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Friday 4 September

Applied Physics

What Photocurrent Versus Effective Voltage Tells Us About Charge Generation in Organic Solar Cells

Ardalan Armin, Austin M. Kay, Drew B. Riley et al. (+2 more)

The photocurrent of an organic solar cell is routinely plotted against an effective voltage and used to extract an exciton dissociation probability, and through it a free-charge generation efficiency. The photocurrent measures neither. Using a drift-diffusion model in which free-charge generation is field independent and losses occur only by bimolecular recombination, we show that the apparent dissociation probability returned by this procedure lies well below unity even for complete dissociation. Across wide variations of carrier mobility and recombination strength it is a single-valued function of the fill factor and coincides with the short-circuit charge collection efficiency, reporting neither the generation yield nor anything about collection beyond what the fill factor already shows. Even for ideal transport the normalised photocurrent saturates below unity at an intensity-independent first-order limit set by recombination of photogenerated carriers with injected equilibrium charge. Applied to four organic solar cells whose generation efficiencies are known independently, the apparent dissociation probabilities track the short-circuit collection efficiency rather than the generation yield. We therefore suggest this construction not be used, particularly in the high-efficiency non-fullerene acceptor systems that are now state of the art.
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All authors Ardalan Armin, Austin M. Kay, Drew B. Riley, Oskar J. Sandberg, Paul Meredith

Applied Physics · Cross-listed from physics.chem-ph

Operando imaging of intercalation memory in MXenes

Franz Groebmeyer, Mohsen Beladi, Christoph G. Gruber et al. (+6 more)

Ion intercalation enables reversible control of charge, structure and function in layered solids. Yet layered materials contain nanosheets with different thicknesses and stacking, whose intercalation pathways are averaged out in ensemble measurements. This becomes critical under Angstrom-scale confinement, where ion transport couples to solvent reorganization and host deformation, leaving the origins of kinetics, reversibility and activation unresolved. Here we combine operando interferometric scattering and interference reflection microscopy to resolve proton-driven dynamics in individual single- and multilayer Ti3C2Tx MXene flakes and show that even a few stacked layers introduce a significant kinetic barrier. Cycling then separates nanosheets into three persistent behaviors: reversible monolayers, restacked multilayers with incomplete recovery and delayed deintercalation, and coherently stacked multilayers that self-stabilize through reversible folding. Operando synchrotron X-ray diffraction shows this memory at electrode scale, where the interlayer structure converges towards a reproducible state. Our findings reveal cycling-induced structural memory that governs subsequent intercalation in layered MXenes.
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All authors Franz Groebmeyer, Mohsen Beladi, Christoph G. Gruber, Ruocun Wang, Pol Salles, Nhu Quynh Nguyen, Jakub Drnec, Yury Gogotsi, Emiliano Cortes

Applied Physics

Noise-Resilient Detection of Neuronal Spikes by a Hopf-Bifurcation Device

Jitendra Kumar, Roberto Fenollosa, Gonzalo Rivera-Sierra et al. (+4 more)

Detecting weak transient signals in noise is a persistent challenge in sensing, communication, and electrophysiology. Here, we demonstrate a physical weak-signal detector based on a semiconductor negative differential resistance (NDR) device operated near a Hopf bifurcation. A coherent input that persists over the response time of the dynamical system can drive a transition between quiescent and oscillatory states, whereas faster stochastic fluctuations are largely suppressed. This nonlinear transformation converts a weak analog threshold crossing into all-or-none voltage spikes and therefore provides asynchronous signal detection without a reference clock. Using a modulated photovoltaic signal, we detect a weak frequency component of 100 Hz as a demonstration, at an input signal-to-noise amplitude ratio as low as 1/500, and reliably recover it in the output spectrum. We then apply the same principle to neuronal multisite extracellular recordings. After standard band-pass filtering, the raw microelectrode signal is transformed by the NDR dynamics, enhancing the distinction between neuronal spikes and background fluctuations. The resulting detected spike times agree closely with those obtained using a traditional spike-detection pipeline. These results establish bifurcation-engineered NDR dynamics as a compact hardware approach to noise-resilient signal discrimination and event-based analog-to-digital conversion that will be useful for neuroprosthetic devices.
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All authors Jitendra Kumar, Roberto Fenollosa, Gonzalo Rivera-Sierra, So-Yeon Kim, Adam Armada-Moreira, Juan Bisquert, Michele Giugliano

Applied Physics · Cross-listed from quant-ph

Microkelvin resolution thermometry at the nanometre scale

Jack W. Hart, Soham Pal, Julien R. E. Roth et al. (+8 more)

Accurate temperature readings of transient events at the nanometer scale are challenging due to the low sensitivity of available sensors. Nanodiamonds containing nitrogen-vacancy (NV) centers have been used for nanoscale thermometry in complex environments, including inside living cells. However, their performance has been limited by short coherence times and low photon counts. In this work, we use isotopically-purified dual-NV nanodiamonds and a bespoke quantum sensing chip to showcase an order of magnitude improvement in temperature measurement sensitivity compared with previous reports. We demonstrate robust temperature measurements with an error of 682 $\mu$K, experimental sensitivities below 50 mK/$\surd \text{Hz}$ and a shot-noise limited sensitivity of 9.6 mK/$\surd \text{Hz}$. To confirm the utility of these high-performance nanothermometers, we quantify the temperature change induced by the thermometry measurement itself, specifically the optical excitation laser used to probe the NV spin state. In addition, we observe directly at the nanometre scale the transient heating caused by the exothermic mixing of dimethyl sulfoxide in water. Sub-millikelvin resolution and millikelvin sensitivity thermometry unlock the possibility of monitoring minute thermal fluctuations in living systems and assessing catalyst performance at the nanometre scale.
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All authors Jack W. Hart, Soham Pal, Julien R. E. Roth, Katie Ninham, Abbie H. Aleksandrova, Xander Peetroons, Soumen Mandal, Oliver A. Williams, Gavin W. Morley, Mete Atature, Helena S. Knowles

Applied Physics · Cross-listed from physics.optics

Modulation-Frequency Dependence of Spatial Resolution in Optical Correlation-Domain Reflectometry

Keisuke Motoda, Takaki Kiyozumi and Yosuke Mizuno

The spatial resolution of optical correlation-domain reflectometry (OCDR) has conventionally been described by an expression that is independent of the modulation frequency $f_m$, source linewidth $\delta\nu$, and receiver resolution bandwidth (RBW). However, our previous measurements showed that the spatial resolution $\Delta z$ improves with increasing $f_m$. Here, we develop a theoretical model for OCDR with a frequency shifter by evaluating the electrical power detected by an electrical spectrum analyzer and explicitly including $\delta\nu$ and the RBW $B$. The model predicts two regimes. At low $f_m$, $\Delta z$ decreases approximately in proportion to $1/f_m$, with the proportionality determined by the combined source and receiver spectral response. At high $f_m$, $\Delta z$ approaches a constant value determined by the modulation amplitude $\Delta f$ and independent of the source linewidth and RBW filter. Measurements at RBW = 1 MHz reproduced the transition between these regimes over correlation orders up to 2048. At RBW = 10 MHz, a Voigt representation of the spectral response overestimated $\Delta z$, whereas direct use of the measured unmodulated beat spectrum reduced the discrepancy to approximately 10 to 20%. These results provide a quantitative description of the modulation-frequency dependence of OCDR spatial resolution and clarify its trade-off with measurement range.
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Applied Physics · Cross-listed from physics.plasm-ph

Activated carbon-assisted anionic dye decomposition using a dual-liquid post-double dielectric barrier discharge plasma reactor

Krishna V S, Surya and Mangilal Choudhary

Non-thermal Plasma (NTP) technology, including the Dielectric Barrier Discharge (DBD) reactor, has emerged as an effective advanced oxidation process (AOP) for treating persistent organic pollutants in industrial wastewater. Based on this objective, a dual-liquid dielectric-barrier-discharge plasma reactor was built and used to degrade a ternary anionic dye mixture comprising Reactive Red 120, Congo Red, and Methyl Orange in its post-discharge configuration. The adjustable length of the outer liquid electrode (grounded) provides flexibility in controlling the discharge length and cooling, thereby achieving higher degradation efficiency and greater reactor stability. The conducting-liquid high-voltage electrode (a dielectric tube filled with a conductive solution) helps mitigate metal corrosion and oxidation caused by reactive species in the discharge zone. A detailed study of the degradation of ternary anionic dye mixtures (simulated dye wastewater samples) in the presence of activated carbon (AC) was performed to explore the potential applications of post-discharge treatment. The activated carbon (granules) enhances the degradation rate of ternary anionic dye mixtures in basic solutions rather than in acidic or neutral solutions. A comparative study of the degradation reaction kinetics of the ternary anionic dye mixtures in acidic and basic solutions was conducted after analyzing UV-Vis absorption spectra. The role of other parameters, such as solution pH, the mass and size of AC particles, dye concentrations, and AC reuse, was investigated in relation to degradation efficiency. The degradation mechanism was elucidated by the formation of strong oxidizer (hydroxyl radicals) on the surface of activated carbon during post-discharge treatment. However, in acidic or neutral anionic dye solutions, the degradation mechanism was dominated by ozone.
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Applied Physics · Cross-listed from cond-mat.mtrl-sci

4 MV/cm (010) $\beta$-Ga$_2$O$_3$ Heterojunction Diodes Realized by Low-Damage e-Beam NiO$_x$ Interlayers

Carl Peterson, Yizheng Liu, Chinmoy Nath Saha et al. (+3 more)

We report on the utilization of a low-damage e-beam NiO$_x$ deposition process to realize field-plated heterojunction diodes (FP-HJDs) on (010) $\beta$-Ga$_2$O$_3$ films with high critical breakdown field strengths beyond 4 MV/cm and power figure of merits (PFOM) of >1 GW/cm$^2$. Diodes were fabricated on a 2.67 $\times$ 10$^{16}$ cm$^{-3}$ intentionally doped 6.2 $\mu$m thick epitaxial layer grown by metalorganic chemical vapor deposition (MOCVD) on a conductive Sn-doped $\beta$-Ga$_2$O$_3$ (010) substrate using TMGa, O$_2$, Ar carrier gas, and SiH$_4$ as the silicon dopant source. The mesa etched FP-HJD devices utilized a thin 7 nm e-beam NiO$_x$ interlayer before the sputtered NiO$_x$ layers to eliminate the effects of sputter-induced ion damage on the (010) epilayers. Current-Voltage measurements resulted in a forward current density of 700 A/cm$^2$ at 4 V, HJD ideality factor of 1.29, a V$_{bi}$ of 2 V, rectification ratio of 10$^{11}$, and a differential specific on resistance (R$_{on,sp}$) value of 2.36 m$\Omega$ $\times$ cm$^2$. Breakdown of the (010) FP-HJDs was 1.64 kV, leading to a parallel plane electric field at breakdown (E$_{||,max}$) of 4.02 MV/cm and a PFOM of 1.14 GW/cm$^2$, which is a state-of-the-art result for diodes on MOCVD-grown (010) drift layers.
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All authors Carl Peterson, Yizheng Liu, Chinmoy Nath Saha, Rachel Kahler, Akhila Mattapalli, Sriram Krishnamoorthy

Thursday 3 September

Applied Physics · Cross-listed from physics.geo-ph

Estimating subsurface water mass changes with ground-based gravimetry

Isabelle Schmidt, Jackson Ang'ong'a, Peter Bauer-Gottwein et al. (+10 more)

Gravitational force is proportional to the mass of an attracting body; therefore, changes in subsurface mass can be detected using gravimetry. In the near subsurface, mass variations are primarily driven by changes in water storage, meaning that gravity measurements provide direct information on variations in water mass. Traditionally, groundwater and soil water dynamics are monitored using point-scale instruments that require direct installation into the subsurface. In contrast, gravimetry is a non-invasive method that integrates signals over a larger spatial extent. Recent advancements in gravimeter technology have significantly improved measurement accuracy and long-term stability under field conditions, thereby expanding their potential for hydrological applications. This review summarizes recent developments in gravimetric instrumentation and provides essential background on gravimeters used for ground-based measurements and their application in hydrology, aiming to facilitate their broader use. In addition, it presents an overview of studies in hydrology and hydrogeology that have successfully applied gravimetry to quantify subsurface water storage changes, estimate hydrological parameters, and constrain numerical models. A perspective on future advancement in gravimetry and resulting potential applications is given.
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All authors Isabelle Schmidt, Jackson Ang'ong'a, Peter Bauer-Gottwein, Stephan Costabel, Christian Freier, Bastian Leykauf, Tina Lundgren, Miguel Martinez-Dorantes, Achim Peters, Ernst Rasel, Enrico Vogt, Gregor Willkommen, Vladimir Schkolnik

Applied Physics · Cross-listed from physics.flu-dyn

Two-phase Temperature Reconstruction in Ice-Water Systems

Zhukun Wang, Daisuke Noto, Douglas J. Jerolmack et al. (+1 more)

What controls heat transport when liquid water interacts with ice? Answering this question is essential for understanding water bodies undergoing phase change. Yet experimental progress remains limited by the lack of a minimally invasive methodology for simultaneously resolving temperature fields in coupled liquid water and non-isothermal ice systems. Here, we introduce a physics-based data-assimilation method for reconstructing temperature fields in buoyancy-driven flows interacting with non-isothermal ice. Particle tracking velocimetry provides the liquid velocity field, while thermal and kinematic boundary conditions constrain the inverse problem. The method couples advection–diffusion in liquid water with conduction in ice to reconstruct simultaneous mean temperature fields and quantify heat transport across the water–ice interface, while remaining minimally invasive and compatible with free-surface systems. We demonstrate the method in laboratory experiments in which the temperature range across the liquid water in contact with ice drives cabbeling-induced convection. This framework enables investigation of coupled thermo-fluid dynamics in cryospheric aquatic systems, including heat exchange at the ice-water interface and liquid-phase energetics, with broader applications to phase-change processes in food and energy industries.
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All authors Zhukun Wang, Daisuke Noto, Douglas J. Jerolmack, Hugo N. Ulloa

Applied Physics · Cross-listed from quant-ph

Proof-of-principle long-distance Sagnac twin-field quantum key distribution network

Reem Mandil, Yen-An Shih, Abhay Verma et al. (+2 more)

Twin-field (TF) quantum key distribution (QKD) offers a promising approach to long-distance QKD networks due to its superior performance over large channel losses. Due to specialized hardware requirements, nearly all long-distance TFQKD demonstrations have only two users exchanging keys, rather than a network with three or more users. In this work, we experimentally demonstrate a proof-of-principle three-user-pair Sagnac TFQKD network spanning 127-km using single-photon avalanche detectors without any active phase stabilization or postcompensation. We implement efficient procedures for maintaining polarization stability and circumventing Rayleigh backscattering noise to achieve a stable Sagnac interference visibility of $93\pm1$% over one hour. A secure key rate of $1.398\times10^{-5}$ bits per pulse is achieved over an asymmetric communication channel with 102-km fiber and 45-dB overall loss. To our knowledge, this is the first TFQKD network without active phase stabilization or postcompensation achieved over long fibers. Our results represent a highly practical and cost-effective approach to long-distance QKD networks.
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All authors Reem Mandil, Yen-An Shih, Abhay Verma, Li Qian, Hoi-Kwong Lo

Wednesday 2 September

Applied Physics

A Langmuir-Like Adsorption Model for MoS2 Gas Sensors: Bridging Theory and Experiment

Z. Geng, M. Ziegler and F. Schwierz

Gas sensors based on layered TMDCs (transition-metal dichalcogenides) such as MoS2 have attracted considerable attention recently and are considered as a promising alternative to the conventional metal-oxide-based gas sensing devices. Recent studies on TMDC gas sensors have mainly addressed either the theoretical description of adsorption mechanisms or the fabrication of test devices and the investigation of their gas sensing performance. However, a suitable approach for modeling the gas sensing behavior of TMDC devices and for establishing a reliable connection between theoretical results and the response of experimental devices when exposed to gases remains absent. To bridge this gap, in the present work a new Langmuir-based adsorption model, capable of calculating both the sensor sensitivity and selectivity, is developed and applied to MoS2-based gas sensors. It is shown that by using appropriate published data for the adsorption energy and the charge transfer as input data, the reported experimental sensitivities of MoS2 gas sensors can be well reproduced by the model.
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Applied Physics · Cross-listed from cond-mat.mtrl-sci

Battery open-circuit voltage is not purely chemical

Andrea Giudici, Christoph Pohl, Alberto Salvadori et al. (+3 more)

Open-circuit-voltage (OCV) curves are commonly treated as intrinsic chemical properties of electrode materials. However, this view is incomplete. In ion-insertion batteries, OCV also depends on mechanical state and microstructure. Using finite-element simulations and asymptotic analysis, we show that particle swelling and external loads promote particle–particle contact that generates compressive stresses, shifting the inserted-ion chemical potential. The OCV correction is nonlinear, follows Hertzian contact scaling, and depends on particle arrangement. Identical materials can therefore exhibit different OCV curves in different electrode microstructures. Furthermore, in full cells, electrodes are mechanically coupled through the common stack stress. Thus, cell OCV is a chemo-mechanical property of the entire battery architecture, not chemistry alone.
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All authors Andrea Giudici, Christoph Pohl, Alberto Salvadori, Colin Please, Manuel Landstorfer, Jon Chapman

Applied Physics · Cross-listed from quant-ph

Exponential speedup of polarization stabilization for long distance DWDM quantum networks

Jinyi Du, En Teng Lim, Xingjian Zhang et al. (+4 more)

Fibre-based quantum networks distributing polarization entanglement require a stable and uninterrupted transmission basis for reliable operation. Bright classical reference light enables rapid polarization feedback but can introduce noise into quantum channels. Entangled-photon-based feedback avoids this noise, but typically interrupts the target entanglement channel during calibration and becomes prohibitively slow over long distances due to the product loss of fibre links. Here, we overcome both limitations by combining wavelength-bracketed probing with switch-enabled path decomposition. Spectrally adjacent entangled-photon sidebands track the polarization response of the central distribution channel without interrupting its transmission, while optical switches and local reference fibres independently determine the signal and idler network transformations. Transferring the resulting compensation settings to the central channel eliminates calibration-induced downtime and changes the acquisition-time scaling from the product of the link losses to the sum of losses. We demonstrate the method on a 133 km fibre testbed and achieve continuous closed-loop stabilization for more than 24 hours without classical reference light. The decomposition of multi-link quantum feedback into single-link measurements provides a scalable stabilization strategy for wavelength-multiplexed quantum networks.
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All authors Jinyi Du, En Teng Lim, Xingjian Zhang, Hongwei Gao, George F.R. Chen, Dawn T. H. Tan, Alexander Ling

Applied Physics · Cross-listed from cond-mat.other

Topological Lithography via External Field: Creating Topological States Anywhere Beyond the Edge

Haoran Nie, Chaoran Jiang, Xiangying Shen et al. (+1 more)

Topological insulators (TIs), recognized for their robust boundary states and unconventional phase transitions, have emerged as one of the most impactful discoveries in recent decades, attracting considerable interest across diverse fields. However, conventional TIs require topological contrasts between adjacent bulk regions, typically achieved by distinct symmetries, which limits their flexibility and broader applicability. In this work, we introduce a "lithography" approach to inducing topological states by applying external fields in time-reversal-symmetric systems. These states transcend the conventional bulk topology design paradigm and offer exceptional tunability: they can exist at geometric edges, within the bulk, or even be induced remotely. Because the external field is highly controllable, the induced states are programmable, reconfigurable, and can be easily tailored into desirable patterns. Theoretically, we demonstrate that the external field modifies the Jackiw-Rebbi mechanism, inducing a real-space topological transition characterized by a local topological marker (LTM). We further establish an extended valley bulk-edge correspondence, which explains both the conventional scenario and our findings. Our results, validated across mechanical, electronic, and acoustic platforms, highlight the broad applicability of this approach to various systems. This work not only advances the theory of topology but also enhances the diversity, tunability, and practical implementation of topological states and materials.
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All authors Haoran Nie, Chaoran Jiang, Xiangying Shen, Lei Xu

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

All-dry processing of 3C-SiC nanomechanical string resonators for extreme aspect ratios and high intrinsic quality factor

Felix David, Philipp Bredol, Yannick S. Klaß et al. (+1 more)

Conventional fabrication of suspended nanomechanical resonators typically relies on wet-chemical process steps and critical point drying, which can compromise sample yield and cleanliness. Here, we present an all-dry fabrication process for strongly stressed 3C-SiC nanomechanical string resonators that entirely avoids wet-chemical etching and cleaning. Using a negative-tone electron-beam resist as an etch mask and a three-step reactive-ion etching process for both structuring and release, we achieve high fabrication yield, clean suspended structures, and extreme aspect ratios of 8.500. We perform full mechanical characterization of the resulting doubly-clamped nanostring resonators, and establish a benchmark intrinsic quality factor for dissipation-diluted 3C-SiC of Qintr = 4.200.
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All authors Felix David, Philipp Bredol, Yannick S. Klaß, Eva M. Weig

Applied Physics · Cross-listed from physics.optics

Multidimensional Light Detection with Symmetry-engineered Heterojunctions

Yucai Lin, Yaoqiang Zhou, Ruijuan Tian et al. (+11 more)

Miniaturised multidimensional light detection, encompassing full-Stokes polarimetry and spectroscopy in ultracompact footprints, is attracting growing interest for its potential to capture a comprehensive set of light properties in portable platforms. Although significant progress has been made in miniaturised schemes for independent polarisation and spectral detection, achieving simultaneous high-dimensional light detection remains an outstanding challenge that limits their development toward full integration. We overcome this limitation by breaking the rotational and inversion symmetries in a symmetry-engineered van der Waals heterojunction to realise an ultracompact multidimensional photodetector. The dual symmetry breaking gives rise to non-trivial quantum geometric and topological features, enabling simultaneous broadband polarisation- and spectrum-resolved light detection, in contrast to previous van der Waals material-based devices, which could detect only one of these modalities. Our device, with an effective area of only 10 micron x 10 micron, reconstructs full-Stokes polarisation with overall root-mean-square errors below 0.05 and resolves spectral peaks separated by 0.4 nm, capabilities not previously achieved in single-pixel detectors. By unifying high-fidelity polarimetry and sub-nanometre spectroscopy in a single electrically tunable junction, our work eliminates the need for cascaded detection architectures and establishes a foundation for multidimensional detector arrays for integrated photonics, quantum information processing, and precision imaging.
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All authors Yucai Lin, Yaoqiang Zhou, Ruijuan Tian, Faisal Ahmed, Andreas C. Liapis, Youqiang Huang, Yawei Dai, Yuwei Chen, Weiwei Cai, Zongyin Yang, Weida Hu, Tawfique Hasan, Luojun Du, Zhipei Sun

Applied Physics

Nanoporous Copper Films as Platform for UV-SERS: Sensitivity and Ability to Perform Chiral Discrimination

Huaizhou Jin, Anastasiia Sapunova, Yanqiu Zou et al. (+8 more)

Surface enhanced Raman spectroscopy (SERS) in the ultraviolet (UV) region offers important advantages for biomolecular detection, including resonance enhancement and reduced fluorescence interference. However, the development of UV SERS substrates that combine low cost, reproducibility, and chemical stability remains challenging. Here, we employ a dry synthesis approach to fabricate nanoporous Cu and copper oxide (CuO) films on silicon substrates and systematically evaluate their UV SERS performance using adenine as a Raman reporter under 325 nm excitation. Among the substrates investigated, nanoporous Cu exhibits the strongest enhancement, enabling adenine detection down to 10 microM. In contrast, no detectable adenine Raman signal is observed under 532 nm excitation, indicating that the enhancement is dominated by a UV induced chemical, charge transfer mechanism rather than conventional electromagnetic enhancement. The Cu substrates further enable the UV Raman spectroscopy of streptavidin, as a test protein, and, more interestingly, the discrimination between L and D tryptophane based solely on differences in UV SERS intensity, without chiral selectors or additional surface functionalization. By varying the substrate rotation speed during metal evaporation, the enantioselective response can be tuned, yielding L over D intensity ratios from 1.10 to 2.35 and demonstrating the critical role of substrate morphology in chiral discrimination. The dry synthesized nanoporous films provide a simpler, scalable, and ligand free fabrication strategy while offering additional capability for enantioselective detection. These findings establish dry processed nanoporous Cu films as promising platforms for UV-SERS biosensing and label free chiral analysis.
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All authors Huaizhou Jin, Anastasiia Sapunova, Yanqiu Zou, Ali Douaki, German Lanzavecchia, Nicolo Maccaferri, Costantino De Angelis, Roman Krahne, Zhenrong Zheng, Shangzhong Jin, Denis Garoli

Tuesday 1 September

Applied Physics · Cross-listed from physics.optics

Multidimensional Double Refraction Microscopy

Jörg König and Christian Cierpka

Multidimensional optical microscopy - extracting 3D location, orientation, and spectral information from nanoscale emitters - is key to modern nanoscience. However, this multidimensional capability requires complex instrumental setups, demanding exceptional stability and specialized expertise in optics. Here, we overcome these barriers by introducing Double Refraction Microscopy (DRM), a passive imaging method providing a straightforward pathway to multidimensional super-resolution imaging. Based on bifocal imaging via a birefringent microscope slide, DRM splits nanoprobe light into a characteristic double image encoding 3D location, orientation, and spectral signature, while requiring zero hardware modifications to standard wide-field microscopes. To showcase its robustness, we implement DRM across three distinct slides of varying materials and thicknesses, calibrating 3D localization with 450-nm nanoparticles and demonstrating high orientation sensitivity using polarization-controlled emitters. Furthermore, we confirm spectral readout using a pinhole array acting as a grid of point emitters back-illuminated at various wavelengths to generate distinct spectral signatures. Finally, we showcase practical utility by performing a two-colour 3D localization measurement within a $10\,\mu\text{m}$ gap, utilizing the single optical pathway of a standard epi-fluorescence microscope. Ultimately, DRM provides the missing optical hardware simplicity to complement established computational tools, promising to transfer advanced super-resolution capabilities from specialized facilities to every laboratory bench.
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Applied Physics

A Constrained Kuramoto Gradient-Flow System Can Perform High-Accuracy Finite-Time Inference

Yi Cheng and Zongli Lin

A central question in physical inference is whether strongly constrained dynamical systems can realize accurate input–output maps through their own finite-time evolution. We study this question in Kuramoto phase networks, whose deterministic dynamics form an input-conditioned gradient flow and whose predictions are read directly from output oscillators. As a constructive training approach, we develop a two-stage teacher–student procedure. A neural teacher is first converted into an explicit phase trajectory whose terminal oscillator activations reproduce the teacher outputs, and the Kuramoto parameters are trained by matching the student vector field along this prescribed path. Because accurate teacher-forced path matching does not ensure accurate autonomous inference, we then differentiate through the autonomous finite-time rollout and directly align its terminal output with the neural target. The resulting oscillator system, with $74$ oscillators, reaches mean test accuracies of $96.711\%$ on MNIST and $86.399\%$ on Fashion-MNIST. This capability persists across neural-teacher architectures, matched system sizes, thermal perturbations, and integration-grid refinement. Together, these results provide a constructive demonstration that a strongly constrained, small-sized Kuramoto gradient-flow system can be trained for high-accuracy finite-time inference through a direct oscillator readout.
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Applied Physics

Quasi-normal-mode signatures of periodicity and hyperuniformity

V. Romero-García, M. Martí-Sabaté, V. F. Dal Poggetto et al. (+2 more)

Hyperuniform materials occupy an intermediate regime between periodic order and conventional disorder, characterized by a suppression of long-wavelength density fluctuations, commonly quantified by the structure factor. This characterization, however, relies on the statistical properties of an extended configuration or supercell representation and does not directly describe the resonant response of a finite structure, as typically encountered in scattering experiments. Here, we introduce the quasi-normal-mode spectrum as a complementary framework for characterizing the wave properties of finite stealthy hyperuniform materials. We analyze the complex eigenfrequencies of both periodic and stealthy hyperuniform structures over a range of the stealthiness parameter ($\chi$), and investigate how the spatial organization of scatterers is encoded in both frequency and spatial profiles of their quasi-normal modes. We find systematic changes in the quasi-normal modes spectrum as the degree of hyperuniformity is varied, revealing signatures that are not captured solely by the structure factor. In particular, the evolution of the resonant states provides a direct connection between structural correlations and the scattering response of finite systems. Our results establish quasi-normal modes as a real-space and open-system characterization of hyperuniformity and provide a unified framework for comparing discrete heterogeneous materials from the perspective of their scattering and resonant dynamics.
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All authors V. Romero-García, M. Martí-Sabaté, V. F. Dal Poggetto, L.M. García-Raffi, M. Lázaro

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

Thickness-Dependent Orbital-to-Spin Torque Signatures in Cr/Gd/Co Thin Films

Tiago de Oliveira Schneider, Michel Heidkamp, Luana Caron et al. (+5 more)

We studied orbital-torque generation in Cr(10nm)/Gd($t_{\mathrm{Gd}}$)/Co(3nm)/TaO$_x$ and the inverted stack Co(3nm)/Gd($t_{\mathrm{Gd}}$)/Cr(10nm) with $t_{\mathrm{Gd}}$ from 0 to $5\,\mathrm{nm}$ by combining electrical harmonic Hall measurements with magnetometry. A detailed understanding of the magnetometric data is obtained by cross-sectional chemical composition mapping. The data show temperature-dependent magnetic compensation points, while elemental analysis provides evidence of pronounced intermixing, in particular of Gd and Co layers. From the harmonic Hall dataset we extract the damping-like (DL) and field-like (FL) torque efficiencies normalized to the applied electric field, $\xi_{\mathrm{DL}}^{E}$ and $\xi_{\mathrm{FL}}^{E}$, and interpret their dependence on the Gd interlayer thickness using two different descriptions: (i) a naive-layer model and (ii) an alloy model that accounts for interfacial mixing. Notably, upon reversing the stack order, the FL contribution changes sign, whereas the DL contribution does not change sign within the harmonic Hall measurements.
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All authors Tiago de Oliveira Schneider, Michel Heidkamp, Luana Caron, Inga Ennen, Matthias Opel, Alexey Arzumanov, Lambert Alff, Markus Meinert

Applied Physics · Cross-listed from nlin.CG

Optical free space extreme learning machine for the implementation of emergent complex systems

Elena Moreno, Fernando Soldevila and Daniel Torrent

Cellular automata conform a set of computational models which evolve with a reduced set of simple rules, yet still are able to show extremely complex emergent phenomena such as fractals and universal computation. Despite their apparent simplicity, they have shown great potential in simulating natural systems and solving challenging computational tasks such as classification and image generation. Instead of implementing cellular automata purely at the software level, it is desirable to design novel analog computing platforms that physically evolve following the automata's underlying rules, thereby reducing power requirements and latency. Here, we introduce an optical extreme learning machine for the simulation of a wide range of cellular automata. Our system operates in free space, and uses a spatial light modulator to encode the evolution rules of the system, while coherent wave propagation performs the corresponding computations. Our results demonstrate a simple, fully-programmable, cost and power efficient, and easy to build and align platform for the implementation of a wide range of complex computational systems such as elementary cellular automata, Conway's Game of Life, and two-dimensional Turing machines.
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Monday 31 August

Applied Physics

A fully coupled electromagnetic-thermal-mechanical model for metal-insulated HTS high field magnets

Anang Dadhich, Nikola Jerance, Tara Benkel et al. (+2 more)

Ultra high field REBCO magnets operate under strongly coupled electromagnetic, thermal and mechanical conditions, where screening currents, localized heating, thermal expansion and Lorentz forces can modify both the structural state and the critical current density of the conductor. In this work, a coupled electromagnetic, thermal and mechanical model is developed for a metal-insulated nested REBCO insert designed for a 40 T class SuperEMFL magnet. The existing electromagnetic formulation resolves the non-uniform screening currents in the REBCO tapes. The thermal model is extended from an explicit Finite Difference Method (FDM) to an implicit Backward Euler scheme with Picard iteration, while a new axisymmetric mechanical FDM solver based on BiCGSTAB is introduced to calculate displacements, strains and stresses in the coil windings and G10 spacer regions. Thermal expansion and Lorentz force contributions are included, and the calculated longitudinal mechanical strain is coupled back to the electromagnetic model through a strain dependent critical current density, which also depends on temperature, magnetic field, and its orientation. A literature-informed Parabolic-Weibull model is used to model reversible and irreversible strain degradation of the REBCO conductor. The numerical methods are benchmarked, and the resulting framework provides a computationally efficient approach for investigating temperature gradients, thermo-mechanical stresses, strain-dependent critical current degradation and quench behaviour in full scale nested high field REBCO magnets.
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All authors Anang Dadhich, Nikola Jerance, Tara Benkel, Philippe Fazilleau, Enric Pardo

Applied Physics · Cross-listed from physics.optics

Modeling and Optimization of Cascaded Four-Wave Mixing toward the VUV in Xenon-Filled Negative-Curvature Fibers

Jose Otavio Rosa, Frédéric Gérôme, Fetah Benabid et al. (+2 more)

In this work, we thoroughly characterize third- and fourth-harmonic generation through cascaded four-wave mixing in a xenon-filled negative-curvature hollow-core fiber operated under a pressure gradient. The experimental results are supported by both a semi-analytical model and numerical simulations, which accurately reproduce the pressure dependence of the harmonic conversion. We identify the optimum pressure conditions for third and fourth-harmonic generation and experimentally observe the cascaded four-wave-mixing process extending into the vacuum ultraviolet (VUV), up to the sixth harmonic at 172 nm, with the accessible spectral range limited by the available VUV filter.
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All authors Jose Otavio Rosa, Frédéric Gérôme, Fetah Benabid, Jonas H. Osório, Jonathas de Paula Siqueira

Applied Physics · Cross-listed from physics.optics

Towards Site-Selective Fabrication of Near-Infrared Emitters in hBN

Tadas Paulauskas, Vakaris Silys, Edgaras Markauskas et al. (+5 more)

Spatial control of near-infrared (NIR) emission from hexagonal boron nitride (hBN) would facilitate coupling atomic-scale light sources to photonic structures, yet oxygen-related NIR emitters are generally formed at stochastic locations. Here, we combine single-shot femtosecond laser writing with annealing in an oxygen-rich environment to bias NIR activation toward predefined coordinates in exfoliated hBN. Spectra acquired with 532, 635, and 785 nm excitations show narrow and multipeak emission extending to a wavelength of 1 um. Among the spectra collected at written sites, over 83% under 785 nm excitation and 71% under 635 nm excitation contain at least one resolved peak above 810 nm. The emission intensity increases monotonically with writing-pulse energy, suggesting tunability and indicating that the optimum for the single-emitter regime may require lower energies. Band-pass-resolved measurements show zero-delay correlation dips with g2(0) values indicative of single-photon emission but also reveal contributions from the spectral background. Spectrally resolved time series under 785 nm excitation show persistent bands over the recorded intervals as well as intermittent emission above 900 nm. This approach demonstrates NIR-emitter activation at predefined sites while identifying residual off-site activation and spectral multiplicity as the principal targets for further optimization.
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All authors Tadas Paulauskas, Vakaris Silys, Edgaras Markauskas, Julius Janusonis, Ifra Bibi, Skirmantas Kersulis, Danielis Rutkauskas, Marek Maciaszek

Applied Physics

Bounds on Shaping Partially Coherent Microwaves with Programmable Scattering Systems

Philipp del Hougne

We derive bounds on manipulating partially coherent microwaves with programmable scattering systems. We first consider the concentration of power from a partially coherent input into a single output port and derive a prototype-aware bound by combining multiport network theory (MNT) with semidefinite relaxation (SDR). We then address general coherency-matrix synthesis, deriving architecture-independent bounds on fidelity and useful strength, prototype-specific scalar refinements thereof, and fully prototype-aware SDR bounds on the useful-strength–fidelity Pareto frontier. The prototype-aware formulations account for mutual coupling, loss, discrete tunability, and static scattering. We evaluate the bounds on four experimental RIS-parametrized MIMO systems with up to 100 1-bit-programmable elements, using proxy-MNT models estimated from measurements. The resulting bounds are generally tight compared with feasible discrete-optimization outcomes. Interestingly, for coherency synthesis, the simpler bounds can in some cases be tighter than the fully prototype-aware bounds, highlighting their complementarity. Our results provide certified limits for wave-domain processing and harvesting of partially coherent microwaves.
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Applied Physics

A model-inversion control strategy to attain desired equilibrium points when synthesizing nonlinear resonators

Maxime Morell, Emanuele De Bono, Emmanuel Gourdon et al. (+3 more)

The initial conditions of a deterministic nonlinear system affects the attained asymptotic solutions. This is a problem when the nonlinear behaviour is achieved by active control synthesis, and the initial conditions are not accessible for pre-constraining. Meanwhile, the generalized impedance control is recently gaining increasing attention in acoustics, and has been enlarged to achieve nonlinear mechanical responses of Electroacoustic Resonators at low excitation levels. Nevertheless, as the initial conditions of Electroacoustic Resonators are hardly accessible, the equilibrium points actually showcased by the synthetic nonlinear dynamics are not advantageous for noise attenuation in cavities excited by realistic noise sources. In this paper, we propose a control strategy being able to attain the desired equilibrium points, due to the definition of an exosystem characterized by a desired nonlinear impedance and a synthetic external excitation. The exosystem dynamics is integrated in real time and its response is enforced in the actual system (in our case, the Electroacoustic Resonator), by a model-inversion feedforward strategy. The proposed algorithm is implemented in the Electroacoustic Resonator, and is validated both numerically and experimentally. In particular, the experimental testing is conducted both in a quasi-open acoustic environment, and in enclosed cavities for acoustic mode attenuation.
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All authors Maxime Morell, Emanuele De Bono, Emmanuel Gourdon, Manuel Collet, Alireza Ture Savadkoohi, Claude-Henri Lamarque

Applied Physics · Cross-listed from cond-mat.soft

Polymer Membrane Tensegrity: Inverse Design of Polymer Films Morphing into Freeform 3D Surfaces with Digital Photopatterning Technique

Shuto Ito, Yuta Shimoda, Haruka Fukunishi et al. (+1 more)

In Metamorphosis of Plants (1790), Goethe traced diverse plant organs to transformations of a common leaf-like structure – a principle modern mechanics attributes to two material ingredients: non-uniform in-plane strain from differential growth or shrinkage, and spatially patterned stiffness. Here we translate this principle into a synthetic fabrication framework called Polymer Membrane Tensegrity (PMT). A flat elastomeric film swollen with a second monomer is selectively UV-cured through a liquid-crystal display (LCD) photomask in a single-side digital photopatterning step, producing rigid rods embedded in a soft, shrinkable membrane. After the unreacted monomer is extracted with a solvent and the film is dried, the membrane shrinks far more than the rods, generating a ~50% in-plane strain differential and a ~2,000-fold modulus contrast – conditions under which the contracting membrane is held in tension by mutually unconnected rods, a tensegrity-inspired arrangement within a single film. An origami-based inverse design algorithm computes the rod layout that morphs the film into a prescribed 3D surface. We demonstrate PMT on a dome, a hyperbolic surface, and a gyroid unit cell – positively and negatively curved targets – reproducing all three with mean deviations of 1.0-2.1% of the target size; perimeter curve optimization halves the mean deviation of the gyroid. Because patterning occurs on one side only, PMT eliminates the front-to-back alignment demanded by bilayer methods, offering a scalable route from flat polymer films to freeform 3D surfaces.
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All authors Shuto Ito, Yuta Shimoda, Haruka Fukunishi, Mikihiro Hayashi

Applied Physics

Towards an effective medium theory for in-vivo Magnetic Resonance: Characterizing Admissible Magnetization Dynamics

Alessandro Sbrizzi, Miha Fuderer and Ray Sheombarsing

The classical Bloch equation forms the foundation of magnetic resonance theory but may be insufficient to capture the complex effective dynamics observed in heterogeneous materials such as biological tissue. Here an effective-medium strategy is proposed to describe magnetic resonance dynamics from observations at macro-scale (order 1 mm$^3$). In this framework, the classical Larmor torque is retained while the relaxation term is replaced by a general nonlinear field (i.e. the effective medium term). This field is required to preserve key physical properties, including rotational symmetry, global stability of thermal equilibrium, and dissipativity. These requirements lead to a complete characterization of admissible relaxation fields and provide a constructive framework for generating nonlinear relaxation models. The resulting theory recovers classical Bloch dynamics as a special case and provides a principled foundation for effective-medium descriptions of magnetic resonance phenomena.
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Applied Physics

Arctic Dispersion Interruption Phenomenon and Sound Source Depth Estimation

Weng Jinbao, Yang Yanming, Chen Benqing et al. (+2 more)

The sound speed profile in the deep Arctic Ocean causes the surface layer to form a normal mode waveguide. When the depth of the sound source or receiver is near a node of the eigenfunction, the source cannot excite the mode, or the receiver cannot detect it, resulting in the modal amplitude at the receiver being approximately zero. This manifests as a dispersion interruption in the dispersion structure, which can be observed through time-frequency analysis of the received acoustic signal. Based on the interruption frequency identified from the received signal, combined with the relationship between node depth and modal frequency calculated from the ocean sound speed profile, the source depth can be estimated if the receiver depth is known. The phenomenon of dispersion interruption and the method of estimating source depth have been validated through simulations and experiments.
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All authors Weng Jinbao, Yang Yanming, Chen Benqing, Xu Dewei, Zhou Hongtao