Optics
Influence of random surface deformations on the resonance frequencies and quality factors of optical cavities and plasmonic nanoparticles
Philip Trøst Kristensen, Thomas Kiel, Kurt Busch et al. (+1 more)
Surface deformations of optical cavities and plasmonic nanoparticles are inevitable in nanophotonics. The random morphology changes of different realizations modify the associated resonance frequencies and quality factors, which may be characterized by specified distributions instead of their nominal values. As an alternative to statistical analyses based on direct numerical calculations, we present an approximate method using first-order perturbation theory with shifting boundaries. For an example resonator in the form of a plasmonic nanowire, the approach explains the bivariate frequency distribution observed in direct numerical calculations involving 1000 realizations of random surface deformations and provides the average and the associated covariance
matrix with relatively high accuracy.
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All authors Philip Trøst Kristensen, Thomas Kiel, Kurt Busch, Francesco Intravaia
Optics
An electrically tunable metaatom for visible light
Janna Wilhelmsen, Longzhu Liu, Harry Miyosi Silalahi et al. (+6 more)
Phase-gradient metasurfaces provide powerful wavefront control through two-dimensional arrangement of nanostructures acting as metaatoms. While dynamic tuning forms a major driver for future breakthroughs and applications in this area, current metaatoms are generally static or limited to operation in the infrared. Here, we present a metaatom that is both electrically tunable and operates in the visible. Its function originates from an excitonic absorption band of a dedoped conducting
polymer, which together with low background permittivity induces optical
metallicity in a selected part of the visible. This allows anisotropic nanostructures to support excitonic resonances along one direction and not the other, promoting polarization-dependent optical response which can be toggled off and on through reversible doping induced by small bias potentials. Our study details the mechanism of these metaatoms and demonstrate their use in electrically tunable phasegradient metasurfaces for visible light, including erasable and rewritable holograms.
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All authors Janna Wilhelmsen, Longzhu Liu, Harry Miyosi Silalahi, Suraya Kazi, Giancarlo Cincotti, Shangzhi Chen, Dongqing Lin, Yulong Duan, Magnus P. Jonsson
Applied Physics · Cross-listed from eess.SP
Enhancing the physical layer security with bending beams
Sotiris Droulias, Giorgos Stratidakis and Angeliki Alexiou
Wavefront engineering for applications in near-field wireless connectivity is gradually becoming common ground. In this landscape, beams that propagate on bent paths are ideal candidates for dynamic blockage avoidance and suppression of potential eavesdropping. In this work we study the physical layer security offered by bending beams, and we demonstrate their capabilities for line-of-sight and non-line-of-sight eavesdropping. We analyze the dependencies between the possible locations of an eavesdropper and the design parameters of such beams, and we introduce metrics to assess their physical layer security performance. Our results demonstrate their superiority with respect to beams generated with conventional beam-forming.
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Instrumentation and Detectors
Predictive drift compensation of multi-frame STEM via live scan modification
Matthew Mosse, Jonathan J.P. Peters, Eoin Moynihan et al. (+4 more)
Scanning transmission electron microscopy (STEM) is widely used tool for materials characterisation. However, being a scanned technique, STEM is susceptible to sample, stage or beam drift, manifesting as distortions within images or movement in the field-of-view during multi-frame imaging. Often this is corrected post-acquisition using image registration of multiple frames, but drift reduces the usable area common to all frames. Here we present a method to mitigate sample drift by analysing past frames to predict the sampling-grid points for the immediately future frame. We present this correction across two time-scales and two lengthscales. By offsetting the scan-grid framewise we remove long-range drift, and offsetting pixelwise we minimise intra-image warping. Examples are presented for both atomic-resolution imaging and lower-magnification in-situ
video capture. The framework is general to raster, serpentine, interlaced and other scan patterns, as well as sequential or scan-rotation series STEM.
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All authors Matthew Mosse, Jonathan J.P. Peters, Eoin Moynihan, James A. Gott, Ana M. Sanchez, Michele Conroy, Lewys Jones
Fluid Dynamics · Cross-listed from astro-ph.SR
Polytropic stellar wind models with strongly localized heating
L. Westrich, B. Shergelashvili, H. Fichtner et al. (+1 more)
Polytropic models of
stellar winds remain to be useful tools because they allow for a simple description of the energy balance of the expanding
plasma without explicitly specifying potentially complex energy transport processes like, e.g., heat conduction or extended wave heating. Among recent applications to
stellar winds and to the
solar wind was a study of the consequences of strongly localized heating in the latter, possibly due to acoustic waves. Such 'nonuniform' heating can result from a time- and space-localized damping of wave modes and allows, as an extreme case, an adiabatic expansion of particular wind streams outside the heating region. The present study generalizes the modeling from the first analytical as well as numerical studies, that were limited to this extreme case, towards a more realistic non-adiabatic behaviour. The additional energy due to heating is demonstrated to be in a plausible range in view of typical
flare energies and low compared to the gravitational energy of the
plasma in this region. The corresponding solutions may be of interest for
stellar winds, in general, and w.r.t. recent observations made with the Parker
Solar Probe, which revealed strongly varying wind streams and the presence of acoustic waves near the Sun, for the
solar wind, in particular. Potential observational evidence for the
solar wind is discussed.
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All authors L. Westrich, B. Shergelashvili, H. Fichtner, V. N. Melnik
Chemical Physics
Different perspectives on the exact factorization for photon-electron-nuclear systems
Claudia Magi, Peter Schuerger, David Lauvergnat et al. (+1 more)
We employ the exact factorization of a multi-component wavefunction to analyze the dynamics of interacting
photons, electrons and
nuclei. We consider physical situations emerging in the regime of strong coupling between light excitations and molecular - electronic excitations, giving rise to the so-called molecular polaritons. Nonadiabatic molecular dynamics techniques, routinely used in the field of chemical physics, have been often employed to simulate photophysical and photochemical phenomena in the presence of molecular polaritons. In this work, we analyze the foundations of these techniques in the eye of the exact factorization and we assess their performance on illustrative model studies.
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All authors Claudia Magi, Peter Schuerger, David Lauvergnat, Federica Agostini
Applied Physics · Cross-listed from nlin.CD
Extreme events in MLC circuit
Tapas Kumar Pa and Dibakar Ghosh
The Murali-Lakshmanan-Chua (MLC)
circuit is a well-recognized prominent nonlinear, nonautonomous, and dissipative electronic
circuit having a versatile chaotic nature. Unraveling the dynamical synergy responsible for the genesis of extreme events in nonlinear dynamical systems is a prolific and spellbinding research area. The present study unveils the dynamical exposition of emerging extreme events in the MLC
circuit concerning two different events being defined in the system. The large expansion of the chaotic attractor following the PM intermittency route plays the crucial role as the precursor behind the emergence of extreme events in the system. Our main finding reveals the prevalence of a force field due to the presence of externally applied periodic force in the system that creates the dynamical synergy that compels the chaotic trajectory traversing in its phase space to be largely deviated from the residing space, and this large deviation shows the signature of extreme events. Apart from the force field explication, we explored another two dynamical aspects that also interpret the mechanism behind the genesis of extreme events as the large deflection of the chaotic trajectory in the system: the decomposition of the phase space in stable and unstable
manifolds concerning slow-fast dynamics and using Floquet multipliers. These two different aspects of calculations of the stable and unstable
manifolds explicate the large excursion of the chaotic trajectory as extreme events from two different perspectives. We also analyzed the rare occurrences of the extreme events statistically using extreme value theory: the threshold excess values follow the generalized Pareto distribution, and the inter-extreme-spike-intervals follow the generalized extreme value distribution.
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Fluid Dynamics
Subharmonic instability of large-scale wavy structures in two-dimensional channels
An-Xiao Han, Peng-Yu Duan, Ming-Ze Ma et al. (+1 more)
A particular interest on two-dimensional
turbulence is the inverse energy cascade from small to large sales, which leads to an energy condensation accompanied by the formation of large-scale vortical structures. Indeed, such a phenomenon is observed in the two-dimensional channel (2DCH) with large Reynolds numbers, where prominent large-scale wavy structures play a central role in the momentum and energy transfer across the inhomogeneous wall-normal direction \citep{Falkovich2018}. Yet, the instability of these wavy structures remains poorly understood, and it is unknown whether they have the capacity to generate
turbulence. To address this, we first conduct the direct numerical simulation (DNS) of Navier-Stokes equations for 2DCH, then extract the large-scale wavy structures through the singular value decomposition, and finally perform a Floquet-based secondary instability analysis. Two bulk Reynolds numbers are examined in particular, i.e. $Re = 3000$ and $Re = 200000$, which lie on opposite sides of the transitional regime near $Re \approx 10000$ and cover the previously reported simulation domain. At $Re = 3000$, the large-scale wavy structure is found to be linearly stable, consistent with the laminar-like DNS flow field. However, at $Re = 200000$, a subharmonic torsional mode is identified, which leads to a definite growth rate ($\lambda_r = 0.18$) for the wavy structures with a half wave-length shift. Temporal reconstruction shows that this unstable mode deforms and splits into multiple wave trains and evolves in the opposite phase. Compared to the TS (Tollmien-Schlichting) wave of laminar flow, the subharmonic mode found here offers a novel understanding for the generation of
turbulence in larger Reynolds number two-dimensional channels.
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All authors An-Xiao Han, Peng-Yu Duan, Ming-Ze Ma, Xi Chen
Optics
Node-reduction through Joint Optimization of Input and Readout Layers in Photonic Reservoir Equalization
Ruben Van Assche, Sarah Masaad and Peter Bienstman
Photonic reservoir computing is a machine learning paradigm in which a recurrent neural network remains fixed while only the output weights are trained. This makes it a well-suited approach for high-speed signal equalisation in optical communication systems, offering a trainable, low-power, and low-complexity solution. However, achieving strong performance typically requires relatively large network sizes, as learning is confined to the output layer. To address this, we investigate the role of trainable input mappings alongside conventional output weight optimisation. Across a range of short- and mid-reach IM/DD transmission scenarios, reaching up to 200 km for a 28 GBd NRZ signal, improvements of over two orders of magnitude in BER are achieved. This enables halving the network size while maintaining comparable performance. Furthermore, we show that this approach effectively extends the
memory of the reservoir, resulting in over three orders of magnitude improvement in
memory-intensive tasks. These results also show that starting at 16 nodes a performance of at least one to two magnitudes better than both a complexity matched FFE and a Volterra filter of second order are reached.
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Fluid Dynamics
Nonisothermal global-pressure exactness in fractured multiphase flow with aperture feedback
Christian Tantardini and Fernando Alonso-Marroquin
Global-pressure formulations recast multiphase Darcy flow in terms of a single pressure driving the total flux. Their exact equivalence to phase-pressure formulations holds only when the constitutive data satisfy the compatibility conditions required for a total-differential structure and its generalized nonisothermal extension. Here, we derive the exactness criterion for temperature-dependent mobilities and capillary pressures. We show that equivalence depends on whether the mobility-weighted capillary contribution is path independent in the saturation–temperature domain, so that it can be absorbed into a scalar global pressure. This yields the classical compatibility conditions within the saturation sector and a distinct mixed saturation–temperature condition that arises only in nonisothermal settings. We then incorporate this structure into a reduced
matrix–fracture model with heat transport,
matrix–fracture thermal exchange, and evolving aperture. Numerical
benchmarks recover the three regimes predicted by the theory: globally exact, exact on each fixed-temperature slice but not on the full saturation–temperature domain, and fully nonexact. In fractured systems, thermal forcing alone can drive transitions between these regimes, while aperture
evolution changes the path through state space. When saturation-sector exactness is lost, a least-squares projection on fixed-temperature slices extracts the nearest gradient component of the mobility-weighted capillary field. This yields a conservative slice-wise scalar-pressure surrogate and a quantitative projection residual. The residual separates saturation-sector nonintegrability from the mixed saturation–temperature incompatibility that controls genuinely nonisothermal loss of exactness. The framework links nonisothermal exactness theory, fractured-flow dynamics, and conservative reduced closure in a global-pressure formulation.
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Optics
Baudrate- and Reach-Flexible All-Optical Equalization with a Co-Packaged Photonic Reservoir and Receiver
Sarah Masaad, Jakob Declercq, Stijn Sackesyn et al. (+10 more)
Intensity-modulation direct-detection links must support increasing baudrates and transmission distances while operating under stringent power and cost constraints. However, as data rates and reaches increase, chromatic dispersion induces stronger inter-symbol interference and, after direct detection, frequency-selective fading, thus requiring increasingly powerful equalization. In conventional receivers, this translates into digital equalization whose complexity scales unfavorably with data rate. Photonic-domain equalization offers a hardware-based alternative that operates naturally at line rate and mitigates frequency fading. However, prior demonstrations were not readily adaptable for different rate and/or reach operation. In this paper, we experimentally demonstrate all-optical equalization across 10-46 Gbaud and 10-250 km SSMF in the C-band enabled solely through retraining of the readout layer, achieving up to four orders of magnitude BER improvement over standard DSP equalization. The demonstrator comprises a 16-node spatially multiplexed reservoir, programmable on-chip readout, and co-packaged receiver front-end. To our knowledge, this is the first co-packaged photonic reservoir receiver and the first demonstration of simultaneous baudrate- and reach-flexible equalization using a fixed-
topology integrated photonic
circuit.
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All authors Sarah Masaad, Jakob Declercq, Stijn Sackesyn, Ruben Van Assche, Hasan Salmanian, Andrzej Polatynski, Dimitrios Chatzitheocharis, Konstantinos Vyrsokinos, Tatiana Buriakova, Christophe Caillaud, Stylianos Sygletos, Xin Yin, Peter Bienstman
Geophysics
OpenWaveLogger v2026 (OWL-v2026): an open source, low cost, easy to build, high performance logger for wave data measurements
Jean Rabault, Joey Voermans, Takuji Waseda et al. (+10 more)
Ocean wave models are critical for weather and
climate forecasting, and accurate in-situ wave observations are essential for validating and improving these models. Open-source, community-driven buoys have democratized wave observations via telemetry in recent years, but these systems transmit only limited amounts of data. Full high-frequency time series, required to study detailed wave physics, can still in most cases only be collected in situ using data loggers. Yet open-source, low-cost logger solutions remain scarce compared to their telemetry-enabled counterparts. Here we present the Openlogartemis Wave Logger (OWL-v2026), an open-source, low-cost, easy-to-build, high-performance logger for wave data measurements. The OWL-v2026 is built from off-the-shelf components from the maker community, requiring only through-hole soldering for assembly, and totals approximately 220USD per unit. Custom firmware enables high-frequency, low-jitter logging of six-axis inertial measurement unit (IMU) data at 208 or 416Hz, and GNSS position and Doppler velocity at 10Hz, with Pulse Per Second (PPS) synchronization for accurate absolute UTC timestamping. We have successfully validated continuous logging over more than 10 days at 208Hz, a power consumption of approximately 80mA (approximately 20 days of autonomy with three D-
cell lithium batteries), and absolute UTC timestamp accuracy typically better than 10ms. Though the OWL-v2026 is a purely technical contribution, it has the potential to substantially expand the availability and affordability of high-frequency in-situ wave time series, similar to how the OpenMetBuoy (OMB) (Rabault 2022) expanded the availability of telemetry-enabled wave observations and helped spark new developments in low-cost open-source buoys.
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All authors Jean Rabault, Joey Voermans, Takuji Waseda, Takehiko Nose, Tsubasa Kodaira, Koya Sato, Alexander Babanin, Gaute Hope, Malte Müller, Lars Willas Dreyer, Øystein Lande, Atle Jensen, Øyvind Breivik
Optics
Maximum Q-factor of planar inductors
Mohamed Ismail Abdelrahman, Matteo Ciabattoni and Francesco Monticone
On-chip inductor design plays a critical role in the advancement of
radio-frequency integrated
circuits (RFICs). Inductors typically occupy a substantial portion of the chip area as their performance metrics, namely, inductance density and Quality factor ($Q$-factor), are fundamentally tied to the available footprint, thereby limiting miniaturization. To better understand and quantify these limitations, we employ rigorous electromagnetic analysis together with convex optimization techniques to derive a fundamental bound on the maximum achievable $Q$-factor of electrically-small planar inductors as a function of the available design area. The analysis yields analytical expressions for the bound and, via modal analysis techniques, identifies and interprets operational regimes and scaling trends with respect to design area and material conductivity. The analysis accounts for both ohmic and radiation losses, with the latter becoming significant as the inductor size increases. A broad set of state-of-the-art inductor designs from the literature is evaluated against the established $Q$-factor upper bound, identifying designs that approach the theoretical limit as well as those with potential for further improvement. The study is extended to include the effect of kinetic inductance, which offers a promising avenue toward next-generation inductors with higher inductance densities and $Q$-factors. By establishing this
benchmark, this work aims to guide and inspire the design of more efficient and compact planar inductors for high-performance RF systems.
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Plasma Physics
The physics of ELM-free regimes in EUROfusion tokamaks
M. G. Dunne, M. Faitsch, O. Sauter et al. (+25 more)
The development of operational scenarios without large Type-I ELMs is of utmost importance for the stable operation and longevity of future tokamaks. The EUROfusion tokamak exploitation program has therefore made the understanding of ELM-free regimes a major topic of exploration across all its contributing devices (ASDEX Upgrade, JET, MAST-Upgrade, TCV, and WEST). An integrated program to investigate a range of Type-I ELM-free regimes has been developed covering the enhanced D-alpha (EDA),
magnetic perturbations (MP), negative triangularity (NT), quasi-continuous exhaust (QCE), quiescent H-mode (QH), the baseline small ELMs (SE), I-mode, and X-point radiator (XPR) regimes. This contribution focuses on the development and understanding of the NT and QCE regimes on ASDEX Upgrade, JET, and TCV. The importance of transport via ballooning modes in both regimes is highlighted, as well as the progress in developing access models based on ideal-MHD. In the case of the QCE, this can also be expressed as a minimum separatrix density, which corresponds well to experimentally measured separatrix densities. Particular focus is paid to the performance of the QCE in terms of the achieved pedestal top values, which, when appropriately normalised, do not differ significantly from ELMy H-mode
plasmas. This, combined with the predicted minimum separatrix density for the 15 MA ITER baseline
plasma, highlight the relevance of the QCE as a potential operational scenario for both ITER and future reactors.
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All authors M. G. Dunne, M. Faitsch, O. Sauter, E. Viezzer, B. Labit, A. Kappatou, D. Keeling, B. Vanovac, I. Balboa, P. Bilkova, P. Bohm, D. Kos, J. Hobirk, E. Lerche, P. Lomas, S. Menmuir, T. Pütterich, L. Radovanovic, S. Saarelma, S. Silburn, D. Silvagni, E.R. Solano, H.J. Sun, A. Tookey, The ASDEX Upgrade Team, The TCV Team, The EUROfusion Tokamak Exploitation Team, JET contributors
Instrumentation and Detectors · Cross-listed from quant-ph
Valley-Aware Optimal Control of Spin Shuttling Using Cryogenic Integrated Electronics
Pau Dietz Romero, Nermine Chaabani, Lammert Duipmans et al. (+4 more)
Electron shuttling is emerging as a key mechanism for enabling long-range coupling in scalable
spin-
qubit architectures. Bringing shuttling waveform generation into the cryostat can improve scalability, but imposes strict area and power constraints on the control electronics. Concurrently, shuttling in Si/SiGe is further limited by a spatially varying valley splitting that induces
spin–valley mixing and degrades coherence. Here, we make three contributions that address these limitations jointly: (i) an end-to-end co-simulation framework that combines disorder-informed valley maps with transistor-level cryogenic
circuit simulations including electronic noise; (ii) a fully integrated cryogenic shuttling-signal generator tailored to velocity modulation, enabling period-wise waveform shaping through discrete
circuit settings stored in on-chip
memory; and (iii) a noise-aware optimization procedure that tunes only these implementable
circuit controls, using one of four discrete resistor settings per period, to generate high-fidelity shuttling sequences. Across simulated valley and noise realizations in our co-simulation framework, the optimized velocity-modulation waveforms improve transport performance, achieving an average shuttling fidelity of $99.99 \pm 0.007\%$ at $v_{\mathrm{avg}} = 20~\mathrm{m\,s^{-1}}$ over a distance of $10~\mu\mathrm{m}$, while maintaining active analog power consumption in the tens of $\mu\mathrm{W}$ during shuttling. This validates on-chip storage and replay of optimized control settings as a practical strategy to mitigate valley disorder in scalable shuttling architectures.
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All authors Pau Dietz Romero, Nermine Chaabani, Lammert Duipmans, Alessandro David, Felix Motzoi, Stefan van Waasen, Lotte Geck
Chemical Physics
Restoring the Conical Intersection Topology using Convex Density Functional Theory
Federico Rossi, Tommaso Giovannini and Henrik Koch
Conical intersections are central to the description of photophysics and photochemistry. Nevertheless, in non-adiabatic molecular dynamics simulations, they are fundamentally challenging for single-reference electronic structure methods. Density functional theory (DFT) and its time-dependent extension (TDDFT) represent the most widely used theoretical approaches in physics, chemistry, and biology. However, the treatment of ground and excited states as separate problems leads to breakdowns in the topological structure of potential energy surfaces near conical intersections. In this work, we solve this long-standing issue by presenting Convex DFT, a framework that, by explicitly enforcing convexity of the variational problem within an appropriately defined subspace, guarantees a unique and continuous electronic solution across regions of degeneracies. We demonstrate that Convex DFT yields smooth and physically meaningful intersection seams by comparison with multireference wave function methods. In this way, we establish the method as a robust and computationally efficient DFT approach for treating electronically degenerate regions. These developments represent a critical step toward reliable non-adiabatic simulations beyond the limitations of conventional TDDFT.
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Atmospheric and Oceanic Physics
Mechanistic Interpretability Tool for AI Weather Models
Kirsten I. Tempest, Matthias Beylich and George C. Craig
Artificial Intelligence (AI) weather models are improving rapidly, and their forecasts are already competitive with long-established traditional Numerical Weather Prediction (NWP). To build confidence in this new methodology, it is critical that we understand how these predictions are generated. This is a huge challenge as these AI weather models remain largely black boxes. In other areas of Machine Learning (ML), mechanistic interpretability has emerged as a framework for understanding ML predictions by analysing the building blocks responsible for them. Here we present an open-source, highly adaptable tool which incorporates concepts from mechanistic interpretability. The tool organises internal latent representations from the model processor and allows for initial analyses, including cosine similarity and Principal Component Analysis (PCA), enabling the user to identify directions in latent space potentially associated with meteorological features. Applying our tool to the graph neural network GraphCast, we present preliminary case studies for mid-latitude synoptic-scale waves and specific humidity. These demonstrate the tool's ability to identify linear combinations of latent channels that appear to correspond to interpretable features.
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Optics
Imaging the transverse component of optical near-fields in resonant photonic structures
Petr Koutenský, Neli Laštovičková Streshkova, Stefanie Kraus et al. (+2 more)
We report on imaging the optical near-fields in resonant periodic photonic structures with nanometer resolution using ultrafast 4D scanning transmission electron microscopy (U4DSTEM). In particular, U4DSTEM is applied to visualize the transverse component of the Lorentz force of a synchronous near-field mode excited by an infrared femtosecond pulse in a periodic silicon nanostructure designed for photonic acceleration of electrons. Our results show that in addition to the accelerating/decelerating force acting on the electrons in the longitudinal direction along the electron propagation, the structures can be efficiently used for transverse electron streaking at optical frequencies when excited by light with polarization perpendicular to the electron trajectory. The measured spatial profile of the excited near-field mode intensity is consistent with the numerical simulations performed using finite-difference time domain technique.
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All authors Petr Koutenský, Neli Laštovičková Streshkova, Stefanie Kraus, Peter Hommelhoff, Martin Kozák
Chemical Physics · Cross-listed from cs.LG
Surrogate Functionals for Machine-Learned Orbital-Free Density Functional Theory
Roman Remme and Fred A. Hamprecht
We introduce surrogate functionals: machine-learned energy functionals for orbital-free density functional theory (OF-DFT) which are defined not by universal fidelity to a physical reference, but merely by the requirement that density optimization with a fixed procedure yields the true ground-state density. Helpfully, training surrogate functionals requires only ground-state densities, no energies or gradients away from the ground state. We here propose a gradient-descent-improvement loss that guarantees exponential convergence of the density to the ground state, and combine it with an adaptive sampling scheme that concentrates learning around the optimization trajectories actually visited during inference. On the QM9 and QMugs
benchmarks, surrogate functionals achieve density errors competitive with or improving upon the state of the art for fully supervised machine-learned OF-DFT, while eliminating the need for the $O(N^3)$ orthononormalization step required by prior work, yielding improved runtime scaling for larger systems.
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Optics
Control Over Fano Parameter in Grating and One-Dimensional Photonic Crystal Cavity
Pratip Ghosh and Akshay K. Naik
Fano resonances are sharp asymmetrical spectral peaks which are now ubiquitous in nanophotonics. The high sensitivity of these resonances to system parameter has been exploited to improve light matter interaction and in applications such as sensing, filters and on-chip processing. The ability to dynamically change the Fano slope and spectral phase would enable optimization of the device parameters post fabrication for various applications. Here we demonstrate such a control over the Fano resonance in a one-dimensional photonics
crystal cavity integrated on a silicon waveguide -grating platform. In our device, Fano resonance arises due to interference between cavity mode and an oscillatory background due to grating coupler. The dynamics tuning of Fano asymmetric parameter is achieved using thermos-optic effect in silicon. We experimentally tune the Fano parameter from ~-3.2 to +1.7 achieving a highest extinction ratio of 21.6 dB and spectral slope of 108dB/nm. All the above is achieved in an ultra-compact design with simple fabrication and with multiple cavities or feedback elements. The steep slope offers distinct advantage over conventional cavity for sensing and modulation applications and the tunability enables dynamic control over gain, dynamic range, bandwidth and noise coupling.
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Computational Physics · Cross-listed from cond-mat.mtrl-sci
Second-order topology in two-dimensional azulenoid kekulene carbon lattices
Xiaorong Zou, Hyeon Suk Shin, Chang-Jong Kang et al. (+5 more)
The discovery of higher-order topological
insulator (HOTI) has established a new paradigm for understanding
symmetry-constrained boundary electronic states. Here, based on first-principles calculations, we demonstrate the emergence of HOTI phase in organic
lattices of two-dimensional azulenoid-kekulene-type carbon allotropes, namely AKC-[3,3] and AKC-[6,0]. Enabled by the $C_6$ rotational
symmetry, the nontrivial bulk
topology is confirmed through the topological invariant and fractionally quantized corner charge, giving $\{[M^{(I)}_{2}],[K^{(3)}_{2}]\}$ = $\{0,2\}$ and $Q_{\mathrm{corner}} = e/3$, respectively, accompanied by the emergence of exotic corner states in nanoflakes. Notably, the structural modifications are explored, revealing that in the derived structure PAK-[6,0], whose corner-localized states are preserved, highlighting the robustness of the higher-order topological phase. These findings highlight azulenoid-kekulene-based carbon allotropes as a promising platform to explore the interplay between structural design, crystalline
symmetry, and higher-order topological boundary responses in two dimensional carbon systems.
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All authors Xiaorong Zou, Hyeon Suk Shin, Chang-Jong Kang, Baibiao Huang, Yanmei Zang, Ying Dai, Chengwang Niu, Chang Woo Myung
Optics · Cross-listed from cond-mat.mes-hall
Quantum Optical Signatures of Band Topology in Solid-State High Harmonics
Denis Ilin, Alexander S. Solntsev and Ivan Iorsh
We develop a general theory of high-harmonic generation (HHG) in solid-state systems, based on a weak-correlation expansion of photonic and matter degrees of freedom. Unlike standard HHG theories, which treat light-matter dynamics through the Schrodinger equation, our approach employs density-
matrix evolution, naturally capturing the mixed-state character of both the field and the matter - a critical aspect for describing complex solid-state band structures. We show explicitly that the properties of the emitted fields are governed by the
quantum statistics and
quantum geometry of the underlying solid. Taking the Su-Schrieffer-Heeger (SSH) model in a one-sided optical cavity as a paradigmatic example and considering the dual regime, we demonstrate that in the topological phase a system exhibits a stronger HHG response and stronger
quantum-light signatures than in the trivial phase. Furthermore, we show that cavity-matter interaction gives rise to squeezed high-harmonic
quantum light, whose properties are directly imprinted by the current-current fluctuations in the material system. Crucially, the observed squeezing does not rely on a separate quartic Kerr mechanism. In the mesoscopic regime, the genuine
quantum Kerr term is higher order in light-matter coupling strength and negligible, while the relevant non-classical effect is governed by current-current fluctuations encoded in the complex susceptibility of the material. This work establishes a direct link between band
topology and
photon statistics, opening new avenues for
topology-sensitive
quantum light generation and
photon statistics based spectroscopy of solid-state systems.
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Computational Physics · Cross-listed from quant-ph
Hessian-vector products for tensor networks via recursive tangent-state propagation
Isabel Nha Minh Le, Roeland Wiersema and Christian B. Mendl
Optimizing tensor networks with standard first-order methods often leads to slow convergence and entrapment in local minima. Although second-order optimization offers enhanced robustness, explicitly constructing the full Hessian
matrix is computationally prohibitive for large-scale systems. In this work, we bypass this bottleneck by introducing an analytical Hessian-vector product kernel designed for arbitrary compositions of linear maps. This two-pass
algorithm leverages recursive tangent-state propagation with a bounded virtual bond dimension to guarantee scalability. We demonstrate the practical utility of this kernel by integrating it into a Riemannian trust-region framework for
quantum circuit compression. Evaluated on time-
evolution circuits for various
spin chains, our second-order approach achieves up to a four-order-of-magnitude improvement in fidelity over naive Trotterization, while delivering significantly smoother, faster convergence than conventional first-order methods such as Riemannian ADAM.
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Fluid Dynamics
Emergence of Transport Regimes from the Axial Field-Induced Interfacial Gradients in Uniform Surface Potential Nanopores
Pramodt Srinivasula and Doyel Pandey
Gate-modulated nanopores have emerged as a promising platform for achieving ion selectivity and ionic current rectification (ICR) with the advantage of active field-based control. However, the mechanistic origin of these experimentally reported phenomena, arising from electrostatic coupling between the prescribed radial pore surface potential and the axial transmembrane electric field, remains insufficiently understood. Here, using coupled Poisson–Nernst–Planck and Navier–Stokes simulations supported by asymptotic analysis, we show that a uniform surface potential inherently interacts with the axial driving field to generate a three-dimensional, axially nonuniform electric double layer (EDL). This field-induced EDL heterogeneity effectively mimics a linear axial variation in
zeta potential, breaking translational
symmetry within an otherwise uniform pore. As a result, the system exhibits coupled electrokinetic responses, including ion selectivity, ionic current rectification, and non-canonical electroosmotic flow, all governed by a single asymmetry parameter $\alpha$ derived from the EDL structure. Critical transitions occur at specific values of $\alpha$; in particular, at $\alpha=0$, the EDL becomes axially antisymmetric, leading to reversal of ion selectivity, significant ICR and the emergence of a peculiar negative electroosmotic flow rectification accompanied by internal vortical structures. These findings establish the electrostatic mechanism for axial
symmetry breaking as the underlying principle for transport in voltage-gated nanopores, enabling a unified framework for designing tunable electrokinetic functionalities beyond geometry- and chemistry-based strategies.
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Fluid Dynamics
AI models of unstable flow exhibit hallucination
Ramdhan Wibawa and Birendra Jha
We report the first systematic evidence of
hallucination in AI models of fluid dynamics, demonstrated in the canonical problem of hydrodynamically unstable transport known as viscous fingering. AI-based modeling of flow with instabilities remains challenging because rapidly evolving, multiscale fingering patterns are difficult to resolve accurately. We identify solutions that appear visually realistic yet are physically implausible, analogous to
hallucinations in large
language models. These
hallucinations manifest as spurious fluid interfaces and reverse
diffusion that violate conservation laws. We show that their origin lies in the spectral bias of AI models, which becomes dominant at high flow rates and viscosity contrasts. Guided by this insight, we introduce DeepFingers, a new framework for AI-driven fluid dynamics that enforces balanced learning across the full spectrum of spatial modes by combining the
Fourier Neural Operator with a Deep Operator Network to predict the spatiotemporal
evolution of viscous fingers. By conditioning on both time and viscosity contrast, DeepFingers learns mappings between successive concentration fields across regimes. The framework accurately captures tip splitting, finger merging, and channel formation while preserving global metrics of mixing. The results open a new research direction to investigate fundamental limitations in AI models of physical systems.
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Geophysics
Extrapolation from historical data cannot reliably predict the time of a potential AMOC collapse
Andreas Morr, Maya Ben-Yami, Brian Groenke et al. (+6 more)
Ditlevsen and Ditlevsen [Nature Communications, 2023] (DD23 hereafter) propose a statistical framework to estimate the timing of a potential collapse of the Atlantic Meridional Overturning Circulation (AMOC) based on extrapolating information from observed sea-surface temperature (SST) variability. By fitting a
stochastic one-dimensional fold-bifurcation model to an SST-based fingerprint of the AMOC using Maximum Likelihood Estimation (MLE), they conclude that a collapse is most likely to occur in the middle of the 21st century, with a reported 95% confidence interval covering the time span from 2037 to 2109. Given the profound implications of such a claim for both
climate and society, it is essential to thoroughly test the robustness of this result, to critically assess the underlying assumptions and uncertainties, and to estimate the extent to which the reported confidence interval reflects the true limits of current knowledge. Here we examine the sensitivity of DD23's results and argue that four types of uncertainty are insufficiently explored in their analysis: (i) structural uncertainty associated with the assumed low-order bifurcation model, (ii) statistical uncertainty in their model fit, (iii) uncertainty in the representativeness of SST-based fingerprints as proxies for the high-dimensional AMOC dynamics, and (iv) uncertainty in the underlying data, arising from non-stationary observational coverage and
dataset preprocessing. Using synthetic experiments and a systematic analysis of alternative fingerprints and observational products, we show that the tipping times estimated by DD23 are highly sensitive to the uncertainties listed above, and extend several millennia into the future when these uncertainties are thoroughly propagated.
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All authors Andreas Morr, Maya Ben-Yami, Brian Groenke, Christof Schötz, Alessandro Cotronei, Eirik Myrvoll-Nilsen, Sebastian Bathiany, Martin Rypdal, Niklas Boers
Space Physics · Cross-listed from astro-ph.EP
Europa's Lyman-$\alpha$ emissions from HST/STIS observations
L. Roth, K.D. Retherford, J. Saur et al. (+13 more)
An image of Lyman-$\alpha$ (Ly$\alpha$) emission from Europa obtained with the Hubble Space
Telescope Space
Telescope Imaging Spectrograph (HST/STIS) has previously provided the first evidence of localized water vapor (H$_2$O) aurora, potentially originating from outgassing. Subsequent STIS observations have revealed the presence of a global atomic hydrogen (H) exosphere. We present a comprehensive analysis of STIS Ly$\alpha$ observations of Europa acquired in 1999 and between 2012 and 2020 to search for localized auroral emissions and constrain the properties of Europa's H exosphere. We analyze the complete
dataset of STIS observations obtained when Europa was sunlit and not transiting Jupiter. A forward model is constructed to account for all known sources of Ly$\alpha$ emission, including resonantly scattered sunlight from Europa's H exosphere. To identify localized anomalies, such as H$_2$O aurora, the modeled Ly$\alpha$ emission is subtracted and the residuals are examined. Emission from Europa's H exosphere is detected at all observing epochs, but is attenuated by absorption in Earth's exosphere when Europa's radial velocity relative to Earth (and thus the Doppler shift) is small. From the velocity dependence of this attenuation, we estimate an H-exosphere temperature of $\sim 1000$ K and derive an upper limit of 5100 K. For the best-constrained epoch in 2014–2015, we infer a vertical H column density of $1.4x10^{12}$ cm$^{-2}$ and an H source rate of $1.1x10^{27}$ s$^{-1}$. No localized emission enhancements are detected in any of the observations, including the image previously interpreted as evidence of H$_2$O aurora near Europa's south pole. The discrepancy with earlier results arises primarily from differences in the assumed position of Europa's disk on the detector, as well as from the inclusion of an H-exosphere signal in the present analysis...
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All authors L. Roth, K.D. Retherford, J. Saur, D.F. Strobel, T. Becker, S. Bergman, A. Blöcker, S.R. Carberry Mogan, C. Grava, M. Ivchenko, S. Joshi, M.A. McGrath, F. Nimmo, L. Paganini, W. Pryor, J.R. Spencer
Classical Physics · Cross-listed from cond-mat.soft
Polymeric Solvents Control Swelling-Induced Surface Creasing
Zechao Jiang, Zhaoyu Ding, Shaohua Yang et al. (+7 more)
Surface creasing in swelling
polymer gels is commonly attributed to compressive strain or interlayer mismatch, yet its general control remains unclear. Here we show that solvent polymerization degree $N_{\rm s}$ provides an independent control parameter for crease onset in surface-bound polydimethylsiloxane gels swollen by silicone oils. Despite nearly identical swelling kinetics and through-thickness solvent concentration profiles, we observe a transition from creased to stable surfaces with increasing $N_{\rm s}$. A theory coupling swelling thermodynamics and mechanical stability reveals that polymeric solvents reduce the mixing
entropy and thereby modify the osmotic pressure, allowing $N_{\rm s}$ to tune separately the
equilibrium swelling and the crease threshold. This framework captures the stability boundary across solvent polymerization degree and network elasticity. These results identify polymeric solvents as active thermodynamic-mechanical regulators of swelling-induced surface.
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All authors Zechao Jiang, Zhaoyu Ding, Shaohua Yang, Ye Xu, Dongshi Guan, Abdelhamid Maali, Joshua D Mcgraw, Thomas Salez, Zaicheng Zhang, Xingkun Man
Computational Physics · Cross-listed from cond-mat.mtrl-sci
Domain-Wall-Mediated Ultralow-Barrier Sliding and Pinning in Ferroelectric Moiré Superlattices Revealed by Machine Learning
Jia-Wen Li, Sheng Meng, Xinghua Shi et al. (+2 more)
Sliding ferroelectrics built from stacked nonpolar monolayers enable out-of-plane polarization and unconventional switching via interlayer sliding, yet the microscopic sliding dynamics remain unclear. Using machine-learning molecular dynamics, we reveal spontaneous thermally driven interlayer sliding in ferroelectric MoS2 moiré superlattices, with relative velocities on the order of 1 m/s at 300 K. Instead of rigid translation of the entire
bilayer, the motion appears as a global drift of the moiré pattern. Such thermally driven sliding is inconsistent with the meV/atom-scale rigid-sliding barrier. In contrast, when constrained relaxation is allowed, the sliding proceeds along an almost barrierless pathway that directly reproduces the global drift of the moiré pattern. Furthermore, sulfur vacancies trigger a sliding-to-pinning transition, with about 0.1% S vacancies already sufficient to convert the long-range sliding into localized oscillations. Notably, these phenomena are not restricted to small twist angles, but arise generically in twisting-induced multidomain structures. These results reveal that the sliding process is governed by a domain-wall-mediated collective reconstruction pathway with an ultralow barrier, rather than rigid layer translation, deepening the understanding of microscopic dynamics in moiré superlattices and sliding ferroelectrics.
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All authors Jia-Wen Li, Sheng Meng, Xinghua Shi, Jin Zhang, Wei-Hai Fang
Optics · Cross-listed from cond-mat.dis-nn
Interplay of Flat-band and Anderson localizations in disordered moire superlattices
Qian Liu, Xiaoshuang Xia, Junjie Wang et al. (+5 more)
Disorder in moire superlattices simultaneously degrades flat-band localization and induces Anderson localization, yet how these two regimes interact has remained unclear. Here, we introduce a combined framework linking localization-length scaling with differential probability density analysis to map localization transitions in partially disordered one-dimensional silicon moire
lattices. It is found that flat bands confined within the interband gap keep their strong localization even as disorder grows. In contrast, flat bands intersecting dispersive bands exhibit rich behaviors: the low-frequency branch undergoes an inverse Anderson transition, while the high-frequency branch supports coexisting flat-band and Anderson localization at strong disorder. Our results deliver the direct evidence of competing localization mechanisms in disordered moire systems and offer guiding principles for engineering robust, nonideal moire photonic devices.
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All authors Qian Liu, Xiaoshuang Xia, Junjie Wang, Peilong Hong, Lei Xu, Lujun Huang, Daohong Song, Yi Liang