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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) - Structured Auxiliary Mesh (SAM) Algorithm for Opto-Thermal Simulation of Laser-Based Lighting Systems
摘要： Laser-based lighting systems are an emerging technology, the next step in solid state lighting that revolutionized the way artificial light is generated. The configuration of interest here is the laser-excited remote phosphor (LRP) scheme that consists of a laser diode as the excitation source of an appropriately chosen phosphor sheet. The phosphor is employed for the down-conversion of the incident laser light and broadening of the output spectrum. Although some commercial applications have already been developed, the optimization of LRP systems has yet to be achieved. A bottleneck in their performance is the thermal dependency of the phosphor’s emission characteristics, a phenomenon also known as thermal quenching. As a result, the need for an opto-thermal simulation strategy arises that will enable the study and optimization of LRP systems [1]. The opto-thermal simulation model discussed here is based on Monte Carlo simulations for the optical part, where the absorbed radiant flux is calculated. These optical thermal losses are subsequently used as a volume heat source to solve the transient heat equation by applying the finite element method (FEM) [2]. As thermal quenching is a time-dependent phenomenon in nature, this is an iterative procedure, where the absorbed flux must be calculated for most of the time steps. Typical Monte Carlo ray tracing algorithms use voxel-based meshes to store any calculated properties / attributes. The problem that arises here is that the computational cost for converting the voxel-based mesh to an FEM mesh would be prohibiting for time-dependent analysis. The solution is to directly store the absorbed flux to the FEM mesh. However, the issue that now emerges is locating the interpolating point, namely the point in space where the absorption occurs, within the unstructured FEM mesh. The processing time of a brute force search would be too long, so more sophisticated solutions must be found. SAM algorithms, which were first introduced in [3], are part of a class of algorithms known as geometric search algorithms [4, 5] that deal with point location in unstructured meshes. We propose here a modified SAM algorithm that uses the optical voxel-based mesh as the auxiliary structured mesh for geometric searching. The two meshes, optical and FEM, are superimposed. As point location in voxel-based meshes is trivial, by mapping which elements of the FEM mesh belong to each voxel, we can easily narrow down the number of searches required. To this end, two maps, implemented as binary search trees, are implemented. The first map, maps the voxel number to the nodes of the FEM mesh that lie within it, while the second map, maps the elements of the unstructured mesh that these nodes belong to. The set-up times of these maps heavily depend on the density of the FEM mesh and the order of the elements used. The use of higher-order elements results in considerable set-up times. As higher order elements are not necessary for thermal analysis, this is not a critical issue here. On the other hand, the denser the optical mesh, the fewer FEM elements are mapped to each voxel. However, the size of voxels should be appropriately chosen, since too small voxels may lead to degenerative cases where there are voxels without any nodes lying in them. A distinct advantage of this method is that once the maps are assembled, the search time of elements is O(1). Simulation plays an increasingly crucial role in the study and optimization of optical systems. Due to the increase in computational capabilities, modelling of more complex phenomena can be included and the need of multi-physics approaches rises. The optical properties of materials often shift to temperature above tolerance levels that may render a particular optical design ineffective. In other cases, structural loads may be the critical issue as they can lead to misalignment of optical elements. The proposed SAM algorithm that enables a more efficient coupling of optical and FEM analysis is a valuable tool to such approaches of optical problems.
关键词： Laser-based lighting systems,opto-thermal simulation,Monte Carlo simulations,SAM algorithm,finite element method,thermal quenching
更新于2025-09-16 10:30:52
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Disorder vs Delocalization: Which Is More Advantageous for High-Efficiency Organic Solar Cells?
摘要： We investigate the combined influence of energetic disorder and delocalization on electron-hole charge-transfer state separation efficiency in donor-acceptor organic photovoltaic systems using an analytical hopping model and Monte Carlo calculations, coupled with an effective mass model. Whereas energetic disorder increases the separation yield at intermediate and low electric fields for low efficiency blends with strongly localized carriers, we find that it reduces dramatically the fill factors and power conversion efficiencies in high efficiency solar cells that require high carrier delocalization within the conjugated segment and high mobility-lifetime product. We further demonstrate that the initial electron-hole distance and thermalization processes play only a minor role in the separation dynamics.
关键词： Charge separation,delocalization,organic photovoltaics,disorder,Monte Carlo simulations
更新于2025-09-12 10:27:22
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Kinetic Monte Carlo simulations of the dynamics of a coupled system of free and localized carriers in AlGaN
摘要： A model for the dynamics of a coupled system of free and localized carriers in semiconductors with strong carrier localization is suggested. Kinetic Monte Carlo technique is exploited for simulations. The model is verified by fitting the simulated and experimental temperature dependences of photoluminescence (PL) band intensity, peak position, and band width, and the carrier density dependence of PL efficiency in AlGaN quantum wells. The influence of carrier localization conditions on the dominating carrier migration and recombination processes is revealed. The efficiency droop effect is shown to be caused by peculiarities of carrier localization without significant influence of Auger recombination.
关键词： AlGaN,nitrides,carrier localization,efficiency droop,Monte Carlo simulations
更新于2025-09-12 10:27:22
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Elucidating the effects of guest-host energy level alignment on charge transport in phosphorescent OLEDs
摘要： The correct choice of guest and host molecules in the light-emitting layer is essential for developing high performance phosphorescent organic light emitting devices. However, the effects of the energy level alignment between the guest and the host are yet to be fully elucidated. In this Letter, we use kinetic Monte Carlo simulations to investigate guest-host systems in which the energy gap of the guest and host is fixed, and only the relative energies of the ionization potential/electron affinity are changed to elucidate their effect on charge transport. It was determined that the mobility balance in the blend was sensitive to the energy level alignment, allowing balanced active layer mobility to be achieved despite the hole and electron mobilities being different by around one order of magnitude. It was also found that the mobility of the faster carrier was more sensitive to the energy level alignment than that of the slower carrier due to reduced slower carrier thermalization under deep charge trapping on the guest.
关键词： guest-host energy level alignment,phosphorescent OLEDs,kinetic Monte Carlo simulations,charge transport
更新于2025-09-12 10:27:22
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Holographic interferences in strong-field ionization beyond the dipole approximation: The influence of the peak and focal-volume-averaged laser intensities
摘要： In strong-?eld ionization, interferences between electron trajectories create a variety of interference structures in the ?nal momentum distributions. Among them, the interferences between electron pathways that are driven directly to the detector and those that rescatter signi?cantly with the parent ion lead to holography-type interference patterns that have received great attention in recent years. In this work, we study the in?uence of the laser magnetic-?eld component onto the holographic interference pattern, an effect beyond the electric-dipole approximation, in experiment and theory. The experimentally observed nondipole signatures are analyzed via quantum trajectory Monte Carlo simulations. We provide explanations for the experimentally demonstrated asymmetry in the holographic interference pattern and its nonuniform photoelectron energy dependence as well as for the variation of the topology of the holography-type interference pattern along the laser-?eld direction. Analytical scaling laws of the interference features are derived, and their direct relation to either the focal-volume-averaged laser intensities, or to the peak intensities are identi?ed. The latter, in particular, provides direct access to the peak intensity in the focal volume.
关键词： laser magnetic-?eld,nondipole effects,strong-?eld ionization,quantum trajectory Monte Carlo simulations,holographic interference
更新于2025-09-12 10:27:22
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Radiation Tolerant Nanowire Array Solar Cells
摘要： Space power systems require photovoltaics that are lightweight, efficient, reliable, and capable of operating for years or decades in space environment. Current solar panels use planar multijunction, III-V based solar cells with very high efficiency, but their specific power (power to weight ratio) is limited by the added mass of radiation shielding (e.g. coverglass) required to protect the cells from the high-energy particle radiation that occurs in space. Here we demonstrate that III-V nanowire-array solar cells have dramatically superior radiation performance relative to planar solar cell designs and show this for multiple cell geometries and materials, including GaAs and InP. Nanowire cells exhibit damage thresholds ranging from ~10-40 times higher than planar control solar cells when subjected to irradiation by 100-350 keV protons and 1 MeV electrons. Using Monte Carlo simulations, we show that this improvement is due in part to a reduction in the displacement density within the wires arising from their nanoscale dimensions. Radiation tolerance, combined with the efficient optical absorption and the improving performance of nanowire photovoltaics, indicates that nanowire arrays could provide a pathway to realize high-specific-power, substrate-free, III-V space solar cells with substantially reduced shielding requirements. More broadly, the exceptional reduction in radiation damage suggests that nanowire architectures may be useful in improving the radiation tolerance of other electronic and optoelectronic devices.
关键词： space environment,irradiation-induced defects,radiation hard,space solar cells,Monte Carlo simulations,nanowire solar cells,high specific power
更新于2025-09-11 14:15:04
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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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Two-dimensional Monte Carlo simulations of coarse-grained poly(3-hexylthiophene) (P3HT) adsorbed on striped substrates
摘要： We investigate the structural phases of single poly(3-hexylthiophene) (P3HT) polymers that are adsorbed on a two-dimensional substrate with a striped pattern. We use a coarse-grained representation of the polymer and sophisticated Monte Carlo techniques such as a parallelized replica exchange scheme and local as well as non-local updates to the polymer’s configuration. From peaks in the canonically derived observables, it is possible to obtain structural phase diagrams for varying substrate parameters. We find that the shape of the stripe pattern has a substantial effect on the obtained configurations of the polymer and can be tailored to promote either more stretched out or more compact configurations. In the compact phases, we observe different structural motifs, such as hairpins, double-hairpins, and interlocking “zipper” states.
关键词： coarse-grained,striped substrates,Monte Carlo simulations,poly(3-hexylthiophene),P3HT,structural phases
更新于2025-09-10 09:29:36
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The metal-free magnetism and ferromagnetic narrow gap semiconductor properties in graphene-like carbon nitride
摘要： In spintronics, if a two-dimensional (2D) organic metal-free material has stable magnetism and narrow gap semiconductor properties, it will have a very bright application prospect. A graphene-like carbon nitride (g-C13N13) that we design just meets these requirements. As a new structure, firstly the stability of the g-C13N13 has been verified. It has stable electron spin polarization and the magnetic moment of each primitive cell is 1μB. It exhibits ferromagnetic narrow gap semiconductor properties through our analysis of energy band structure and charge density. Ferromagnetic ordering between two adjacent primitive cells is stable. The Monte Carlo simulation using the Ising model shows the Curie temperature material is 204K. Our research is an inspiration for the applications of this kind of materials in spintronics devices.
关键词： Metal-free magnetism,Electron spin polarization,Ferromagnetic ordering,Narrow gap semiconductor properties,Monte Carlo simulations
更新于2025-09-10 09:29:36
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Phase-field method for epitaxial kinetics on surfaces
摘要： We present a procedure for simulating epitaxial growth based on the phase-field method. We consider a basic model in which growth is initiated by a flux of atoms onto a heated surface. The deposited atoms diffuse in the presence of this flux and eventually collide to form islands which grow and decay by the attachment and detachment of migrating atoms at their edges. Our implementation of the phase-field method for this model includes uniform deposition, isotropic surface diffusion, and stochastic nucleation (in both space and time), which creates islands whose boundaries evolve as the surface atoms "condense" into and "evaporate" from the islands. Computations using this model in the submonolayer regime, prior to any appreciable coalescence of islands, agree with the results of kinetic Monte Carlo (KMC) simulations for the coverage-dependence of adatom and island densities and island-size distributions, for both reversible and irreversible growth. The scaling of the island density, as obtained from homogeneous rate equations, agrees with KMC simulations for irreversible growth and for reversible growth for varying deposition flux at constant temperature. For reversible growth with varying temperature but constant flux, agreement relies on an estimate of the formation energy of the critical cluster. Taken together, our results provide a comprehensive analysis of the phase-field method in the submonolayer regime of epitaxial growth, including the verification of the main scaling laws for adatoms and island densities and the scaling functions for island-size distributions, and point to the areas where the method can be extended and improved.
关键词： epitaxial growth,reversible and irreversible growth,kinetic Monte Carlo simulations,island-size distributions,phase-field method
更新于2025-09-10 09:29:36