Computational Physics · Cross-listed from cond-mat.mtrl-sci
Bulk Photovoltaic Effect in Two-Dimensional Perovskite Oxides
Chunmei Zhang, Jian Zhou and Liang Si
Perovskite
oxides ABO$_3$ host a rich interplay of charge,
spin,
lattice, and orbital degrees of freedom, giving rise to diverse
quantum phenomena. In low-dimensional ABO$_3$, reduced
symmetry can induce exotic
quantum effects such as the two-dimensional electron gas and unconventional superconductivity. Using first-principles density functional theory, tight-binding modeling, and
symmetry analysis, we show that ultrathin two-dimensional (2D) ABO$_3$ films – exemplified by SrTiO$_3$ – naturally break inversion
symmetry, producing a spontaneous out-of-plane bulk photovoltaic (BPV) effect. This differs from previous studies on in-plane BPV current signals and is more applicable and experimentally detectable. Such an effect is highly tunable via thickness, strain, surface termination, crystallographic orientation, and Moiré twisting. These findings are broadly applicable to a wide range of 2D perovskite and other layer-resolved
oxides.
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Optics
Induced Directional Switching of Platicon Microcombs in Photonic Crystal Ring Resonators
Erwan Lucas, Su-Peng Yu, Jizhao Zang et al. (+1 more)
Microcombs in normal-dispersion photonic
crystal ring resonators (PhCRs) are versatile building blocks for next-generation integrated photonic
circuits, but their inherent backward-propagation bias necessitates optical circulators or complex filtering for comb extraction, creating a significant bottleneck for full on-chip integration and precluding self-injection locking schemes. In this work, we introduce Side-mode Induced Forward Forcing (SIFF), a robust method to control and reverse this directionality. By engineering auxiliary mode splittings on resonances adjacent to the pump, we steer the nonlinear dynamics to favor stable, forward-propagating platicon states. We identify an optimal coupling condition that ensures forward-comb dominance across a wide parameter range. Our findings, validated numerically and experimentally, enable circulator-free, integrated normal-dispersion microcombs compatible with self-injection locking, offering a scalable architecture for compact telecommunications and sensing systems.
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All authors Erwan Lucas, Su-Peng Yu, Jizhao Zang, Scott B. Papp
Plasma Physics
Geometric numerical discretization of a quasineutral hybrid model of drift-kinetic electrons and fully kinetic ions
Nishant Narechania, Guo Meng, Emil Poulsen et al. (+1 more)
We extend the
geometric electromagnetic particle-in-
cell (PIC) framework, GEMPICX, to solve the quasineutral hybrid Vlasov-Maxwell equations with drift-kinetic electrons and fully kinetic ions. A structure-preserving finite difference method that employs dual grids is used. The discrete action principle for the hybrid model is derived, using the dual nature of the grids. The dynamical system for this hybrid quasineutral model does not explicitly involve the temporal
evolution term for the electric field. A curl-curl equation is therefore used to implicitly obtain the component of the electric field that is parallel to the background
magnetic field, at every timestep. The perpendicular component of the electric field is obtained using the quasineutral Ampere's equation without the displacement current, combined with the definition of the current in the drift-kinetic model. The discretized versions of the electric field equations are large, sparse linear systems. A fully explicit time-stepping scheme as well as two implicit-explicit (IMEX) schemes are tested. The numerical model is validated by verifying the various waves obtained from the dispersion relation.
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All authors Nishant Narechania, Guo Meng, Emil Poulsen, Eric Sonnendruecker
Plasma Physics
Geometric numerical discretization of electromagnetic quasineutral models
Nishant Narechania, Emil Poulsen and Eric Sonnendrucker
In this work, the
geometric electromagnetic Particle-in-
Cell (PIC) framework, GEMPICX, is extended to solve the quasineutral, fully kinetic Vlasov-Maxwell equations on dual grids using mimetic finite differences. The discrete action principle is derived, taking into account the duality between the grids. The temporal derivative of the electric field does not directly appear in the dynamical system for the quasineutral model. Hence, a discretized curl-curl equation is used to implicitly obtain the electric field at every time-step. This also circumvents the need to obtain electric potentials. A Lagrange multiplier is used to maintain the discretized divergence of the current density at machine zero.
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Optics
Blind Gradient-Ascent Phase Alignment for Multi-Aperture Coherent Digital Combining Under Aperture-Dependent Phase Disturbance
Cheng Chen, Tong Luo, Jiayin Xue et al. (+5 more)
Multi-aperture reception can provide spatial diversity in free-space optical (FSO) communication by collecting signal replicas at separate apertures. When the branches are accurately phase-aligned, their received optical fields can also be added constructively to obtain coherent-combining gain. In this paper, we propose blind gradient-ascent phase
alignment (BGAPA), which iteratively adjusts one phase correction per aperture by directly maximizing the combined output power. Closed-form analytical gradients provide a deterministic update that requires no symbol decisions, unlike the
stochastic perturbation-based estimate of SPGD or the decision-directed feedback of DD-LMS. To isolate phase-tracking capability, the numerical model includes independent aperture-dependent phase disturbance but excludes amplitude scintillation and polarization-dependent distortion. Under this controlled phase-only setting, BGAPA obtains an SNR improvement closer to the ideal 6.02~dB coherent-combining gain than block-wise cross-correlation, SPGD, DD-LMS, and CMA/RDE-based equalization when the aperture count is increased by a factor of four. In particular, increasing the aperture count from 64 to 256 yields an SNR improvement of about 5.7~dB. In a separate amplitude-tolerance test with $N=16$ and $f_{\max}=1$~MHz, the first observed BGAPA trial above the HD-FEC threshold of $3.8\times10^{-3}$ occurs at an actual phase RMS of approximately 278~rad, whereas DD-LMS becomes unreliable at substantially smaller phase excursions. The reported step size is optimized separately at each operating point. BGAPA is fully blind and updates its phase parameters directly from the received aperture fields without training symbols, pilots, or decision-directed feedback.
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All authors Cheng Chen, Tong Luo, Jiayin Xue, Siyu Gong, Qun Zhang, Linsheng Fan, Qi Wu, Yanfu Yang
Fluid Dynamics
Enhanced Heat Transfer through Density- and Pressure-Driven Flow at Fracture Intersections With Dead-Ends
Lisa Maria Ringel, Yves Méheust, Caroline Darcel et al. (+2 more)
Heat transport in fractured media is governed by coupled thermal-hydraulic (TH) processes. This study evaluates TH processes at fracture intersections, focusing on T-intersections where one horizontal fracture is subjected to a pressure gradient while the other forms a vertical dead-end fracture. Using numerical simulations, we investigate the influence of the inlet velocity, thermal Péclet, and Rayleigh numbers, and the impact of a pressure gradient along the T-intersection, on the resulting heat transport. The model domain consists of a fluid and a solid region. Fluid flow and heat transport in the fractures are described by the conservation equations for mass, momentum, and energy. The rock
matrix is considered impermeable, therefore, it is governed by heat conduction. The simulations consistently show that heat transfer from the fluid to the
matrix is enhanced when fluid flow occurs within the dead-end fracture, since such fluid flow maintains a higher temperature difference between the
matrix and the fluid. This flow arises either from buoyancy-driven natural convection due to temperature-dependent fluid density or from a pressure gradient imposed by the orientation of the dead-end fracture with respect to the flow direction in the horizontal fracture. Natural convection dominates at high flow rate, Rayleigh, and Péclet numbers, whereas pressure-driven flow becomes the controlling mechanism for an increasing deviation from the orthogonal configuration of the two fracture planes and under higher flow rates. At low flow rates, Péclet, or Rayleigh numbers, no flow develops in the dead-end fracture, and heat transport in the dead-end fracture becomes conduction-dominated.
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All authors Lisa Maria Ringel, Yves Méheust, Caroline Darcel, Philippe Davy, Maria Klepikova
Optics · Cross-listed from cs.CV
OSOG: A Differentiable, Physics-Informed Synthetic Data Engine for Micro-Optical Environments
Caio Silva
Deep learning in computational microscopy is severely constrained by the scarcity of densely annotated
datasets. While synthetic data generation has bridged this gap in macroscopic computer
vision, traditional graphics engines rely on
geometric ray-tracing, failing to capture the micro-optical phenomena required for microscopy. Conversely, while wave-optics formulations exist, rendering them computationally tractable at the scale required for deep learning remains a massive systems challenge. To address this, we introduce the Optical Synthetic Object Generator (OSOG), a high-performance, fully differentiable forward-modeling engine. Drawing on established physical models of diffraction and phase retardation, OSOG maps continuous Optical Path Difference (OPD) calculations into a highly optimized, PyTorch-native Structure-of-Arrays (SoA) architecture. We validate this computational framework across three axes: First, object detection models (YOLOv11-OBB) trained purely on OSOG-generated data achieve robust zero-shot transfer to real-world highly occluded Lysozyme micrographs. Second, we introduce DiffOSOG, demonstrating that the engine's end-to-end differentiability allows for the exact recovery of continuous optical parameters via curriculum-guided inverse rendering. Finally, OSOG bypasses the $\mathcal{O}(N)$ bottlenecks of sequential ray-tracing, demonstrating sub-linear scaling by synthesizing 40,000 complex wave-optic particles in under 50 milliseconds (\>20 FPS). By providing a fast, scalable, and physically grounded tensor pipeline, OSOG enables true real-time, on-the-fly
dataset generation.
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Biological Physics
Computationally guided modifications of CviUPO to improve catalytic activity
Hanna-Friederike Poggemann, Tim Dirks, Timo Jacob et al. (+1 more)
Unspecific peroxygenases (UPOs) are promising biocatalysts that selectively oxyfunctionalize saturated hydrocarbons using only hydrogen peroxide as a co-substrate. Peroxide-induced enzyme inactivation makes targeted enzyme engineering essential to mitigate this effect and also enhance catalytic performance. To meet this need, systematic approaches are used, including extensive database studies for rational enzyme design, as well as computational enzyme engineering. In this study, we followed the latter strategy and explored the possibility for computationally-guided modification of UPOs. Specifically, our focus was on uncovering the influence of active site amino acids on the catalytic activity of the enzyme CviUPO. Two mutations were introduced close to the active center, and the changes in the energy barriers leading to the activated complex were investigated in detail by
Quantum Mechanics/Molecular Mechanics Nudged Elastic Band simulations. Our studies revealed that a change of the glutamic acid, assisting the catalytic cycle, by the shorter aspartic acid, leads to an increased reaction barrier, probably decreasing the catalytic activity of the enzyme. Exchanging the heme-anchoring cysteine group by a histidine exhibited promising behavior as the energy barriers decreased significantly. However, it is possible that the histidine modification also alters the reaction behavior of the peroxygenase, turning it into a peroxidase, an aspect that so far could not be confirmed beyond doubt. Simulations alone cannot conclusively determine whether substrate specificity and reactivity are maintained in the modifications tested. Nevertheless, our results highlight the importance of
spin states and active pocket hydration for the catalytic reaction and demonstrate why a synergistic approach of theoretical predictions and experimental verifications is required for efficient enzyme engineering.
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All authors Hanna-Friederike Poggemann, Tim Dirks, Timo Jacob, Christoph Jung
Atomic Physics · Cross-listed from quant-ph
Quantum Beam-Splitter Cooling and Thermometry in Large Trapped-Ion Crystals
Kirthik Rajakumar, Ansh Das, Abhinay Pandey et al. (+1 more)
We propose and characterize a protocol for rapid near-ground state cooling of the center-of-mass (c.m.) mode of a large trapped ion
crystal. When the initial mean thermal occupation of the mode $\bar{n}_i$ is small compared to the number of ions $N$, a red sideband drive implements a beam-splitter type SWAP operation between the mode and the collective
spin of the $N$ ions, with the latter effectively serving as a
quantum harmonic oscillator. Subsequently, a reset of the
spins removes the
entropy, leading to near-ground state cooling of the c.m. mode. We term this protocol as
quantum beam-splitter cooling (QBSC). We analyze the impact of several practical imperfections on the final temperature achievable under QBSC, including finite ion number, off-resonant carrier and blue-sideband contributions, and the impact of the sideband drives arising from spectator modes. In addition, we outline practical strategies to eliminate the carrier drive. Furthermore, we show that measuring the population statistics of the ions at the end of the SWAP operation can enable near-optimal
quantum beam-splitter thermometry (QBST), with the classical Fisher information approaching the
quantum Fisher information of a thermal state. We discuss the connection of QBSC with continuous sideband cooling and compare QBST with a recently proposed rapid adiabatic passage-based thermometry scheme. Our work constitutes an example of harnessing many-body effects to open new routes to
laser cooling and thermometry in large trapped ion
crystals.
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All authors Kirthik Rajakumar, Ansh Das, Abhinay Pandey, Athreya Shankar
Instrumentation and Detectors
Enhanced electron-beam lithography to reduce the frequency scatter of 200-400 GHz superconducting microstrip resonators for on-chip filterbank spectrometers
L. G. G. Olde Scholtenhuis, D. J. Thoen, J. J. A. Baselmans et al. (+4 more)
Integrated superconducting spectrometers (ISSs) provide the instantaneous bandwidth, sensitivity, and scalable architecture for large-scale spectroscopic surveys in submillimeter-wave astronomy and cosmology. However, the accuracy with which the resonant frequencies of superconducting microstrip band pass filters can be spaced, has limited the spectral resolution for these spectrometers with continuous spectral coverage to $\frac{F}{\Delta F} < 500$. The origin of this frequency scatter has been largely unknown. In this work, we demonstrate a four-fold improvement in the frequency spacing of superconducting microstrip resonators by optimizing electron-beam lithography. We find that reducing the beam step size (BSS) on the nanometer scale reduces the random frequency scatter, and that avoiding main-field stitching across filter patterns can eliminate a systematic frequency shift between groups of resonators, indicating the different origins of these two modes of frequency deviation. These findings demonstrate that nanometer-scale control of lithographic processes is imperative for the realization of the next-generation integrated superconducting spectrometers with higher spectral resolution.
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All authors L. G. G. Olde Scholtenhuis, D. J. Thoen, J. J. A. Baselmans, K. Karatsu, L. H. Martin, S. Vollebregt, A. Endo
Optics
Mitigating Outages in a 4.6-km FSO Link via Mode-Diverse Reception: An Experimentally Validated Digital Twin Approach
Jonas Krimmer, Vincent van Vliet, Menno van den Hout et al. (+3 more)
Terrestrial coherent FSO requires mitigating
turbulence-induced coupling losses. We experimentally validate a "Digital Twin" channel model against fading statistics from a 4.6-km link. Combining long-term scintillometer data with mode-diverse receiver simulations, we demonstrate that a 6-mode receiver reduces
turbulence-induced outage probabilities to 2.02e-5.
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All authors Jonas Krimmer, Vincent van Vliet, Menno van den Hout, Eduward Tangdiongga, Chigo Okonkwo, Sebastian Randel
Medical Physics
MORSE-PI – Flexible and artefact-free image reconstruction for structural and functional QSM and other phase-critical imaging applications
Barbara Dymerska, Oliver Josephs, Benjamin James et al. (+3 more)
Phase imaging applications such as QSM are highly sensitive to noise amplifications, phase singularities, and other artefacts, particularly in challenging scenarios such as ultra-high field (7T), under-sampled or single-echo acquisitions. We present a novel image reconstruction method, MORSE-PI, designed to produce high-SNR, artefact-free, and singularity-free phase images for both structural and functional phase-based
brain imaging. MORSE-PI extends our previous approach, MORSE, by introducing a Virtual Reference Coil (VRC). The VRC is constructed as a linear combination of coil sensitivity maps, with correlations enhanced between coil elements using the noise covariance
matrix. Such a VRC ensures robust signal support across the entire
brain and is used to correct phase offsets in the MORSE-derived coil sensitivity estimates, resulting in artefact-free, high SNR phase. Compared to GRAPPA with ASPIRE phase correction, MORSE-PI demonstrates greater robustness to artefacts such as noise amplification and aliasing, and shows improved reproducibility in structural imaging at both 3T and 7T. Unlike ESPIRiT and GRAPPA combined with adaptive coil combination methods, MORSE-PI yields singularity-free phase maps. MORSE-PI enables high-SNR reconstructions even for the most challenging scenarios, such as single-echo EPI at 7T. Its efficient, containerised implementation using the Gadgetron framework supports deployment on the MRI scanner console during measurements. MORSE-PI offers a flexible and computationally efficient solution for generating high-SNR, artefact- and singularity-free phase images in both single- and multi-echo GRE and EPI acquisitions. This makes it particularly well-suited for structural and functional QSM, as well as other phase-based MRI applications. Its robustness and rapid computational time facilitate efficient deployment on scanners across field strengths.
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All authors Barbara Dymerska, Oliver Josephs, Benjamin James, Vahid Malekian, Nadine N. Graedel, Martina F. Callaghan
Medical Physics
Patient-specific immobilisation for radiotherapy treatment of extensive lower-limb carcinoma
Tanya Kairn, Tania Poroa, Jenna Luscombe et al. (+3 more)
When non-melanoma skin cancers extend over large areas of skin, effective radiotherapy treatments can be delivered using volumetric modulated arc therapy (VMAT) beams with narrow segments that rotate around the affected surfaces. For lower-limb carcinoma treatments, careful immobilisation is needed to achieve the degree of reproducible and stable positioning required to ensure the narrow field segments treat the targeted tissue and avoid underlying anatomy. To meet this need, a 3D printed patient-specific foot support was created in the form of a solid box containing a deep “footprint”, shaped to match the outline of the patient's relaxed foot when lying supine, supported by a
vacuum bag. For our first patient treated with a 3D printed foot support, image guidance data and clinical notes were evaluated against corresponding information from all recent lower-leg VMAT treatments. The patient feedback was recorded as “good”, with no complaints of discomfort or poor fit, and the proportion of treatment fractions requiring shifts greater than 5 mm after setup imaging (20%) compared favourably to the proportions for patients without a patient-specific foot support (23%-84%). The patient-specific foot support was particularly useful for minimising longitudinal shifts and leg rotations. Evidently, 3D-printed patient-specific immobilisation devices have the potential to enhance positioning stability, and therefore potentially improve accuracy and effectiveness of VMAT treatments, for patients with extensive lower-limb carcinomas.
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All authors Tanya Kairn, Tania Poroa, Jenna Luscombe, Shaun Chan, Michelle Grogan, Scott B. Crowe
Fluid Dynamics
Flow mechanisms governing oscillation in a sonic fluidic oscillator
Chris J. Nicholls, Michael R. Fenelon, Yang Zhang et al. (+1 more)
Two factors that influence the oscillation mechanism of a sonic fluidic oscillator are investigated: the geometry of the feedback channel connections (control ports) and the influence of flow restrictions in the oscillator outlets. Phase-averaged planar PIV measurements are performed inside the oscillator, synchronised with unsteady pressure measurements, and analysed using space-only proper orthogonal decomposition (POD). The POD analysis reveals two coupled modes: a Sweeping Mode capturing lateral jet displacement and a Bending Mode capturing jet
curvature during switching, the latter being the primary driver of outlet mass flux modulation. Flow separation at the control port entrances is shown to throttle the feedback flow and progressively limit oscillation strength at higher inlet flow rates. Restrictive outlet paths induce a differential back pressure that is shown to cause the jet to separate from its attachment wall and bend towards the splitter tip (`secondary separation'). The secondary separation reduces the differential outlet mass flux and introduces a flow
curvature that limits the upstream propagation of the back pressure and thus shields the primary jet attachment. The consequence of these effects is that strong oscillations are sustained down to the smallest outlet apertures investigated. The principal contribution is to demonstrate that the assumed coupling between upstream jet attachment and outlet flow split is broken when the outlet aperture is reduced, with significant implications for the design of fluidic oscillators operating with downstream flow impedances.
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All authors Chris J. Nicholls, Michael R. Fenelon, Yang Zhang, Louis N. Cattafesta III
Medical Physics
Adapting a 3D scanning water phantom for use in brachytherapy dosimetry
Rachael Wilks, Samuel C. Peet, Tanya Kairn et al. (+1 more)
In external beam radiotherapy, 3D scanning water phantoms are the gold standard for obtaining relative dosimetry data. These phantoms, consisting of a water tank and mechanical arm, along with accompanying software, are de-signed to acquire dose profiles along and orthogonal to the beam axis. In brachy-therapy, the acquisition of analogous dose profiles is more difficult, and is generally achieved with complex custom-built phantoms or chemical dosimeters such as film or gel. In this study, a low-cost 3D-printed jig was designed and fabricated within a clinical department, to allow precise brachytherapy dose measurements using a PTW BeamScan water phantom. Specifically, this jig al-lowed applicators to be suspended securely and reproducibly within the water phantom. Protocols were developed to relate the scanning system coordinates to the physical source position, and to obtain isodose planes both parallel and radial to the source axis. The developed solution has the potential to be used for physical verification of TG43 dose calculation parameters (e.g. anisotropy functions), the characterization of dose for a single dwell position in a complex applicator containing non-water equivalent materials, or the collection of point dose measure-ments for treatments incorporating multiple dwell positions or catheters.
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All authors Rachael Wilks, Samuel C. Peet, Tanya Kairn, Scott B. Crowe
Plasma Physics · Cross-listed from cond-mat.stat-mech
From Landau Equation and Large Deviations to Efficient Simulations of Dynamical Fluctuations
Anwar El Rhirhayi, Jean-Baptiste Fouvry and Julien Barré
The (deterministic) Landau equation captures the mean long-term
evolution of dynamically hot long-range interacting finite-$N$ systems. Though successful, this kinetic equation fundamentally ignores dynamical fluctuations. Building upon Large Deviation Theory, we present a physically-consistent system of Langevin equations that simultaneously recovers the mean Landau dynamics and accurately captures the corresponding fluctuations among different realizations. We show in particular how these Langevin equations can be derived from Rostoker's principle in the limit of weak two-body deflections. We extensively validate these equations against tailored direct $N$-body simulations, showing an exquisite level of agreement.
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Optics
Dynamic dark-field FFOCT and dynamic reflection differential phase contrast for label-free functional imaging at reflective biomaterial interfaces
Tual Monfort
Strong reflections from metallic and engineered substrates severely limit label-free functional imaging of living
cells at biomaterial interfaces, neural electrodes, and implantable devices. Here we introduce two complementary approaches for recovering intracellular dynamic contrast at highly reflective interfaces. Dynamic dark-field full-field optical coherence
tomography (D-dFFOCT) suppresses the dominant substrate reflection and restores intracellular visibility through selective detection of scattered light. In parallel, asymmetric illumination generates a distinct directional dynamic contrast that is most consistently interpreted as dynamic reflection differential phase contrast (D-RDPC). Both approaches reveal intracellular activity that remains poorly visible with conventional dynamic full-field optical coherence
tomography. D-RDPC exhibits characteristic signatures of phase-gradient imaging, including contrast reversal upon illumination inversion, enhancement with increasing illumination asymmetry, and recovery of spatial localization through directional
Hilbert-transform reconstruction. Together, these results establish new strategies for functional imaging at reflective interfaces and suggest that differential phase contrast signals can support temporal fluctuation analysis.
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Medical Physics
Characteristics of a 9 MeV electron beam for total skin electron radiotherapy evaluated in comparison to a 6 MeV electron beam
Tanya Kairn, Craig M. Lancaster and Scott B. Crowe
Characteristics of a 9 MeV electron beam were investigated, to evaluate potential benefits for patients with extensive mycosis fungoides lesions that are deeper than commonly treated with 6 MeV total skin electron therapy (TSET or TSE/TSEI/TSEB/TSEBT). A comprehensive commissioning measurement program was completed for TSET delivery using high-dose-rate 6 MeV and 9 MeV electron beams from a Varian TrueBeam linac. Various phantoms and dosimeters were set up 300 cm from isocenter, behind a 6 mm PMMA spoiler screen, and used for optimising beam-pair gantry angles, measuring depth-dose, lateral and horizontal profiles and B-factors, as well as performing end-to-end tests. The key clinical distinctions observed for the 9 MeV TSET beam pair compared to the 6 MeV TSET beam pair were that the 80% dose depth increased to nearly 1 cm and the practical range increased to 3.5 cm, while the maximum dose remained within 0.2 cm of the surface. Vertical and horizontal dose profiles measured with the two energies were almost indistinguishable. The 9 MeV TSET beam has been shown to achieve greater depth penetration compared to the commonly used 6 MeV TSET beam, without detrimentally affecting the dose uniformity achievable in the patient plane. TSET treatments with 9 MeV electrons may be advisable for patients with extensive lesions that are too deep for effective treatment with a 6 MeV beam.
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Chemical Physics
Property-Specific Molecular Representations via Feature-Space Transfer Compression
Ali Banjafar and Guido Falk von Rudorff
In many machine learning applications, molecules need to be transformed into representations, i.e. mathematical objects. Those representations are typically considered to be property-agnostic and as such are expected to be over-complete: for different physical properties, different parts or the representation may be relevant. In this work, we propose a method to sub-select and re-weight the representation by adapting it to the property in question. We find that in most cases this makes representations shorter and more accurate at the same time. The feature selection itself uses cheap semi-empirical data instead of high-quality labels. We study four properties (total energy, heat capacity, dipole moment, and polarizability) for three representations (cMBDF, FCHL19, and MACE-MP-0 descriptors) on two
datasets (QM9 and VQM24). We can reduce the number of dimensions of a representation in the median by 72\,% (range 36-98\,%) while retaining the accuracy. Tuning for accuracy instead we can increase the learning efficiency for dipole moments such that the same accuracy can be reached with 19\,% of the training data. Our approach yields data-driven interpretations of feature importance, lossless compact representations, and increased data efficiency, requiring only expendable surrogate data.
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Optics
A dressed polarizability framework for interface-coupled meta-atoms and large-scale metasurfaces
Peng Fu, Jean-Paul Hugonin, Maxime Bertrand et al. (+2 more)
The modeling of large collections of interacting meta-atoms remains a central challenge in photonics because it requires the simultaneous treatment of complex scatterer geometries, multiple scattering, and heterogeneous environments. Here, we introduce the dressed Global Polarizability
Matrix (dGPM), a reduced-order electromagnetic framework in which complex meta-atoms are represented by compact scattering operators that explicitly incorporate the influence of nearby interfaces. The dGPM accurately reconstructs near- and far-field electromagnetic responses for interface-coupled structures, including geometries beyond the practical applicability of conventional T-
matrix approaches. The framework naturally combines interface-coupled and free-space scatterers within a unified multiple-scattering formalism and enables simulations of large ordered and disordered metasurfaces. More broadly, the dGPM extends operator-based electromagnetic modeling to heterogeneous photonic environments while preserving a compact scatterer-based representation, providing an efficient framework for the simulation and design of large-scale photonic systems.
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All authors Peng Fu, Jean-Paul Hugonin, Maxime Bertrand, Kevin Vynck, P. Lalanne
Instrumentation and Detectors
Characterization of GaN:Si and ZnO:Ga for position-resolved fast timing applications
Julius Meyer, Joshua W. Cates, Woon-Seng Choong et al. (+7 more)
We present the characterization of two fast, crystalline inorganic scintillators, silicon-doped gallium nitride (GaN:Si) and gallium-doped zinc
oxide (ZnO:Ga), and compare their performance with cerium-doped yttrium aluminium perovskite (YAP:Ce) for in-
vacuum alpha-detection applications that require high-performance timing, position, and energy resolution, such as 3D elemental mapping, medical imaging, and homeland security applications. In this paper, we propose ZnO:Ga and GaN:Si as high-performance drop-in replacements for the alpha detector in Associated Particle Imaging (API) systems. However, the results reported here also have wide applicability. Prior work has reported on polycrystalline forms of ZnO:Ga, which suffer from self-absorption. To our knowledge, GaN:Si has not been proposed to be used in API systems. We present room-temperature scintillation time constants obtained via X-ray-induced time-correlated single-
photon counting for both proposed materials. They both exhibit exceedingly fast rise times of <15ps, and high brightness >1000ph/MeV with resolved alpha-peaks. Single-
crystal ZnO:Ga and single-
crystal GaN:Si yield single-component decays of 805ps and 32ps, respectively. Using a plastic scintillator reference setup, coincidence timing resolution (CTR) and detector timing resolution (DTR) measurements demonstrate a >3x improvement in timing resolution compared to traditional YAP:Ce. GaN:Si and ZnO:Ga exhibit (35(9))ps and (49(5))ps DTR, respectively, compared to(144(2))ps for conventional, single-
crystal YAP:Ce. Finally, we evaluate their position resolution in an experimental setup designed for API and measure better than 0.2mm for YAP:Ce and approximately 1mm for GaN:Si. We obtain a position resolution of 0.3mm for ZnO:Ga from simulations. We also present alpha-induced ionoluminescence emission spectra that reveal direct, red-shifted near-bandgap emission.
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All authors Julius Meyer, Joshua W. Cates, Woon-Seng Choong, Juan Cristhian Luque Gutierrez, Federico Moretti, Ryan Pavlovsky, Mauricio Ayllon Unzueta, Weronika W. Wolszczak, Markus Roth, Arun Persaud
Atmospheric and Oceanic Physics
Using Distributional Regression Networks to Retrieve Cloud Properties from Solar Satellite Channels for Data Assimilation
Stefano Franzoni, Christopher Bülte, Leonhard Scheck et al. (+2 more)
Satellite observations in the
solar spectrum (including visible and near-infrared channels) offer high-resolution information on clouds and atmospheric properties valuable for data assimilation. While forward operators for a direct assimilation of
solar images have become available recently and a first visible channel is already used operationally, their assimilation remains challenging due to strong non-linearities, ambiguities and high inter-channel correlations. This study addresses two central questions: what is the potential impact of assimilating multiple
solar channels jointly, and can observed reflectances be transformed into physically meaningful, uncertainty-quantified variables better suited to assimilation than the raw reflectances themselves? As a proof of concept, we assess the joint information content of six
solar channels from the Flexible Combined Imager (FCI) onboard Meteosat Third Generation and introduce a novel "Backward Operator" (BO) for probabilistic
retrievals of cloud-related variables. The BO is implemented in a machine learning approach as a distributional regression network that is trained on synthetic images from a NWP regional model run and produces multivariate Gaussian estimates of total optical thickness, column cloud fraction, ice fraction, and effective radii of water and ice. The BO predictions are unbiased and well-calibrated, with realistic, situation-dependent and non-trivial covariance structures. The retrieved variables can be overall usefully constrained. Despite strong inter-channel correlations, combining multiple channels yields substantial performance improvements. As the BO does not require prior information, is consistent with an existing forward operator, and yields cloud variables more linearly related to the NWP model state, assimilating these variables could be a viable alternative to direct reflectance assimilation.
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All authors Stefano Franzoni, Christopher Bülte, Leonhard Scheck, Christian Keil, George C. Craig
Plasma Physics
Reconnection-induced electron energization in magnetospheric Kelvin-Helmholtz dynamics
Silvia Ferro, Fabio Bacchini, Giuseppe Arrò et al. (+2 more)
The Kelvin-Helmholtz instability (KHI) is a major driver of multiscale
plasma dynamics at velocity shear layers, where it can promote the formation of current sheets and the onset of
magnetic reconnection as well as drive
plasma energization. While recent kinetic studies have shown efficient electron heating during nonlinear KH
evolution, the connection between reconnection dynamics and localized electron energization is still not fully understood. We investigate this link using two-dimensional fully kinetic simulations of KHI developing in a double-periodic system with two velocity shear layers and a uniform guide field, initialized from a finite-Larmor-radius
equilibrium. During the nonlinear stage, initially coherent
vortices evolve into layers populated by fragmented current sheets containing active
magnetic reconnection sites. The global energetics reveal distinct species-dependent energization pathways. Ions act as the primary energy reservoir, transferring energy to the electromagnetic fields, while electrons receive the dominant net positive energy input. Electron energization is strongly anisotropic ($T_{\parallel,e} > T_{\perp,e}$) and localized within intermittent reconnecting current sheets associated with enhanced field-particle energy exchange and elevated agyrotropy. These regions also show the development of suprathermal tails in the electron energy distributions, providing evidence for nonthermal electron energization. Despite opposite vorticity orientations, the two shear layers exhibit similar statistical behavior. Together, these results establish a direct connection between reconnection-associated current structures and localized electron energization in collisionless KHI dynamics.
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All authors Silvia Ferro, Fabio Bacchini, Giuseppe Arrò, Francesco Pucci, Pierre Henri
Atomic Physics
Direct evidence for projectile electronic structure effects in slow multielectron capture collisions
Akash Srivastav, Sumit Srivastav and Bhas Bapat
We investigate the role of the electronic structure of the projectile in ionization and subsequent fragmentation of CO$_2$ induced by multielectron capture in collisions at 0.31 a.u. impact velocity. Focusing on the $\text{CO}_2^{3+} \rightarrow \text{O}^+:\text{C}^+:\text{O}^+$ break-up channel as a representative channel, we report kinetic energy release distributions (KERDs) for collisions with equi-velocity N$^{q+}$ and O$^{q+}$ projectiles. We consider two complementary categories of measurements. In the first category, in which different projectiles of the same charge are considered, we find that KERDs obtained with N$^{q+}$ and O$^{q+}$ impact ($q=4,6$) are broadly similar, but they differ significantly from the earlier reported KERD with Ar$^{q+}$ impact. In the second category, pronounced differences are observed between the KERDs obtained with isoelectronic N$^{q+}$ and O$^{(q+1)+}$ ($q=3,5,7$) projectiles. These results provide direct evidence that projectile electronic structure plays a critical role in multielectron capture collisions.
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Computational Physics
MADField: Multi-fidelity Amortized Density Field for Adsorption in Nanoporous Materials
Yoonho Kim, Seongsu Kim, Sungsoo Ahn et al. (+1 more)
High-throughput computational screening of nanoporous materials for gas storage and separation requires fast and accurate characterization of adsorption
equilibrium. Particle-based grand canonical Monte Carlo (GCMC) and density-based classical density functional theory (cDFT) provide simulation-based estimates of gas uptake and adsorbate density fields, but their speed-accuracy tradeoff remains insufficient for large-scale screening. In this work, we address this gap with Multi-fidelity Amortized Density Field for Adsorption in Nanoporous Materials (MADField), which reframes adsorption prediction as
equilibrium density-field estimation. MADField learns from two complementary fidelities, combining broad and scalable cDFT density supervision with higher-fidelity GCMC density labels, and recovers gas uptake by integrating the predicted density field. MADField improves uptake accuracy over the strongest baselines by 6.0x for cDFT and 15.4x for GCMC, and its predicted fields accelerate cDFT solvers with 2.0x fewer iteration steps while recovering 42 percent of cases that fail under standard settings. Finally, we evaluate MADField for conventional CH4 working capacity screening on the 270k-structure ARC-MOF database. Within this space of extremely rare high-capacity targets, 167 in total, the model achieves 56x higher average precision than the strongest baseline and accelerates inference by five orders of magnitude compared to GCMC. By prioritizing the MADField rankings, selecting the top 1.7 percent of candidates recovers 95 percent of all targets, while the top 6 percent ensures 100 percent recall.
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All authors Yoonho Kim, Seongsu Kim, Sungsoo Ahn, Honghui Kim
Atmospheric and Oceanic Physics
Elastic Non-Uniform FFT (ENUFFT) spectral reconstruction of irregularly sampled orography on unstructured grids
Tridib Banerjee, Felix Jochum and Ulrich Achatz
Subgrid-scale orography remains a leading source of uncertainty in numerical modeling because terrain spectra must be recovered from irregularly sampled elevation data and then reduced to a flow-dependent launch budget for parameterizations. Existing approaches are limited either by assuming regular samples on rectangular grids or by fitting coefficients whose truncation and regularization effects become embedded in the spectrum. None achieves dynamic, flow-dependent truncation. This study introduces an Elastic Non-Uniform Fast
Fourier Transform (ENUFFT) framework that computes local
Fourier coefficients directly from irregularly sampled orography on unstructured grids, without interpolation or fitting. It combines a type-1 NUFFT with local windowing, quadrature weights, and a new Elastic Mode Selection (EMS)
algorithm for retaining a local flow-dependent subset of modes. ENUFFT is compared with the strongest relevant existing method in a monochromatic and a real Alpine terrain test. In both cases, it recovers peak amplitude and direction comparably while significantly compacting the spectra (monochromatic ~25%, Alpine ~60%). It also satisfies the Parseval condition more closely with its spectral variance (energy) deviating from reference by ~14-24% versus ~500-122,000% for the existing method. Its EMS is additionally tested in a mountain-wave test where it reduces the launch spectrum by >=75% while keeping launch-power loss <=7%. Along with better compute scaling, ENUFFT is thus a computationally efficient, physically interpretable framework that can make
Fourier-based orographic source descriptions practical for spectral-budget-aware parameterizations.
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Optics
Nested active pointing control for interspacecraft laser interferometry
Daikang Wei, Christoph Bode, Yihao Yan et al. (+3 more)
Precise pointing control is critical for interspacecraft
laser interferometry, as angular misalignment introduces measurement noise and can even lead to
laser link loss. We present a nested control architecture that uses differential wavefront sensing signals to drive a fast steering mirror (FSM) to track the incoming beam, while feeding the FSM deflections back to the attitude and orbit control system to suppress angle-dependent optical path variations. This scheme is experimentally validated in our hexapod-based setup. Relative to standalone FSM actuation, the nested configuration reduced the low-frequency root-mean-square pointing jitter by approximately a factor of 2 over 3–70 mHz in both directions. Additionally, tilt-to-length coupling was suppressed by an order of magnitude below 5 mHz and by two orders of magnitude below 0.5 mHz. These results demonstrate the feasibility of nested active pointing control for future interspacecraft
laser interferometry missions.
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All authors Daikang Wei, Christoph Bode, Yihao Yan, Vitali Müller, Juan José Esteban Delgado, Gerhard Heinzel
Optics
Non-line-of-sight imaging with arbitrary relay surface geometries via 3D Gaussian Transient Rendering
Yi Wang, Ziyu Zhan, Yuran Wang et al. (+5 more)
Imaging objects hidden outside the direct line of sight expands the effective field of view and is critical for applications such as autonomous driving and robotic perception. Despite impressive progress in time-of-flight (ToF)-based non-line-of-sight (NLOS) imaging, real-world deployment remains challenging because practical measurements are often collected over spatially limited, arbitrarily shaped relay regions-conditions that violate the planar-wall and dense-sampling assumptions made by most existing methods. To address these limitations, we propose a LOS-guided NLOS imaging pipeline that imposes no
geometric assumptions on the relay surface and naturally supports both confocal and non-confocal configurations. Our method represents the hidden scene using 3D Gaussian primitives and couples them with an efficient, differentiable transient rendering model, enabling end-to-end optimization directly from measured transients. We validate our approach on real-world measurements from both a public
dataset and a custom-built capture system. Across settings, our method achieves state-of-the-art reconstruction fidelity under spatially limited, sparsely sampled conditions, and significantly outperforms existing methods on complex, arbitrary relay surface geometries.
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All authors Yi Wang, Ziyu Zhan, Yuran Wang, Hao Wang, Qiang Liu, Zuoqiang Shi, Lingyun Qiu, Xing Fu
Plasma Physics
Semi-local Floquet theory for active azimuthal magnetic modulation of Hall-thruster high-frequency instabilities
Yinjian Zhao and Changzheng Hu
We develop a semi-local Floquet model to screen active azimuthal
magnetic modulation as a control approach for high-frequency drift instabilities in
Hall thrusters. The uniform-field kinetic response supplies a local kernel for a finite sideband
matrix, and stability is measured by the upper growth envelope over the Bloch zone. Root searches include both frequency branches, continuation, singular-value validation, and contour root counts. At the reference normalized parallel wave number k_z = 0.045, for sinusoidal profiles with periods of 10, 20, and 40 mm, every nonzero sampled modulation amplitude increases both the peak and integrated positive Bloch-zone growth. The uniform-field reference is already unstable, so these increases describe faster linear amplification rather than the onset of instability from a stable state. At a period of 20 mm and relative amplitude 0.5, the peak and integral increase by 26.74 and 26.44 percent, respectively; this large amplitude requires caution within the semi-local ordering. Selected projected wave-number windows still show reductions, but these coexist with enhanced global growth and do not establish stabilization. A supplementary parallel-wave-number scan finds weak global growth reduction at k_z = 0.09: at 20 mm and amplitude 0.3, the peak and integral decrease by 0.300 and 0.299 percent, respectively, while the spectrum remains unstable. The sign and magnitude of the modulation effect therefore depend on the tested
plasma parameters; neither these small reductions nor the local projected changes establish improved nonlinear transport.
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Accelerator Physics
Role of the Local Electric-Field in High-Field Conditioning of DC Electrodes:Numerical and Experimental Insights
Victoria Madeleine Bjelland, William Lee Millar, Morten Kildemo et al. (+1 more)
Conditioning, the progressive increase of voltage-holding through the controlled application of fields, is an important and widely used process for bringing high-field and high-voltage devices up to their full operating parameters. Here, a study is presented on how conditioning can vary within a device, specifically, when there is a spatial variation in the surface electric field. What has been observed in high-field pulsed direct-current electrodes and
radio-frequency structures is that locations exposed to higher fields exhibit a greater tendency to breakdown, but this increase is counteracted by an increased conditioning rate. This interplay explains the observed breakdown locations and provides important insights into the mechanisms underlying both conditioning and breakdown. This study combines Monte Carlo simulations with experimental results from pairs of high-field electrodes with a radially varying surface electric field. Results are presented from a high-field pulsed DC system, in which the position of each breakdown during conditioning was recorded by triangulation using a pair of cameras, and the results are compared with Monte Carlo simulations.
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All authors Victoria Madeleine Bjelland, William Lee Millar, Morten Kildemo, Walter Wuensch