the field and the Coulomb interaction strength. Using the phase structure of a preceding unrestricted Hartree-Fock calculation for phase fixing, we find a rich variety of correlated states, often completely different from the prediction of mean-field theory. These are finite temperature results, but sometimes the correlations saturate with decreasing temperature, providing insight into the ground-state properties.
关键词： magnetic field,electron correlations,Hartree-Fock,quantum dots,path-integral Monte Carlo
更新于2025-09-11 14:15:04
-
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
-
Monte Carlo simulation of a transversely excited solar-pumped fiber laser
摘要： A Monte Carlo ray-tracing simulation was developed for a transversely excited solar-pumped fiber laser without focusing optics. To make this possible, the fiber is immersed in a liquid sensitizer and sandwiched between highly reflective mirrors. The top reflector is dichroic to transmit solar radiation but reflect fluorescence to confine photons that match the absorption band of the active fiber. Simulation was used to evaluate the validity of the concept and optimize device performance. For comparison with the calculations, preliminary experiments were conducted by illuminating a 30 cm aperture, laser module with a solar simulator. The observed gains were in good agreement with the calculations for various conditions, such as sensitizer concentration and mirror reflectivity. Finally, we show that the predicted output power reaches 29 mW when the fiber length is optimized, and it will be enhanced to 150 mW with 0.21% solar-to-laser efficiency under the assumption of a reabsorption-free sensitizer.
关键词： unconcentrated,dichroic mirror,Monte Carlo simulation,sensitizer,solar-pumped fiber laser,transversely excited
更新于2025-09-11 14:15:04
-
Species discrimination and individual tree detection for predicting main dendrometric characteristics in mixed temperate forests by use of airborne laser scanning and ultra-high-resolution imagery
摘要： This study aims to investigate the combined use of two types of remote sensing data — ALS derived and digital aerial photogrammetry data (based on imagery collected by airborne UAV sensors) — along with intensive field measurements for extracting and predicting tree and stand parameters in even-aged mixed forests. The study is located in South West Romania and analyzes data collected from mixed-species plots. The main tree species within each plot are Norway spruce (Picea abies L. Karst.) and Beech (Fagus sylvatica L.). The ALS data were used to extract the digital terrain model (DTM), digital surface model (DSM) and normalized canopy height model (CHM). Object-Based Image Analysis (OBIA) classification was performed to automatically detect and separate the main tree species. A local filtering algorithm with a canopy-height based variable window size was applied to identify the position, height and crown diameter of the main tree species within each plot. The filter was separately applied for each of the plots and for the areas covered with Norway spruce and beech trees, respectively (i.e. as resulted from OBIA classification). The dbh was predicted based on ALS data by statistical Monte Carlo simulations and a linear regression model that relates field dbh for each tree species with their corresponding ALS-derived tree height and crown diameter. The overall RMSE for each of the tree species within all the plots was 5.8 cm for the Norway spruce trees, respectively 5.9 cm for the beech trees. The results indicate a higher individual tree detection rate and subsequently a more precise estimation of dendrometric parameters for Norway spruce compared to beech trees located in spruce-beech even-aged mixed stands. Further investigations are required, particularly in the case of choosing the best method for individual tree detection of beech trees located in temperate even-aged mixed stands.
关键词： Monte Carlo simulation,ALS,Forest inventory,UAV,OBIA
更新于2025-09-11 14:15:04
-
Scale Accuracy Evaluation of Image-Based 3D Reconstruction Strategies Using Laser Photogrammetry
摘要： Rapid developments in the field of underwater photogrammetry have given scientists the ability to produce accurate 3-dimensional (3D) models which are now increasingly used in the representation and study of local areas of interest. This paper addresses the lack of systematic analysis of 3D reconstruction and navigation fusion strategies, as well as associated error evaluation of models produced at larger scales in GPS-denied environments using a monocular camera (often in deep sea scenarios). Based on our prior work on automatic scale estimation of Structure from Motion (SfM)-based 3D models using laser scalers, an automatic scale accuracy framework is presented. The confidence level for each of the scale error estimates is independently assessed through the propagation of the uncertainties associated with image features and laser spot detections using a Monte Carlo simulation. The number of iterations used in the simulation was validated through the analysis of the final estimate behavior. To facilitate the detection and uncertainty estimation of even greatly attenuated laser beams, an automatic laser spot detection method was developed, with the main novelty of estimating the uncertainties based on the recovered characteristic shapes of laser spots with radially decreasing intensities. The effects of four different reconstruction strategies resulting from the combinations of Incremental/Global SfM, and the a priori and a posteriori use of navigation data were analyzed using two distinct survey scenarios captured during the SUBSAINTES 2017 cruise (doi: 10.17600/17001000). The study demonstrates that surveys with multiple overlaps of nonsequential images result in a nearly identical solution regardless of the strategy (SfM or navigation fusion), while surveys with weakly connected sequentially acquired images are prone to produce broad-scale deformation (doming effect) when navigation is not included in the optimization. Thus the scenarios with complex survey patterns substantially benefit from using multiobjective BA navigation fusion. The errors in models, produced by the most appropriate strategy, were estimated at around 1% in the central parts and always inferior to 5% on the extremities. The effects of combining data from multiple surveys were also evaluated. The introduction of additional vectors in the optimization of multisurvey problems successfully accounted for offset changes present in the underwater USBL-based navigation data, and thus minimize the effect of contradicting navigation priors. Our results also illustrate the importance of collecting a multitude of evaluation data at different locations and moments during the survey.
关键词： navigation fusion,laser scalers,uncertainty estimation,structure-from-motion,Monte Carlo simulation,laser spot detection,metrology,underwater 3D reconstruction,multiobjective BA,scale drift evaluation,photogrammetry
更新于2025-09-11 14:15:04
-
Monte Carlo model incorporating many-body effects for determining the gain spectra of quantum dot lasers
摘要： We present a Monte Carlo model that simulates the gain spectra of a QD laser material that empirically includes free-carrier effects. We compare simulation results of both Fermi–Dirac and random carrier populations, and compare them with experimental data. The free-carrier effects are highlighted as being more important than the choice of carrier statistics, and routes to improve this simple model are discussed.
关键词： many-body effects,free-carrier effects,Monte Carlo model,gain spectra,quantum dot lasers
更新于2025-09-11 14:15:04
-
Ultrafast polarization of an electron beam in an intense bichromatic laser field
摘要： Here, we demonstrate the radiative polarization of high-energy electron beams in collisions with ultrashort pulsed bichromatic laser fields. Employing a Boltzmann kinetic approach for the electron distribution allows us to simulate the beam polarization over a wide range of parameters and determine the optimum conditions for maximum radiative polarization. Those results are contrasted with a Monte Carlo algorithm where photon emission and associated spin effects are treated fully quantum mechanically using spin-dependent photon emission rates. The latter method includes realistic focusing laser fields, which allows us to simulate a near-term experimentally feasible scenario of an 8 GeV electron beam scattering from a 1 PW laser pulse and provide a measurement that would verify the ultrafast radiative polarization in high-intensity laser pulses that we predict. Aspects of spin-dependent radiation reaction are also discussed, with spin polarization leading to a measurable (5%) splitting of the energies of spin-up and spin-down electrons.
关键词： spin-dependent radiation reaction,Boltzmann kinetic approach,radiative polarization,Monte Carlo algorithm,electron beams,bichromatic laser fields
更新于2025-09-11 14:15:04
-
Kinetic Monte Carlo Study of the Role of the Energetic Disorder on the Open-circuit Voltage in Polymer:Fullerene Solar Cells
摘要： One major factors limiting the e?ciency in organic solar cells (OSCs) is the low open-circuit voltage (Voc). Existing theoretical studies link the Voc with the charge transfer (CT) state and non-radiative recombination. However, also morphology and energetic disorder can have a strong impact on the Voc within realistic bulk-heterojunction OSCs. In this work, we present a kinetic Monte Carlo study on the role of the energetic disorder on the maximum Voc. We compute the quasi-Fermi level splitting for di?erent energetic disorder and analyze the impact of the energetic disorder at the donor-acceptor interface as well as correlations in the site energies on the Voc. Our results show that the interface strongly controls the maximum Voc. For a higher interface disorder, charge densities and non-geminate recombination increases, and the Voc is reduced. Furthermore, the correlated morphologies show an increase in the maximum Voc and a reduced impact of the energetic disorder.
关键词： Open-circuit Voltage,Kinetic Monte Carlo,Energy Conversion and Storage,Plasmonics and Optoelectronics,Polymer:Fullerene Solar Cells
更新于2025-09-11 14:15:04
-
[IEEE 2018 11th International Symposium on Communication Systems, Networks and Digital Signal Processing (CSNDSP) - Budapest (2018.7.18-2018.7.20)] 2018 11th International Symposium on Communication Systems, Networks & Digital Signal Processing (CSNDSP) - Spatial Interpolation of Optical Wireless Impulse Responses
摘要： In this paper, spatial interpolation is proposed as a time-saving technique for the estimation of the different Optical Wireless Impulse Responses which result from the nodes’ position changes in the mobility modeling of wireless optical communications. Typically, in this kind of analysis, the time-dependent impulse response is calculated numerically at each point of the path. However, this approach may result prohibitive due to the computation time. In this work, a support domain distributed on the moving area is defined, and accurate impulse responses for the domain’s points are obtained using a Monte Carlo Ray Tracing algorithm. Then, using this support, Lagrange interpolation method is applied for quick calculation out-of-the-support positions’ impulse responses. In this way, the impulse response for those points included in nodes’ path can be obtained without a high time and computational cost. The Lagrange interpolants were essayed with successful results, suggesting that the proposed technique may become relevant in upcoming statistical modeling in mobility situations.
关键词： Spatial Interpolation,Impulse Response,Lagrange Interpolation,Monte Carlo Ray Tracing,Optical Wireless Communication
更新于2025-09-11 14:15:04
-
Photoneutronic aspects of the molybdenum-99 production by means of electron linear accelerators
摘要： Monte Carlo simulations were used to study photonuclear production of Molybdenum-99 isotope by means of electron linear accelerators. Simulation showed that direct production in the cylindrical target provides higher values of specific activity compared to production using separate bremsstrahlung converter. For 50 MeV 100 kW electron beam we obtained the maximum value of 61 Ci/g specific activity of produced Molybdenum-99 at saturation. Calculations for the targets with optimized shape showed that even higher levels of specific activity are achievable: for optimized target the calculated value of Molybdenum-99 specific activity at saturation exceeds values of 145.89 Ci/g for setup with bremsstrahlung converter and 170.07 Ci/g without the converter.
关键词： Electron linear accelerator,Mo-99 production,Monte Carlo simulation,Medical isotopes,PHITS
更新于2025-09-11 14:15:04