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Physics Physics

physics Watch Physics

Friday 8 May

Fluid Dynamics · Cross-listed from cond-mat.soft

Droplet Deformation and Emulsion Rheology in Two-Dimensional Odd Stokes Flow

Thomas Appleford, Hugo França and Maziyar Jalaal

We study the deformation of a two-dimensional viscous droplet in simple shear in the presence of odd viscosity. We derive an analytical solution for the droplet shape and surrounding flow field within the framework of odd Stokes flow, allowing for differences in both even and odd viscosity between the droplet and the surrounding fluid. This solution yields closed-form expressions for the macroscopic apparent even and odd viscosities of a dilute emulsion. We show that, provided all viscosity differences remain moderate, the steady-state Taylor deformation parameter satisfies $D_T^\infty = \text{Ca} + \mathcal{O}(\text{Ca}^2)$ so that the leading-order droplet deformation is unchanged from the classical (even-viscous) result. Nevertheless, pronounced effects emerges beyond leading order, where our direct numerical simulations reveal odd-viscous differences to the droplet deformation. In addition, we show that the flow is influenced only by the difference in odd viscosity between the droplet and the medium and not on their individual values. Our analysis clarifies how odd viscosity might modify the effective rheology of dilute emulsions and provides a framework for interpreting droplet-based measurements of odd-viscous response. Key words: odd viscosity $|$ droplets $|$ emulsions $|$ surface tension $|$ chiral fluids
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Physics Education

Strategies for Collecting Multi-Institutional Data in Discipline-Based Education Research

Meagan Sundstrom, David Esparza, Justin Gambrell et al. (+2 more)

Multi-institutional studies are critical for advancing discipline-based education research (DBER) because they allow us to determine where and for whom research findings are applicable. Despite this benefit, such studies remain relatively rare due to the complexities of coordinating data collection across different institutions. In this paper, we describe key challenges and propose actionable strategies for implementing multi-institutional DBER studies. We focus on navigating Institutional Review Board procedures, recruiting participants from a range of institution types, standardizing data sources across institutions, and managing logistics. We also provide an applied example of these strategies from a national research project in which we collected concept inventory data, social network surveys, and classroom observations from 31 introductory physics instructors at 28 institutions in the United States.
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All authors Meagan Sundstrom, David Esparza, Justin Gambrell, Adrienne Traxler, Eric Brewe

Computational Physics

Physics-Informed Reduced-Order Operator Learning for Hyperelasticity in Continuum Micromechanics

Hamidreza Eivazi and Henning Wessels

Physics-informed operator learning is an attractive candidate for surrogate modeling of microstructures, especially in multiscale finite-element simulations. Its practical use, however, is often limited by the high cost of loss evaluation. We address this bottleneck by combining the Equilibrium Neural Operator (EquiNO) with the QR-based discrete empirical interpolation method (Q-DEIM). EquiNO learns only the modal coefficients of reduced displacement-fluctuation and first Piola-Kirchhoff stress representations built from periodic and divergence-free bases, thereby enforcing periodicity and mechanical equilibrium by construction. Q-DEIM then identifies a small set of spatial points through a column-pivoted QR factorization of the stress basis and restricts constitutive evaluations during training to these points alone. This makes full-batch second-order optimization practical for three-dimensional representative volume elements (RVEs). Homogenized first Piola-Kirchhoff stresses are recovered directly from the offline-averaged reduced stress modes, without the need to reconstruct the full stress field at inference time. We validate the framework on two three-dimensional finite-strain hyperelastic RVEs. Q-DEIM reduces the per-step training cost by roughly three orders of magnitude relative to full-field loss evaluation, while reduced homogenization achieves speed-up factors of order $10^3$ to $10^4$ over direct full-field computations. Despite relying on only a small number of offline snapshot loading paths for basis construction, the method accurately interpolates and extrapolates both microscopic stress fields and homogenized stresses, with prediction quality improving systematically as more snapshots are added.
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Atomic Physics

Quadratic Zeeman effect in light boron-like ions

V. A. Agababaev, A. V. Volotka, D. A. Glazov et al. (+3 more)

The quadratic Zeeman effect is calculated for the ground $^2P_{1/2}$ state of light boron-like ions in the range of nuclear-charge numbers $Z = 10-24$. The calculations are performed in the Furry picture using three models for the zeroth-order approximation potential: pure nuclear Coulomb potential and two effective screening potentials $-$ core-Hartree and Kohn-Sham. First-order perturbation-theory contributions are considered: the one-photon-exchange correction and the radiative corrections associated with the self-energy and vacuum-polarization diagrams. The dominant contributions from the self-energy diagrams are calculated within a rigorous QED approach. The vacuum polarization corrections are obtained within the electric-loop approximation in the leading order, which is given by the Uehling potential. As a result, theoretical predictions for the contribution of the quadratic Zeeman effect to the binding energy of the valence electron in the $^2P_{1/2}$ state are obtained. The results can be used for the analysis of high-precision $g$-factor and fine-structure splitting measurements in boron-like highly charged ions.
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All authors V. A. Agababaev, A. V. Volotka, D. A. Glazov, A. V. Malyshev, M. M. Osiptsov, V. M. Shabaev

Physics and Society · Cross-listed from cs.DL

LLM hallucinations in the wild: Large-scale evidence from non-existent citations

Zhenyue Zhao, Yihe Wang, Toby Stuart et al. (+3 more)

Large language models (LLMs) are known to generate plausible but false information across a wide range of contexts, yet the real-world magnitude and consequences of this hallucination problem remain poorly understood. Here we leverage a uniquely verifiable object - scientific citations - to audit 111 million references across 2.5 million papers in arXiv, bioRxiv, SSRN, and PubMed Central. We find a sharp rise in non-existent references following widespread LLM adoption, with a conservative estimate of 146,932 hallucinated citations in 2025 alone. These errors are diffusely embedded across many papers but especially pronounced in fields with rapid AI uptake, in manuscripts with linguistic signatures of AI-assisted writing, and among small and early-career author teams. At the same time, hallucinated references disproportionately assign credit to already prominent and male scholars, suggesting that LLM-generated errors may reinforce existing inequities in scientific recognition. Preprint moderation and journal publication processes capture only a fraction of these errors, suggesting that the spread of hallucinated content has outpaced existing safeguards. Together, these findings demonstrate that LLM hallucinations are infiltrating knowledge production at scale, threatening both the reliability and equity of future scientific discovery as human and AI systems draw on the existing literature.
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All authors Zhenyue Zhao, Yihe Wang, Toby Stuart, Mathijs De Vaan, Paul Ginsparg, Yian Yin

Optics

Transmission resonances in silicon subwavelength grating slot waveguide with functional host material for sensing applications

S. Hadi Badri

A highly sensitive and selective CO2 gas sensor is presented based on a subwavelength grating (SWG) slot waveguide. Polyhexamethylene biguanide (PHMB) as a functional material fills the slot gap as well as the space between the silicon pillars of the SWG structure. Beyond the photonic bandgap of the SWG slot waveguide, there are transmission resonances sensitive to the refractive index changes of PHMB due to the infiltration of CO2 molecules into the functional material. The numerical simulations indicate that the sensitivity of the structure is S=12.9 pm/ppm which is considerably higher than the previously designed gas sensors based on functional materials. The higher sensitivity of the proposed sensor is attributed to the strong confinement of the light in the slot gap filled with functional material while previous designs have limited light-matter interaction by placing the functional material in the cladding. The proposed structure may be used to design various sensors by utilizing different functional material sensitive to the desired analyte.
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Optics

Subwavelength grating waveguide filter based on cladding modulation with phase-change material grating

S. Hadi Badri and Saeid Gholami Farkoush

Subwavelength engineering and utilizing phase-change materials with large contrast in their optical properties have become powerful design tools for integrated silicon photonics. Reversible phase-transition of phase-change materials such as Ge2Sb2Te5 (GST) provides a new degree of freedom and opens up the possibility of adding new functionalities to the designed devices. We present an optical filter based on a silicon subwavelength grating (SWG) waveguide evanescently coupled to phase-change material loading segments arranged periodically around the SWG core. The effect of the GST loading segments' geometry and their distance from the SWG core on the filter's central wavelength and bandwidth are studied with three-dimensional finite-difference time-domain simulations. The employment of GST in the structure adds a switching functionality with an extinction ratio of 28.8 dB. We also examine the possibility of using the proposed structure as a reconfigurable filter by controlling the partial crystallization of the GST offering a blueshift of more than 4 nm.
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Chemical Physics · Cross-listed from cond-mat.mtrl-sci

Selectivity- and Activity-Aware Catalyst Descriptors for CO$_2$ Hydrogenation on Alloy Nanocatalysts using Machine-Learned Force Fields

Prajwal Pisal, Ondřej Krejčí and Patrick Rinke

Adsorption energy distributions (AEDs) have emerged as a powerful and increasingly adopted descriptor for catalytic performance in high-entropy alloys and, more recently, in conventional metallic alloy nanocrystal catalysts. By accounting for diverse adsorption sites and crystallographic facets, AEDs more fully represent nanoparticle-based catalytic surfaces and show strong promise for accelerating rational design and discovery of heterogeneous catalysts, especially for CO$_2$ hydrogenation. However, previous high-throughput screenings have not provided simultaneous facet-level insight into catalytic activity, product selectivity, and thermodynamic stability. Here, we implement and extend the AED framework to individual crystallographic facets, combining facet-resolved adsorption fingerprints with Wulff-derived facet abundances and an interpretable latent-space analysis of C1-product selectivity. We employ universal machine-learned force fields trained on Open Catalyst Project data to compute adsorption energies across 226 experimentally observed metals, binary alloys, and previously unexplored ternary alloys, encompassing ~1.4 million adsorption configurations on >2,600 crystallographically distinct surfaces. Using distribution-based similarity analysis and principal component analysis of AED moments, we identify composition-facet combinations with adsorption landscapes associated with activity and qualitative selectivity trends toward methanol, methane, formic acid, and CO. Our framework links surface structure to adsorption-based catalytic trends, providing experimentally testable composition-facet candidates for further investigation and validation.
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Optics

High-Q Fano resonance in all-dielectric metasurfaces for molecular fingerprint detection

S. Hadi Badri, M. M. Gilarlue, Sanam Saeidnahaei et al. (+1 more)

We present and numerically investigate a high-quality factor (high-Q) meta-atom with Fano resonance. Numerical simulations indicate that the designed meta-atom has a single sharp Fano resonance in the 1350-1750 1/cm range. Moreover, the frequency of the single resonance can be tuned in this frequency range by scaling the meta-atom. We exploit these properties to design a pixelated metasurface for spectrometer-less molecular fingerprint retrieval. The proposed meta-atom with an average quality factor of 2000 makes it possible to decrease the scaling step of metapixels without introducing any resonance overlap between the metapixels leading to higher precision in label-free and non-destructive identification of the molecular fingerprints.
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All authors S. Hadi Badri, M. M. Gilarlue, Sanam Saeidnahaei, Jong Su Kim

Optics

Active Control of Topological Exceptional Points in Non-Hermitian Metasurfaces

Parul Sharma, Sobhan Subhra Mishra, Yash Gupta et al. (+4 more)

Active control and ultrafast switching of non-Hermitian photonic systems are essential for next-generation reconfigurable optical technologies. Here, we demonstrate dynamic temporal manipulation of EPs in the terahertz (THz) regime using optically excited germanium (Ge) as an active medium. By exploiting pump-probe delay as a continuous tuning parameter, we achieve sub-picosecond eigenmode switching (~0.5 ps) and realize a complete time-resolved EP encirclement within ~2 ps, enabling direct observation of topological phase accumulation. At EP, the metasurface exhibits highly asymmetric transmission for circularly polarized light, characteristic of chiral mode response. Furthermore, we observe ultrafast eigenmode switching and topological phase evolution within ~1 ps, achieving >99% cross-polarization modulation depth. The measured results show strong agreement with theoretical modeling, with a high Petermann factor of approximately 10^3, confirming the effectiveness of the design. Our work establishes pump-probe delay time as a dynamical control parameter for EP topology, introducing a new regime of ultrafast non-Hermitian photonics for high-speed switching, enhanced sensitivity, and tunable polarization control in the THz domain.
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All authors Parul Sharma, Sobhan Subhra Mishra, Yash Gupta, Brijesh Kumar, Ranjan Singh, Abhishek Kumar, Anshuman Kumar

Fluid Dynamics

Cassie-Wenzel transition induced by localized freezing after droplet impact on supercooled micro-patterned surfaces

Jun Fang, Mengqi Ye, Huafeng Liu et al. (+3 more)

Micro-patterned surfaces have attracted significant attention in numerous applications owing to their potential to enhance hydrophobic and icephobic properties. A Cassie state of final wetting of a droplet upon impact on a micro-patterned surface, which is highly favorable for anti-icing applications, is achieved in this study through rapid localized freezing in the droplet-surface contact region via tuning the coupled interplay among droplet spreading kinetics, interfacial heat transfer, and solidification dynamics. Synchronized high-speed imaging and infrared thermography are employed to probe droplet impact and freezing dynamics, with particular emphasis on the transition of wetting state and its effect on the resulting freezing morphology. Experimental results reveal that variations in impact velocity and wall temperature lead to a final frozen wetting-state transition of the droplet from the Wenzel to the Cassie regime, accompanied by pronounced changes in freezing time, final spreading diameter, and frozen height. The transition of wetting states is attributed to rapid localized freezing at the droplet bottom, which suppresses liquid penetration into the micro-pattern. At lower impact velocities and surface temperatures, droplets tend to maintain the Cassie state with extended freezing durations, whereas higher velocities or higher temperatures promote rapid penetration and accelerated freezing. This study elucidates the coupled penetrating-freezing mechanism governed by micro-pattern design and provides fundamental insights into the rational design of anti-icing and icephobic surfaces.
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All authors Jun Fang, Mengqi Ye, Huafeng Liu, Yupeng Jiang, Tianyou Wang, Zhizhao Che

Atomic Physics · Cross-listed from cond-mat.str-el

Ground states of quantum XY dipoles on the Archimedean lattices

Marcus Bintz, Ahmed Khalifa, Vincent S. Liu et al. (+4 more)

We report numerical ground states for the dipolar XY spin model, which describes extended antiferromagnetic interactions in two-dimensional arrays of polar molecules and two-level Rydberg atoms. Carrying out large-scale density matrix renormalization group (DMRG) calculations, we compute ground state properties on nine of the eleven Archimedean lattices–tilings of the plane by regular polygons. Four of these host trivial paramagnets, while another four develop collinear Neel magnetic order, as was found previously for the square lattice. For the ordered states, we calculate the hydrodynamic parameters (magnetization, susceptibility, and stiffness) and compare to linear spin wave theory. We also investigate the triangular lattice, for which we find several competing phases including coplanar magnetism, stripe density wave order, and a possible spin liquid; their relative stability is sensitive to the long-range couplings present in our dipolar model. Finally, the Archimedean classification is completed by the kagome lattice, which we argue in a companion work is likely to be a Dirac spin liquid.
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All authors Marcus Bintz, Ahmed Khalifa, Vincent S. Liu, Johannes Hauschild, Michael P. Zaletel, Shubhayu Chatterjee, Norman Y. Yao

Instrumentation and Detectors

Test-Beam Performance of the AstroPix Silicon Sensor for Imaging Calorimetry

Yoonha Hong, Jeongsu Bok, Geunpil An et al. (+35 more)

AstroPix is a high-voltage CMOS HVCMOS monolithic active pixel sensor MAPS developed for future space-based gamma-ray missions. It is also a candidate technology for the imaging layer of the Barrel Imaging Calorimeter BIC in the ePIC experiment at the future Electron-Ion Collider EIC. We report the first AstroPix test-beam results obtained at the KEK Photon Factory Advanced Ring PF-AR and the CERN Proton Synchrotron PS T10 beam line in 2025, using the third prototype AstroPix-v3. AstroPix-v3 sensors were operated as both standalone tracking layers and imaging layers interleaved with prototype lead/scintillating-fiber Pb/SciFi calorimeter modules, using electron and hadron beams in the few-GeV/c momentum range. Event synchronization between the continuous readout of AstroPix-v3 and the trigger-based readout of the Pb/SciFi calorimeter was achieved using a common timestamp. The AstroPix-v3 sensors exhibit stable performance, reaching a maximum hit efficiency of 68 percent at a bias voltage of -400 V under pion-dominated beam conditions. When combined with the Pb/SciFi calorimeter, the AstroPix layers successfully capture the development of electromagnetic showers. Using Cherenkov-based particle identification, electron-induced events exhibit significantly higher hit multiplicities and broader spatial distributions than pion-induced events, thereby providing clear discrimination between electromagnetic and hadronic showers. These results demonstrate that AstroPix-v3 provides effective, high-granularity imaging of shower development and is well suited as an imaging layer in future calorimeter systems for both collider and space-based experiments.
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All authors Yoonha Hong, Jeongsu Bok, Geunpil An, Joonsuk Bae, Yunseul Bae, Regina Caputo, Yun Eo, Wooseok Ham, Woohyeon Heo, Yoonha Hong, Manoj Jadhav, Seo Yun Jang, Jinryong Jeong, Hyon-Suk Jo, Sylvester Joosten, Beomkyu Kim, Bobae Kim, Chong Kim, Dongguk Kim, Minsuk Kim, Shin Hyung Kim, Woojong Kim, Wonjun Ko, Changhui Lee, Hyungjun Lee, Jason Sang Hun Lee, Jongwon Lee, Kyeongpil Lee, Sehwook Lee, Sangwoo Park, Jaehyeok Ryu, Bogyeong Seo, Jessica Metcalfe, Minsub Shim, Junseop Shin, Hwidong Yoo, Maria Żurek, Sanghoon Lim

Atomic Physics · Cross-listed from quant-ph

A Unified Local Light-shifts Encoding For Solving Optimization Problems on a Rydberg Annealer

Kapil Goswami and Peter Schmelcher

Combinatorial optimization problems play a central role in computer science with many real world applications. A number of relevant problems remain computationally difficult to solve as they lie in the NP-hard complexity class. We present a unified framework for solving such optimization problems represented in the quadratic unconstrained binary optimization (QUBO) formalism, namely two-SAT, XOR-SAT, mixed-two-XOR-SAT, set packing, quadratic assignment, binary clustering, and protein folding, by expanding the domain of applications of PRR, 6(2), 023031. A direct mapping from the QUBO form of these problems onto the Rydberg quantum platform is demonstrated as our first step. This mapping to the Rydberg system depends on distance-dependent long-range interactions and configurable local detuning, thus reducing resource overhead and improving scalability. Following-up on the encoding, the solution is reached by steering the system toward the ground state of the target Hamiltonian using an optimized quantum annealing protocol that controls the time-dependent detuning and Rabi frequency profiles. The framework can handle a variety of problems, each with different complexity. To quantify the complexity of any problem, a generalized hardness parameter is introduced that compares different problems based on the structure of their optimization landscapes. This is a proceedings contribution to the Athens Workshop in Theoretical Physics: 10th Anniversary, held at the National and Kapodistrian University of Athens on December 17-19 2025.
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Computational Physics · Cross-listed from quant-ph

Entanglement-informed distributed wavefunction approach to scalable quantum many-body systems

Adriano Amaricci

We show that the entanglement structure of quantum many-body states defines a natural and optimal distributed representation for their simulation. An arbitrary entanglement cut induces a bipartite decomposition of the wavefunction, mapping its distribution onto that of the entanglement spectrum. In this representation the Hamiltonian application, the core of Krylov-subspace methods, reduces to local contractions and communication-optimal operations. Using benchmarks from different methods and models, we demonstrate near-linear scaling for sufficiently large systems and identify entanglement spectrum fragmentation as a key factor controlling computational cost. This establishes entanglement as an organizing principle and unified, method-independent, route for scaling up quantum many-body simulations.
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Computational Physics

foap4: Adaptive mesh refinement with OpenACC/OpenMP, MPI, and p4est

Jannis Teunissen, Héctor R. Olivares Sánchez, Jesse Vos et al. (+10 more)

GPUs and other accelerators are increasingly used for scientific computing. We want to add GPU support to existing parallel adaptive mesh refinement (AMR) codes written in Fortran. To understand which changes are necessary to obtain good performance we have developed foap4, an AMR framework implemented in Fortran that uses OpenACC/OpenMP, MPI, and the p4est library. We discuss the design and implementation of the framework. Several benchmark problems are considered, in which Euler's equations of gas dynamics are solved using explicit time integration. These benchmarks are performed in both 2D and 3D, using static and adaptive meshes, for varying problem sizes on different hardware. Our results show that AMR simulations can be carried out efficiently on GPUs, even when using relatively small grid blocks of $8^3$ or $16^3$ cells. With NVIDIA's nvfortran compiler similar performance is obtained with OpenACC and OpenMP offloading. Initial results with the gfortran compiler and OpenACC demonstrate the portability of foap4.
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All authors Jannis Teunissen, Héctor R. Olivares Sánchez, Jesse Vos, Leon Oostrum, Johan Hidding, Victor Azizi, Yuhao Zhou, Hao Wu, Adrian Kelly, Olaf Willocx, Chun Xia, Rony Keppens, Oliver Porth

Instrumentation and Detectors

Optimisation of TES design for the CRESST experiment

G. Angloher, S. Banik, A. Bento et al. (+54 more)

The CRESST experiment aims at the direct detection of sub-GeV dark matter particles via elastic scattering off nuclei in different target crystals at cryogenic temperatures. The advancement in W-TES sensors allowed the CRESST detectors to reach energy thresholds of 10 eV and lower, opening the way to the exploration of dark matter masses as low as 70 MeV/c2. This work presents optimisation studies of W-TESs aimed at further improving the signal-to-noise ratio and overall detector performance. In particular, we investigate the thickness, dimensions and material composition of phonon collectors and assess their impact on detector response. The results demonstrate a significant performance enhancement and establish new benchmarks for the sensors used within CRESST.
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All authors G. Angloher, S. Banik, A. Bento, A. Bertolini, R. Breier, C. Bucci, J. Burkhart, L. Burmeister, L. Canonica, E. Cipelli, S. Di Lorenzo, J. Dohm, F. Dominsky, A. Erb, E. Fascione, F. v. Feilitzsch, S. Fichtinger, D. Fuchs, V.M. Ghete, P. Gorla, P.V. Guillaumon, S. Gupta, D. Hauff, M. Ješkovský, J. Jochum, M. Kaznacheeva, H. Kluck, H. Kraus, B. von Krosigk, A. Langenkämper, M. Mancuso, B. Mauri, V. Mokina, C. Moore, P. Murali, M. Olmi, T. Ortmann, C. Pagliarone, L. Pattavina, F. Petricca, W. Potzel, P. Povinec, F. Pröbst, F. Pucci, F. Reindl, J. Rothe, K. Schäffner, J. Schieck, S. Schönert, C. Schwertner, M. Stahlberg, L. Stodolsky, C. Strandhagen, R. Strauss, I. Usherov, M. Zanirato, V. Zema

Instrumentation and Detectors

Track and Vertex Reconstruction with the ATLAS Inner Detector

ATLAS Collaboration

Charged-particle reconstruction is a fundamental part of the event reconstruction in modern multi-purpose high-energy physics detectors. This paper describes the algorithms used to reconstruct charged particles and primary vertices with the ATLAS Inner Detector. The most recent software configuration deployed for data-taking is described, and the performance obtained when this software is used to process Run 2 (2015-2018) data, a subset (from 2022) of Run 3 (2022-2026) data, and corresponding simulated data is presented. The ATLAS track and vertex reconstruction performance is shown for up to 80 simultaneous proton-proton interaction. It maintains a high efficiency, good resolution for key parameters, and low rates of mis-reconstructed candidates for Run 2 and Run 3 conditions.
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Optics

Boundary-dominated optomechanics in silicon metamaterial membranes

David González-Andrade, Paula Nuño Ruano, Jianhao Zhang et al. (+10 more)

Stimulated Brillouin scattering in integrated photonic waveguides enables coherent coupling between optical photons and gigahertz acoustic phonons, providing a powerful mechanism for on-chip microwave photonics and opto-acoustic signal processing. Despite theoretical predictions of ultra-strong Brillouin interactions arising from enhanced light-sound coupling at device boundaries, most state-of-the-art integrated demonstrations remain governed by bulk photoelastic effects. This limitation stems from trade-offs between optical loss, interaction with waveguide boundaries and accessible phonon frequencies associated with the use of transverse-electric optical modes coupled to horizontally breathing mechanical modes. Here we demonstrate a new approach based on transverse-magnetic optical modes coupled to vertically breathing mechanical modes in suspended silicon membranes engineered with subwavelength metamaterial claddings. In this geometry, the interaction is dominated by the moving-boundary effect occurring at smooth top and bottom interfaces, while the phonon frequency is set primarily by the membrane thickness rather than its width. We observe forward Brillouin interactions at a record frequency of 12 GHz with a gain of 7200 W$^{-1}$ m$^{-1}$ and a mechanical quality factor of 620, yielding the highest Brillouin gain-to-quality-factor ratio reported in silicon waveguides. The devices exhibit net Brillouin amplification in millimeter-scale waveguides with pump powers below 15 mW, establishing a scalable platform for high-frequency integrated opto-acoustic signal processing.
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All authors David González-Andrade, Paula Nuño Ruano, Jianhao Zhang, Paul Joseph Robin, Hiba El Batoul Ferhat, Samson Edmond, Pavel Cheben, Daniele Melati, Eric Cassan, Laurent Vivien, Delphine Marris-Morini, Norberto Daniel Lanzillotti-Kimura, Carlos Alonso-Ramos

Optics

Asynchronous Event-Based Spectroscopy for Microsecond-Resolved Spectral Reconstruction

Joana M. Teixeira, Tomas Lopes, Tiago D. Ferreira et al. (+3 more)

Many physical and chemical processes of interest evolve on timescales that push the limits of conventional spectroscopic instrumentation. Indeed, the temporal resolution of standard spectrometers is often insufficient to track these dynamics, which is connected to the fact that most systems rely on frame-based sensors, imposing fundamental constraints on acquisition speed, sensitivity, and data efficiency, frequently limiting practical operation to the kHz regime. In this work, we present an approach to circumvent this limitation by developing an event-based spectrometer to enable spectral reconstruction with microsecond temporal resolution by leveraging a Czerny-Turner configuration combined with asynchronous and event-driven sensing. A dedicated signal processing pipeline converts the resulting stream of binary events into calibrated spectra through temporal accumulation, geometric correction, and vertical spatial integration of the spectral line, covering a 234nm bandwidth in the visible range with a spectral resolution of approximately 0.18nm per pixel. Performance characterization under temporally modulated illumination demonstrates that the event-based spectrometer can reconstruct spectra at probing rates of up to tens of kilohertz, far exceeding the practical limits of a conventional frame-based spectrometer operated in parallel, while accurately preserving spectral peak positions and relative spectral features. Finally, to further illustrate its potential applications, the system is validated in a microfluidic experiment integrated into an inverted microscope, where spectral changes induced by an absorbing dye are tracked with higher temporal fidelity and resolution compared with the frame-based approach. These results establish event-based spectroscopy as a promising paradigm for real-time, high-temporal-resolution spectral measurements in dynamic and low-light applications.
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All authors Joana M. Teixeira, Tomas Lopes, Tiago D. Ferreira, Catarina S. Monteiro, Pedro A.S. Jorge, Nuno A. Silva

Optics · Cross-listed from cond-mat.mes-hall

Exciton-mediated optical control of liquid-solid friction

Timur Pryadilin, Alexey Kavokin and Baptiste Coquinot

Interfacial friction in nanofluidic systems can arise from fluctuation-induced coupling between liquid charge fluctuations and the internal excitations of the confining solid. Here, we develop a microscopic theory of exciton-mediated solid-liquid friction based on the coupling between optically generated excitons and charge fluctuations in water. We distinguish between static excitons, localized by disorder or functionalization, and dynamic excitons, which interact with water through polarization fluctuations. In both cases, we derive analytical formulas for the excitonic friction, which is experimentally tunable and can significantly reduce the slip length and thereby the hydraulic permeability of nanochannels. Applying our framework to carbon nanotubes, we quantitatively reproduce the recent measurements of Kistwal et al., showing a reduction of nanotube diffusion under optical excitation, without fitting parameters. More broadly, our results establish excitons as a mechanism to optically control nanofluidic transport and suggest that excitonic photoluminescence could provide an optical probe of flow velocity inside nanochannels.
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Instrumentation and Detectors

Measurements of the micro-spill structure of medical cyclotron and synchrotron beams and its impact on pulse pileup

Matthias Knopf, Simon Waid, Stefan Gundacker et al. (+10 more)

Detector characterization and instrumentation testing are often performed at cyclotron and synchrotron facilities, many of which were originally developed for medical applications in cancer therapy. For particle physics experiments requiring a single-particle resolution, pileup can significantly degrade data quality, making precise knowledge of the beam time structure essential for selecting appropriate readout parameters. However, such information is often unavailable from the facilities and challenging to determine experimentally. Here, we report measurements of the spill time structure at two medical accelerator facilities using a silicon carbide (SiC) particle sensor coupled to a high-frequency readout system. Owing to its high carrier saturation velocity and the tolerance to large bias voltages, SiC is well suited for fast readout and measurements requiring precise timing. Using a 6 GHz readout with custom SiC diodes, we characterize the micro-spill structure of both cyclotron and synchrotron beams on a sub-nanosecond timescale. The measured arrival-time distributions exhibit modulation with the accelerator RF frequencies, reflecting features of the extraction process. The resolved micro-spill structure enables quantitative estimation of pileup contributions and provides design constraints for future readout electronics. The presented results emphasize the importance of the characterization of the beam time-structure characterization for the development of precise readout systems.
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All authors Matthias Knopf, Simon Waid, Stefan Gundacker, Sebastian Onder, Daniel Radmanovac, Philipp Gaggl, Giulio Bordieri, Francesco Cordoni, Marta Missiaggia, Enrico Verroi, Giulio Magrin, Thomas Bergauer, Albert Hirtl

Fluid Dynamics

A causal model of drop breakup in turbulence

Daniel Morón, Ianto Cannon, Alberto Vela-Martín et al. (+1 more)

Fragmentation of drops and bubbles in turbulence controls interfacial area generation, mixing, and transport in environmental and engineering flows. Reduced-order models of breakup are highly sought after, but the bidirectional, nonlinear coupling between the interfacial and hydrodynamic stresses is an obstacle to their development. By leveraging a decomposition of the flow into outer and inner regions introduced by Vela-Martín & Avila (2021), we demonstrate that at low Weber numbers breakup is caused by outer eddies that produce extreme events of interfacial stretching. Capillary forces oppose stretching and transfer the interfacial energy back to the velocity field by generating inner eddies as the drop relaxes. Hence, for breakup to occur, outer stretching events must inject energy faster than the interface can convert it into inner eddies. Numerical simulations initialized with ellipsoidal drops reveal that the energy transfer to the inner eddies is governed by the capillary time, whereas the outer forcing acts on the eddy-turnover time scale. Building on these observations and the governing equations, we derive a simple model for the breakup rate. Although the underlying assumptions are strictly valid only in the limit of large surface tension, the resulting equation quantitatively captures both drop and bubble breakup rates over a wide range of Weber numbers using only a single fitting parameter. Our results establish a direct causal link between turbulent intermittency and the memoryless nature of breakup, and suggest a universal mechanism governing drop and bubble breakup at low Weber numbers.
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All authors Daniel Morón, Ianto Cannon, Alberto Vela-Martín, Marc Avila

Optics

Static SERS with near-minus-one-epsilon substrate

Alexey P. Vinogradov and Evgeny S. Andrianov

A mechanism for additional enhancement of SERS in the nanoparticle-on-mirror scheme is proposed. This new mechanism is based on the use of a substrate made of material with a near-minus-one permittivity. The setup involves a plasmonic nanoparticle in the form of an oblate ellipsoid positioned above the substrate and a Raman-active molecule located between them. In the conventional nanoparticle-on-mirror scheme, the plasmonic dipole resonance frequency coincides with the Stokes frequency of the Raman-active molecule. Consequently, due to the Purcell effect, the molecule's near fields mostly excites a dipole mode in nanoparticle. This dipole moment is many times greater than the dipole moment of the molecule by itself. If the real part of substrate permittivity is near minus one, the image of the nanoparticle dipole moment in the mirror-substrate is a dipole moment pointed in the same direction but approximately $ 1/{\rm{Im}} \varepsilon_{_{\rm{ENZ}}}$ times larger in magnitude. The simultaneous radiation of these two dipoles additionally increases the SERS intensity in $ 10^4$ times.
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Chemical Physics

On the single-Hessian Gaussian wavepacket dynamics

Davide Barbiero and Jiří J. L. Vaníček

Single-Hessian Gaussian wavepacket dynamics (GWD) significantly reduces the computational burden of Heller's local harmonic GWD, while maintaining comparable accuracy in approximating vibronic spectra. Here, we provide a new, symplectic derivation of the equations of motion of single-Hessian GWD and show that, unlike the local harmonic version, this method conserves the non-canonical symplectic structure on the manifold of Gaussian wavepackets and$-$for bounded dynamics in smooth potentials$-$avoids the drift of energy. Our numerical results suggest that, despite being much more efficient than the local harmonic variant, the single-Hessian GWD exhibits the same $\mathcal{O}(\hbar)$ asymptotic error in averages of observables. To further accelerate numerical simulations, we implement high-order time-stepping geometric integrators that are time-reversible and conserve the norm and symplectic structure exactly, regardless of the time step. In addition, we present explicit expressions for the exact evolution of the width of a single-Hessian Gaussian wavepacket in a general potential, as well as for the exact evolution of the whole wavepacket in a global harmonic potential. Using on-the-fly ab initio Gaussian wavepacket dynamics on the first excited-state surface of ammonia, we numerically confirm the conservation of geometric properties by these integrators and demonstrate that high-order integrators can enhance both accuracy and computational efficiency. We also compute the photoelectron spectrum of the difluorocarbene anion and the absorption spectrum of methylamine, and find that, in comparison with experiment, single-Hessian GWD outperforms global harmonic models and matches the accuracy of local harmonic GWD. Finally, we identify which spectral features are sensitive to the choice of reference Hessian.
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Instrumentation and Detectors

Characterization of a Two-Channel Optical and Near-infrared Transition Edge Sensor System for Rare-Event Searches

Manuel Meyer, Katharina-Sophie Isleif, Friederike Januschek et al. (+5 more)

Transition edge sensors (TESs) are superconducting energy-resolving microcalorimeters that have demonstrated low background rates as well as quantum efficiencies close to unity for photons at optical and near-infrared wavelengths. This makes these detectors well suited for rare-event searches. We report on the comprehensive characterization of a two-channel detector module consisting of two tungsten TESs optimized for the detection of photons with a wavelength of 1064nm. The devices achieve a system detection efficiency of $(86\pm1)$%, an energy resolution better than 7%, and a background dark-count rate of photon-like events below 6mHz when coupled to an optical fiber. Using an unbinned likelihood framework, we find the dark count rate to be compatible with blackbody radiation from the room-temperature laboratory environment. Thanks to the energy resolution of the TESs, we show that it is possible to detect monochromatic signals at 1064nm with photon rates $\geqslant 2.7_{-0.6}^{+0.8} \times10^{-5}$Hz, which corresponds to a power of $\geqslant(5.0_{-1.1}^{+1.4})\times10^{-24}$W, within 20 days of measurement time at the 5$\sigma$ confidence level. This makes our detectors well suited for searches for hypothetical axions and axion-like particles with experiments such as the Any Light Particle Search II (ALPS II) or axion interferometers. The developed methodologies are not only applicable to axion searches, but are also relevant for rare-event searches with TESs in general.
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All authors Manuel Meyer, Katharina-Sophie Isleif, Friederike Januschek, Axel Lindner, Gulden Othman, Elmeri Rivasto, Jose Alejandro Rubiera Gimeno, Christina Schwemmbauer

Instrumentation and Detectors

Design and Characterization of Racetrack 3D-Trench Silicon Sensor Based on 8-Inch Process with Excellent Time Resolution

Huimin Ji, Manwen Liu, Kuo Ma et al. (+5 more)

In the extreme environments of high-luminosity colliders, traditional planar silicon sensors suffer severe radiation-induced performance degradation and fail to satisfy the stringent demands of high-precision tracking and high-speed timing in particle physics. 3D silicon sensors enhance radiation hardness by shortening charge collection distance, yet conventional designs with columnar or square-cell trench electrodes exhibit non-uniform electric fields, including saddle points and low-field regions, which degrade charge collection efficiency and timing resolution. This work presents a novel racetrack 3D-trench silicon sensor with continuous racetrack electrodes surrounding a long central collection electrode, aiming to eliminate electric field inhomogeneities. For the first time, a 23 $\mu $m shallow-etched device was fabricated on an 8-inch platform, which provides a promising basis for its subsequent mass production and engineering applications. The device performance was systematically evaluated through theoretical analysis, 3D TCAD simulations, and characterization using semiconductor parameter analyzers and transient current technique (TCT) measurements. The sensor achieves leakage current below 0.2 nA, breakdown voltage above 110 V, full depletion voltage as low as a few volts, capacitance as low as 650 fF, collected charge of 4 fC, time response of about 640 ps, and time resolution of 50 ps. This large-scale manufacturable, shallow-etched racetrack 3D-trench silicon sensor provides a competitive device solution for portable radiation detection and next-generation 4D tracking under high-radiation and high-event-rate conditions.
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All authors Huimin Ji, Manwen Liu, Kuo Ma, Chuan Liao, Yanwen Liu, Zheng Li, Zhihua Li, Jun Luo

Instrumentation and Detectors

Review of germanium-silicon single-photon avalanche diodes

Neil Na, Erik Chen, Gerald S. Bullerb et al. (+2 more)

While it took about a decade for a germanium (Ge) thin film grown on a silicon (Si) substrate to be successfully applied as a detector material for high-speed optical fiber communication application, it took about another decade to further expand its usage as a sensor material for active optical sensing and imaging applications. In this paper, we shall review the progress of a shortwave infrared (SWIR) single-photon detection (SPD) with germanium-silicon (GeSi) single-photon avalanche diode (SPAD), ranging from the first demonstration at cryogenic temperature (Z. Lu et al., 2011) to the recent demonstration at room temperature (N. Na et al, 2024). Potential new applications will also be discussed.
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All authors Neil Na, Erik Chen, Gerald S. Bullerb, Robert H. Hadfield, Richard A. Sorefd

Chemical Physics · Cross-listed from cond-mat.mtrl-sci

Water adsorption on a model silicate surface: wollastonite (100)

Luca Lezuo, Andrea Conti, Alexander Hoheneder et al. (+7 more)

Water adsorption on silicate surfaces is a critical yet poorly understood process relevant to, e.g., mineral weathering and cement hydration. This study investigates the structure of water overlayers on a model calcium silicate, the lowest-energy (100) surface of wollastonite (CaSiO3). It combines atomically resolved non-contact atomic force microscopy (nc-AFM), acquired with qPlus sensors and functionalized tips in ultrahigh vacuum (UHV), with density functional theory (DFT) calculations employing the metaGGA r2SCAN+rVV10 functional. Adding incremental doses of water to the sample at cryogenic temperatures produces distinct structures governed by the competition between water-surface and water-water interactions. With two water molecules per surface unit cell, water-surface interactions dominate: In line with previous theoretical predictions, adsorbates follow the surface lattice. As the coverage increases, intermolecular hydrogen bonding competes with bonding to the surface, leading to the emergence of complex, coexisting patterns. While their small energy differences prevent an unambiguous identification of the most stable structure by DFT, the experimentally observed symmetries help constrain plausible structural models. Above a critical density of four water molecules per unit cell, water-water interactions prevail, and water clusters are formed. The results provide an atomic-scale framework for understanding water interactions with calcium silicate surfaces.
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All authors Luca Lezuo, Andrea Conti, Alexander Hoheneder, Elena Vaníčková, Domitilla Alessandra Aloi, Rainer Abart, Florian Mittendorfer, Michael Schmid, Ulrike Diebold, Giada Franceschi

Computational Physics · Cross-listed from math.NA

Sparse Random-Feature Neural Networks with Krylov-Based SVD for Singularly Perturbed ODE

Kevin Kurian Thomas Vaidyan and Siddharth Rout

Random-feature neural networks (RFNNs), including architectures with fixed hidden layers and analytically determined output weights, offer fast training but often suffer from issues due to dense representations of the hidden layer activation. Their reliance on dense feature mappings and least squares solvers can limit scalability and numerical stability, particularly for high-dimensional or stiff systems. Specifically, the activation matrix is observed to be low-rank and extremely ill-conditioned. In this work, we propose a sparse framework for RFNNs that integrates structured sparsity into the hidden layer activations that increases the rank and employs Sparse Singular Value Decomposition (sSVD) for solving the resulting linear least squares problem scalably and efficiently while catering to the bad condition number. We explore the theory behind Lanczos-Golub-Kahan Bidiagonalization technique for sparse SVD and conduct some experiments to identify some limitations and justify the requirement for orthogonalization step in our application. Then, we demonstrate that the proposed method maintains or improves solution accuracy for solving the benchmark one-dimensional steady convection-diffusion equations case having stronger advection, while achieving substantial gains in training efficiency and robustness compared to standard dense implementations.
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