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Tuesday 6 October

Applied Physics

An associated model of Coulomb friction based on the real area of contact

Manon Thbaut and Laura De Lorenzis

Coulomb friction is a non-associated law: it contains a threshold that depends on the contact pressure, i.e. a stress that is not conjugate to any internal variable, hence it admits no dissipation potential in the sense of generalized standard materials (GSM), and the incremental frictional contact problem is not a minimization problem. In this paper, we propose an alternative friction model that retains Coulomb-like behavior while being associated and fully GSM-compatible. Key to our formulation is the introduction of a new internal variable motivated by the microscopic contact state of rough surfaces, whose evolution is driven by the plastic deformation of the asperities. Under a non-decreasing contact pressure and below a threshold pressure, the model recovers Coulomb friction up to an asperity-scale penetration depth. This depth is physically meaningful and can be calibrated to reproduce known tribology results. Under decreasing contact pressures, the model exhibits a memory effect due to irreversible plastic deformation of the asperities, whereas above the pressure threshold it yields a saturation of the frictional traction. We present both penalty and Lagrange multiplier formulations of the model. A 2D finite element implementation of the penalty approach confirms excellent agreement with the classical Coulomb predictions across two benchmark tests.
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Applied Physics · Cross-listed from quant-ph

A 3D-Printed GHz Microwave-Resonator Paul Trap for Electron Confinement and Millisecond Spin-Qubit Coherence

Niklas V. Lausti, Vineet Kumar, Ivan Hudák et al. (+4 more)

We report a 3D-printed microwave-resonator Paul trap that operates at the GHz frequencies required for electron confinement while preserving an open geometry suitable for laser access and imaging ion Coulomb crystals. The resonator exhibits moderate quality factors of approximately 1000, enabling large confining electric-field amplitudes at low input power, and is in good agreement with finite-element-method (FEM) predictions despite manufacturing imperfections, which we experimentally characterize. The validated FEM model is then used to calculate the electromagnetic fields in the trapping region. These field distributions, together with surface-roughness measurements, are incorporated into a model of electron spin-qubit decoherence to estimate the coherence time. Our analysis predicts that electron spin-qubit coherence times on the order of 10 ms should be achievable under the assumption that motional heating is dominated by Johnson noise, making such systems relevant for quantum information processing. The method of obtaining the coherence time is applicable to any electron Paul trap.
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All authors Niklas V. Lausti, Vineet Kumar, Ivan Hudák, Jiří Hajnyš, Peter Kúš, Radek Plašil, Michal Hejduk

Applied Physics

Towards Accurate MHz Core Loss Measurements

Nick J. Kirkby and Mike K. Ranjram

Miniaturized magnetic components play an important role in achieving high efficiency and high power density in power converters. Accurate core loss data are essential when optimizing magnetic components, but making accurate core loss measurements remains challenging, especially at MHz frequencies. As a result, many research groups have developed their own testers in pursuit of improved accuracy, and the amount of published core loss data has increased. However, large variations in these data shed light on fundamental barriers to accurate MHz core loss measurement. This paper identifies these barriers and presents methods for overcoming them.
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Monday 5 October

Applied Physics · Cross-listed from physics.optics

Compact Circuit Models for Nanoantenna-Based Lightwave Electronics

Adina Bechhofer, Felix Ritzkowsky, Karl K. Berggren et al. (+1 more)

As the field of nanoscale lightwave electronics matures from centering around fundamental science to focusing on device engineering, there is a growing need for compact models that can provide rapid and scalable quantitative analysis of the performance of petahertz-frequency optical-field-driven electronics. In this work, we developed a circuit model framework for describing nano-scale lightwave electronics. Our framework captures the physics governing nanoscale optical devices, such as electrically-connected nanoantennas, using compact circuit models that predict their electromagnetic response, their electron emission, and their charge transfer dynamics. This approach allows scalable simulation of large integrated systems containing networks of nanoscale lightwave electronic components, which is not possible using full-wave electromagnetic and particle-in-cell simulation methods. We implemented the compact model in an free commercial circuit solver (LTspice) and validated its electromagnetic response against a full-wave electromagnetic solver (MEEP). We propose an experiment to fully benchmark the model's ability to capture non-linear charge-transfer-based coupling between devices. The circuit model implementation speeds up the electromagnetic analysis from hours to seconds and accounts for charge-driven coupling, enabling rapid quantitative studies of device operation which provide new insights into how femtosecond signals propagate through nanoscale lightwave electronic structures. We anticipate that the methods we introduce here will become essential to the development and performance analysis of nanoscale lightwave electronics for communication, computation, and signal processing at optical frequencies.
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All authors Adina Bechhofer, Felix Ritzkowsky, Karl K. Berggren, Phillip D. Keathley

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

Mechanics of a Model Frictional Knitted Fabric

Laura Michel, Antoine Faulconnier, Audrey Steinberger et al. (+2 more)

We present a study of the shapes and mechanical properties of a knitted loop made of elastic and frictional yarns. First, for non-frictional yarns, we investigate how the elastic energy of a loop varies with its shape. This allows us to quantify the energetic contributions of stretching and bending, and to identify energetically favorable loop configurations. We then consider the case of thin frictional yarns. In this case, the stability of a loop can be reduced, even in the presence of friction, to a simple planar elastica problem. Friction gives rise to a multiplicity of stable loop shapes in a knitted fabric at rest, i.e., in the absence of externally applied forces. We show that, in the limit of thin yarns, a 2D elastica model can predict these rest shapes. Furthermore, the force-deformation curves under uniaxial stretching can be obtained straightforwardly within this framework. Measurements of loop dimensions and forces during uniaxial stretching of nylon knitted fabrics allow us to assess the qualitative and quantitative validity of the planar elastica model. Finally, we examine the limitations of this model for dense knitted fabrics.
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All authors Laura Michel, Antoine Faulconnier, Audrey Steinberger, Samuel Poincloux, Jérôme Crassous

Applied Physics · Cross-listed from cond-mat.mes-hall

Contact-Governed Macroscopic Signatures of Strain-Induced Valley Sorting in MoS$_2$ Field-Effect Transistors

Suhas Suresh Bharadwaj

Strain engineering is a leading route to control valley-selective transport in monolayer transition metal dichalcogenides. Whether this quantum effect survives inside a fully contacted device remains an open question. Quantum transport models capture the underlying valley deflection but are limited to isolated nanoscale fragments, leaving it unclear if a full device registers strain-induced valley sorting in the bulk, at the contacts, or at all. Here, the quantum valley-deflection tensor of strained monolayer MoS$_2$ is embedded directly into a macroscopic Poisson-drift-diffusion model, extending this physics to a complete, contact-inclusive device. Applying this framework to a 300~nm monolayer MoS$_2$ transistor with a 3~nm HfO$_2$ gate dielectric at 77~K, under a localized 6.14~T pseudo-magnetic strain field, shows the resulting 63.6% suppression of the ON-state current is not a bulk property. It is set almost entirely by the source and drain metal-semiconductor interfaces. The full suppression is reproduced when only the 15~nm contact regions are strained and nearly vanishes when the channel interior alone is strained, identifying the metal contact as the dominant site of strain-induced valley physics. Observing it requires tunneling-transparent, degenerately-doped contacts, since classical Ohmic barriers overwhelm the topological signal. A transmission-line analysis reduces this behavior to a single valley-topological contact resistance ($R_{c,V}$) and a predictive inverse-length scaling law. This framework applies to any strain-engineered 2D-material heterostructure and can be tested with standard multi-length contact structures. Ultimately, these results identify the metal-semiconductor interface as the decisive location for engineering and detecting valley-selective transport in real 2D-material devices.
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Applied Physics · Cross-listed from cond-mat.mes-hall

Adjoint-State Identifiability of Piezo-Tunable Valley Splitting in 2D Magnetic Heterostructures

Suhas Suresh Bharadwaj

Controlling valley degrees of freedom with mechanical strain is a promising approach for solid-state information processing. Current theoretical literature routinely predicts strain-tuned valley splitting at the microscopic level but rarely evaluates whether these quantum predictions remain statistically recoverable in realistic macroscopic devices. This manuscript establishes a fully classical, partial differential equation-constrained multiscale inverse framework for quantifying the device-level identifiability of predicted strain-tunable valley effects in two-dimensional magnetic heterostructures, demonstrated here for a molybdenum disulfide and chromium tribromide heterostructure. First-principles structural relaxations confirm a chiral $C_3$ point-group symmetry which mathematically reduces the relevant exchange-strain coupling tensor to a single scalar. A partial differential equation-constrained adjoint-state architecture successfully bridges continuum elastodynamics to valley-resolved anomalous Hall transport. Density functional theory yields a coupling estimate of $\eta \approx -0.07$ meV whose 95% confidence interval is consistent with zero. Evaluating this specific coupling magnitude against established thermal noise and velocity saturation limits defines a safe operating window bounded between 262.0 and 22,337.6 V/cm. Rather than asserting a confirmed nonzero material property this bounded operational window functions as a precise diagnostic threshold. Deploying this rigorous statistical identifiability framework provides a necessary mathematical filter to determine the true experimental viability of theoretically predicted two-dimensional materials before complex physical fabrication.
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Applied Physics · Cross-listed from eess.SP

Goal-Oriented Wave Computing for Direction-of-Arrival Estimation

Mert Kalfa

Programmable metasurfaces—arrays of elements whose electromagnetic response can be tuned electronically—can process signals as a wave propagates through or reflects off them, a paradigm known as wave-domain analog computing. The design strategy is typically indirect: the surface is fitted to a prescribed linear operator, such as a discrete Fourier transform, and the result is read from output power measurements. Using direction-of-arrival (DoA) estimation as a canonical benchmark, we study what this indirection costs across analog computing topologies, namely reflective reconfigurable intelligent surfaces (RIS), beyond-diagonal RIS (BD-RIS), and stacked intelligent metasurfaces (SIM). We first derive Cramér–Rao bounds for the complete physical estimation chain, including one that accounts for output detectors measuring only intensity. We then formulate a goal-oriented alternative where the surfaces are trained directly through the deployed readout, with no operator target. We observe that goal-oriented training lets sparsely connected or shallower surfaces reach the task performance of their more complex operator-fitted counterparts—despite nearly identical Fisher information—and that the advantage persists under coarsely quantized (1–3 bit) control. From 16 to 64 elements, the depth saving persists, while the connectivity a beyond-diagonal RIS needs grows with the aperture. Under tuner and per-layer loss, the goal-oriented designs keep their advantage.
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Applied Physics · Cross-listed from cond-mat.mtrl-sci

Structural, electronic, and magnetic properties of HoCo$_2$ thin films grown by pulsed laser deposition

Ajay Kumar, Christopher Burgio, Anis Biswas et al. (+1 more)

We report the growth and physical properties of HoCo$_2$ thin films on Si substrate by pulsed laser deposition. Well-defined Kiessig fringes in the x-ray reflectivity data yield a film thickness of $35 \pm 1$~nm and demonstrate smooth film surface and interface profiles. Atomic force microscopy confirms the high morphological quality of the films, revealing subnanometer surface roughness of $\sim$170~pm. Two independently prepared films exhibit a reproducible ferrimagnetic transition at $T_{\mathrm{C}}=77\pm3$~K, demonstrating retention of the characteristic magnetic ordering of bulk HoCo$_2$. However, the absence of resolvable thermal hysteresis indicates a predominantly continuous, second-order like transition in the films, in contrast to the first-order transition of bulk HoCo$_2$. At 5~K, the films exhibit clear magnetic hysteresis with a coercive field of $\sim$600~Oe, substantially larger than the negligible coercivity of bulk HoCo$_2$. X-ray photoelectron spectroscopy reveals comparable Ho $4d$ and Co $2p$ core-level line shapes and broadly similar valence-band spectra for the bulk and film, indicating preservation of the essential local electronic environment and occupied electronic structure.
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All authors Ajay Kumar, Christopher Burgio, Anis Biswas, Yaroslav Mudryk

Applied Physics · Cross-listed from physics.optics

Digital confocal imaging in holographic optical coherence tomography at high space-bandwidth product

N. P. Klooster, S. A. Thakur, F. Zeisberger et al. (+4 more)

Full-field Fourier-domain optical coherence tomography (FF-FD-OCT) enables rapid volumetric imaging but lacks the confocal gating inherent to point-scanning OCT. By acquiring volumes under multiple oblique illuminations, a digital confocal volume can be synthesized in post-processing, a technique originally developed within the framework of reflection matrix imaging (RMI). Existing implementations, however, are often time-domain, off-axis, or use a Michelson interferometer, which reduces sensitivity, speed and the usable space-bandwidth product (SBP). Here, we present an oblique-illumination FF-FD-OCT system in a Mach-Zehnder configuration. This setup enables optimal use of SBP, providing up to a fourfold increase in effective acquisition throughput. In addition, we pose the scattering and aberrations as an overdetermined system, allowing a closed-form solution of an alternating least squares problem. The resulting iterative optimization is equivalent to the state-of-the-art closed-loop accumulation of single scattering (CLASS) algorithm. We demonstrate the setup and reconstruction method in lens tissue, a PSF phantom, a USAF target, and a model eye. The resulting digital confocal volumes provide locally optimized focusing and detection quality, and show improved resolution and signal-to-noise ratio (SNR) compared to conventional FF-FD-OCT. The technique combines the advantages of FF-FD-OCT and confocal OCT while mitigating the limitations of both approaches.
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All authors N. P. Klooster, S. A. Thakur, F. Zeisberger, J. R. Hagen, S. Eggert, U. Najar, D. Hillmann

Applied Physics · Cross-listed from physics.optics

Electro-Optical Metalens for Near-Surface Cooling of a Levitated Particle

Pedro Rosso Gomez, Bruno Melo, Adeel Afridi et al. (+3 more)

Advanced levitation optomechanics in the quantum regime requires sophisticated quantumengineered protocols operating at shorter length and time scales, demanding an increasing control over both optical and electrical potentials. Integrated chip-scale platforms offer a natural route toward such control. Here, we demonstrate optical levitation in vacuum using a miniaturized hybrid electro-optical metalens. Motion control is achieved using a high numerical aperture metalens combined with electrical active feedback using planar electrodes, enabling cooling to low phonon occupations. Reheating measurements enable us to rule out any significant influence from electric field noise; an encouraging result for the future development of on-chip levitation platforms operating close to surfaces. This compact integrated platform facilitates the precise manipulation and control of levitated nanoparticles while laying the ground toward exploiting the full capacity of metaoptics to engineer complex optical potentials, marking a significant step forward in technologies for quantum levitation optomechanics.
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All authors Pedro Rosso Gomez, Bruno Melo, Adeel Afridi, Mohamed Sabry Mohamed, Nadine Meyer, Romain Quidant

Applied Physics · Cross-listed from cond-mat.mes-hall

Giant Conductance Enhancement in Graphene Nanoscrolls via Interlayer-Coupled States

Jia-Cheng Li, Ying-Je Lee, Yu An Cheng et al. (+6 more)

Graphene nanoscrolls form a unique radial superlattice whose geometry enables transport behaviors distinct from their flat counterparts. We theoretically demonstrate that rolling a graphene nanoribbon into a few-turn nanoscroll (1.0-1.2 turns) triggers a giant enhancement in longitudinal conductance by an order of magnitude (nearly twentyfold). We show that this effect stems from interlayer hopping between open boundaries, which not only transforms localized edge states into one-dimensional-like conducting channels and also induces a significant Fermi energy shift. Our results reveal an inverse scaling of conductance with turn number, establishing GNSs as a platform for geometry-driven quantum transport and offering a tunable approach to engineering nanoarchitectural electronic states.
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All authors Jia-Cheng Li, Ying-Je Lee, Yu An Cheng, Yu-Hsiang Fu, Xuan-Fu Huang, Yu-Jie Zhong, Ion Cosma Fulga, Carmine Ortix, Ching-Hao Chang

Sunday 4 October

Applied Physics · Cross-listed from physics.optics

Photonic architectures for synthetic chiral light-driven chiral sensing

Alexander Löhr, Justas Terentjevas, Patricia Vindel-Zandbergen et al. (+6 more)

Molecular chirality gives rise to left- and right-handed enantiomers whose balance underpins biochemical function. Deviations from this enantiomeric excess are promising biomarkers of disease. Despite its importance, rapid and sensitive detection of enantiomeric excess in trace amounts of small chiral molecules remains an outstanding challenge. Synthetic chiral light introduces new opportunities for chiroptical spectroscopy, with the potential to enhance light-matter interactions by several orders of magnitude. Here we propose a new class of photonic architectures for synthetic chiral light, in which photonic structures generate, guide and optimize chiral light-matter interactions. As a first realization, we show that hollow dielectric waveguides support phase-matched propagation of multicolor fields enabling coherent accumulation of the enantio-sensitive nonlinear response over centimeter-scale distances. By combining microscopic quantum-response simulations with Maxwell's equations, we predict that incident pulse energies of around 1 uJ produce hundreds of picojoules of third-harmonic emission from only picomole quantities of carvone gas at 50 mbar over just 2 cm of propagation, with enantio-sensitive contrast approaching 5%. Phase-resolved Fourier analysis isolates a background-free chiral signal exhibiting the maximum possible dichroism of 200%. Beyond efficient chiral sensing, photonic architectures for synthetic chiral light establish a scalable platform for integrated nonlinear chiral spectroscopy, molecular fingerprinting based on ultrafast electron dynamics, and topologically robust chiroptical observables.
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All authors Alexander Löhr, Justas Terentjevas, Patricia Vindel-Zandbergen, Aude Rodriguez, David Ayuso, Laura Rego, Serguei Patchkovskii, Misha Ivanov, Olga Smirnova

Saturday 3 October

Applied Physics · Cross-listed from eess.SP

Toward a Lean Formalization of Analog Computing with Microwaves

Matteo Nerini, Xuekang Liu and Bruno Clerckx

Analog computing with microwave signals can perform linear transformations directly in the analog domain, as the signals propagate through a microwave network. A fundamental question is which transformations can be computed with a given set of microwave components. In our previous work, we answered this question for networks of hybrid couplers and phase shifters by deriving a necessary and sufficient condition on the transformations these networks can compute, and we showed that the discrete Fourier transform (DFT) satisfies it. In this paper, we take a first step toward the formalization of analog computing with microwaves in Lean, a programming language and proof assistant that is increasingly adopted in mathematics. We formalize the considered components, their series and parallel connections, and the class of networks they can implement. Then, we formally prove the necessary and sufficient condition characterizing these networks, as well as the implementability of the DFT of any size power of two. All proofs are checked by the Lean kernel, and the code is openly available at: https://github.com/matteonerini/formalizing-analog-computing.
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Applied Physics · Cross-listed from physics.class-ph

The dual-array RopeComb: a guide-balanced transmission with synthesisable rising mechanical advantage for impedance-matched launch

Rami N. Mahdi

Efficiently transferring kinetic energy from a slow, high-force driver to a light payload, with a near-uniform output force, requires a transmission whose mechanical advantage rises along a steeply convex profile. Continuous-contour mechanisms (variable-radius drums, cams) become prohibitive at scale through diameter span, inertia and rim burst. The RopeComb bypasses these limits by synthesising profiles ($G = 0 \to 100+$) from discrete constant-radius sheaves deflecting tension members into staggered spans. Each fold's ratio vanishes at first contact, enabling snatch-free engagement at 10 m/s without clutches or dampers. Because the array ratio is closed-form, bounded least squares matches uniform-force targets to within 0.6-1.5% across mass ratios of 100:1 to 10,000:1 using 8-13 members. To carry large carriage reactions without the overturning moment a single array imposes, two independent arrays flank a central guide and drive a partitioned fixed-ratio stage ($n_1 + n_2 = k$). Sizing array ratios and member counts in the fall-count ratio $n_2 : n_1$ cancels 97.7% of the single-array reaction and 95% of its overturning moment in simulated 1,000:1 launches at $k=7$, reducing compliant peak-to-mean payload force from 1.40 to 1.13. All results are numerical.
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Friday 2 October

Applied Physics · Cross-listed from quant-ph

Accelerated search for global extrema in structured quantum signals

Sachin S. Bharadwaj and Katepalli R. Sreenivasan

Extreme values of a physical system are rare by definition, yet they can govern some of its most consequential and anomalous behavior. Their sparsity makes them particularly costly to identify in large data sets. Quantum computation offers a potential way to reduce this cost. The challenge is that quantum amplitudes cannot simply be read, copied, compared, or marked using conventional search oracles. Here we introduce an algorithm for searching amplitude-encoded quantum signals that exploits their underlying structure. Through a nonlinear amplitude transformation, the algorithm identifies and certifies a global extremum while exponentially reducing the effective search space for structured signals. We establish a polylogarithmic query complexity in the data size $N$, together with global completeness, efficient classical verification, and a measurable criterion for quantifying exploitable structure. We demonstrate the framework using computational turbulence data. Applied to line cuts of velocity gradients from a $1024^3$ simulation containing more than one billion grid points, the algorithm consistently isolates the extreme tail and reduces the median search space to $O(\log N)$. The clustered structure characteristic of turbulence further accelerates the search. These results establish a direct connection between physical structure and quantum advantage in extreme-value search. For accessibility, the main text emphasizes the central ideas and results, while the theoretical derivations are provided in the Appendices.
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Applied Physics

A Novel Iterative-Fitting Approach for the Thermal Characterization of Thin Fibers

William Spirnock, Alison Hake, Dihui Wang et al. (+3 more)

The transient electrothermal technique (TET) is widely used to measure the thermal properties of thin fibers and films, but its accuracy declines when radiative heat losses are significant and surface emissivity is unknown. These conditions are difficult to avoid at the microscale, while conventional analyses either neglect them or rely on separate measurements that introduce sequential errors. We present a TET implementation that determines multiple thermal properties and effective surface emissivity from only two transient measurements. An analytical model incorporating thermal radiation and variable heat generation supports sensitivity-guided, multiparameter fitting. Two forms of the temperature solution yield thermal conductivity and diffusivity, and thermal conductivity and volumetric heat capacity, respectively. An iterative fitting procedure then recovers intrinsic properties corrected for environmental effects while reducing the required sample count and measurement time. Measurements of amorphous silicon dioxide (SiO2) optical fiber and Hi-Nicalon Type S silicon carbide (SiC) fiber agreed with literature values to within an average error of approximately 10% across all samples. Experimental and numerical uncertainty analyses quantify measurement variability and relate it to sample geometry. This method simultaneously resolves multiple intrinsic thermal properties, corrects radiative-loss errors, and provides quantitative guidance for sample geometry selection.
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All authors William Spirnock, Alison Hake, Dihui Wang, Timothy Sullivan, Dana He, Heng Ban

Applied Physics · Cross-listed from physics.optics

Beyond the PT-Broken Phase: Constant-Frequency Oscillation from Floquet Hybridization

Xiaoke Gao, Xiaoyin Sha, Xunyan Sun et al. (+3 more)

Constant-frequency oscillation is usually tied to the broken phase of a spatially coupled PT-symmetric dimer, where the eigenfrequencies form a complex-conjugate pair with a fixed real part. This work shows that an analogous locking can instead come from Floquet hybridization in a synthetic frequency dimension. Periodic gain–loss modulation folds the spectrum into replicas spaced by the drive frequency $\Omega$. Coupling between neighboring replicas opens an additional real-part degeneracy at the edge of the Floquet Brillouin zone. The hybridized Floquet mode at $\omega_0\pm\Omega/2$ requires the minimum gain and is therefore selected for the self-oscillation. A Floquet PT-symmetric circuit with a JFET time-varying negative resistance confirms the prediction. The observed constant-frequency oscillation contains two frequency components of comparable amplitude, separated by the modulation frequency, which identifies its origin from Floquet hybridization rather than ordinary PT-symmetry breaking or dispersion flattening. More broadly, once a suitably symmetry-constrained spectral degeneracy is established, self-sustained frequency pinning can arise beyond the conventional spatial PT-broken phase.
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All authors Xiaoke Gao, Xiaoyin Sha, Xunyan Sun, Xianglin Hao, Huaiqing Zhang, Tianyu Dong

Applied Physics · Cross-listed from physics.ins-det

Controlled-illumination smartphone imaging for quantitative turbidity measurement and cross-device transfer

Sebastian Olivares, Addles Castillo, Patricia Velasquez et al. (+2 more)

Smartphone imaging could provide a low-cost turbidity measurement method if illumination and acquisition geometry are controlled. We evaluated a 3D-printed enclosure with fixed illumination and a submerged optical target using 162 milk-based suspensions spanning 0.16–50.2 nephelometric turbidity units (NTU), with a portable turbidimeter as the reference. An EfficientNetB0 regression model evaluated using repeated five-fold grouped cross-validation achieved a mean absolute error (MAE) of 1.208 NTU and $R^2=0.9816$, indicating that predictions followed the reference measurements closely. The MAE was 0.335 NTU below 1 NTU and 0.449 NTU from 1 to below 5 NTU; classification accuracy at the operational 5-NTU threshold was 0.9753. When applied without retraining to 103 paired images from a second smartphone, prediction error remained comparable to that on the development smartphone below 10 NTU, but turbidity was increasingly underestimated at higher values. The results demonstrate the measurement performance attainable with a controlled smartphone acquisition system, quantify an important device-dependent transfer limitation, and motivate standardized multi-device acquisition and prospective natural-water validation.
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All authors Sebastian Olivares, Addles Castillo, Patricia Velasquez, Roxana Arce, Nicolas Richardson

Applied Physics · Cross-listed from physics.optics

Active Phase and Amplitude Modulation in Perovskite Polaritonic Waveguides

Andrea Zacheo, Marco Marangi, Tim Colin Meiler et al. (+6 more)

Integrated photonic technologies require active materials capable of providing large optical modulation within compact footprints and low losses. The large optical nonlinearities of exciton-polaritons offer a promising route to meet these requirements, yet their integration into photonic circuits remains challenging because strong light-matter interactions must be combined with low-loss transport and spatially localized control. Here, we demonstrate active phase and amplitude modulation in halide perovskite polaritonic waveguides integrated with electrical microheaters. Local electrothermal actuation drives a structural phase transition in the perovskite, modifying the complex refractive index dispersion near the excitonic resonance. By tuning the excitonic fraction of the propagating mode, the waveguides can therefore operate either as a phase shifter with a tuning efficiency of 0.2 mW mm, approximately one order of magnitude lower than silicon modulators with similar geometries, or as an intensity switch , providing 1 dB/um modulation depth at 20 mW. Our work establishes phase-change excitonic reconfiguration as a route towards compact polariton circuits for nonlinear photonic applications.
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All authors Andrea Zacheo, Marco Marangi, Tim Colin Meiler, Filippo Martinelli, Antonio Fieramosca, Vincenzo Ardizzone, Giorgio Adamo, Daniele Sanvitto, Cesare Soci

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

Second-order Raman and anharmonic lattice dynamics in lead-halide perovskites

Chao Shen, Florian Kluibenschedl, Ayan A. Zhumekenov et al. (+4 more)

We explore the microscopic origin of THz-driven lattice dynamics in methylammonium lead bromide CH$_3$NH$_3$PbBr$_3$ using two-dimensional THz Kerr-effect spectroscopy. The analysis of the experiment enables discrimination between competing nonlinear excitation mechanisms and reveals that the dominant pathway is phonon-mediated rather than purely photonic. In particular, we argue that the commonly observed 1.15~THz Raman-active $A_g$ mode is not excited directly by the THz electric field. Instead, it emerges through cubic anharmonic coupling to an infrared-active phonon at 0.68~THz. Furthermore, a satellite feature at 1.36~THz is identified as a two-phonon Raman response of the same infrared-active mode. The observation of strong quadratic coupling involving the infrared-active phonon suggests an unconventional form of electron-phonon dressing present in lead-halide perovskites.
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All authors Chao Shen, Florian Kluibenschedl, Ayan A. Zhumekenov, Mikhail Lemeshko, Osman M. Bakr, Artem G. Volosniev, Zhanybek Alpichshev

Applied Physics

Lithium Niobate Surface-Acoustic-Wave Resonator for Very-High-Frequency Isolated Power Conversion

Liyang Jin, Zhiyang Cao, Zichen Xi et al. (+4 more)

Piezoelectric devices offer a promising magnetic-free alternative for high-frequency power electronics, enabling compact energy transfer with inherent galvanic isolation. Here we demonstrate a lithium niobate surface acoustic wave (SAW) resonator that enables isolated power transfer and addresses the challenges in existing piezoelectric devices for power applications, including operation frequencies, power delivery figure of merit (FOM), power handling, and electrical isolation. Our application-optimized 40-MHz SAW resonator, with the two electrically isolated interdigital transducers (IDTs) and acoustic Bragg mirrors, achieves a 25.7 W output power at a less than 15 {\deg}C temperature rise with only passive cooling through the back mounting side, and an isolation voltage of 2.17 kV. In DC-AC power conversion, we demonstrate a peak efficiency of 93.8 %, a power delivery FOM of 13.47 mW/V$^2$ and a peak chip power density of 81.7 W/cm$^3$. DC-DC conversion is also demonstrated by connecting a diode rectifier to the output IDT, despite efficiency degrading to 52.7 %. Our results advance SAW devices as one of the core components for next-generation very-high-frequency, miniaturized, high-density, magnetic-free, isolated power converters.
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All authors Liyang Jin, Zhiyang Cao, Zichen Xi, Jun Ji, Yizheng Zhu, Liyan Zhu, Linbo Shao

Thursday 1 October

Applied Physics · Cross-listed from physics.optics

Thin-capping layer epitaxial quantum dots for near-field quantum photonics

Yuting Guo, Jonathan Bar-David, Pasquale Cilibrizzi et al. (+3 more)

Epitaxial quantum dots (QDs) are widely recognised as one of the best quantum light sources, given their good stability, brightness, quantum efficiency and coherence. To reach such properties, QDs are protected from potentially detrimental surface states by a relatively thick capping layer, typically exceeding 50nm. This prevents the implementation of near-field effects, like plasmonic-based ones, that can dramatically increase the light-matter interaction, since the control of the spontaneous emission dynamics and directionality of the emission can only occur if the emitter is in close proximity (typically tens of nanometers or less) from the metallic structures. In this work, we report the growth and optical characterisation of InAs/GaAs QDs with GaAs capping layer thickness of 10nm, 20nm, and a more standard 95nm. Remarkably, we observe emission linewidths up to 5 times smaller than previously reported, with unperturbed excitonic lifetimes. These results demonstrate that the epitaxial QD's high optical quality can be maintained even when the emitters are at reduced distances from the surface, opening the path for the exploration of near-field light-matter interactions with coherent and stable emitters, suitable for quantum technology applications.
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All authors Yuting Guo, Jonathan Bar-David, Pasquale Cilibrizzi, Sung-Yul L. Park, Jin Dong Song, Luca Sapienza

Applied Physics · Cross-listed from physics.optics

Programmable nonlinearity within nanophotonic waveguides

Benjamin A. Ash, Ryotatsu Yanagimoto, Mandar M. Sohoni et al. (+8 more)

Nonlinear nanophotonics enables the engineering of optical functions using light-matter interactions in nanoscale structures with carefully designed geometries. Phase matching in nonlinear optics is typically essential for realizing high efficiency for desired processes and engineering of sophisticated functions, but it can be challenging to reliably achieve in nanophotonic waveguides. Conventionally, phase matching in nanophotonics is achieved through nanofabrication, e.g., by poling a device's nonlinear material using electrodes with fixed geometry. This approach has two drawbacks: the optical functions a device can perform are fixed at the time of fabrication, and the device's performance can be degraded by fabrication imperfections. Here, we circumvent these limitations by engineering programmable $\chi^{(2)}$ holograms inside nanophotonic waveguides, enabling reconfigurable quasi-phase matching that can be updated after device fabrication in approximately one second. By projecting structured optical illumination onto photoconductive electrodes, we generated spatial patterns of electric-field-induced $\chi^{(2)}$ to exert spectral, modal, and polarization control over three-wave mixing processes. This programmability also allowed us to employ closed-loop in situ optimization to compensate for phase mismatch in complex waveguide structures, such as width-modulated waveguides and spiral waveguides exceeding 10 cm in length. Furthermore, we show that programmable nonlinearity enables powerful in situ diagnostics, including direct dispersion measurements of various transverse modes and on-chip wavefront phase tomography. Our work opens up the possibility of building cascaded nonlinear nanophotonic systems where yield issues due to fabrication variations can be sidestepped while retaining the benefits of etched waveguides such as confinement and dispersion engineering.
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All authors Benjamin A. Ash, Ryotatsu Yanagimoto, Mandar M. Sohoni, Yang Xu, Yiqi Zhao, Martin M. Stein, Fan O. Wu, Marc Jankowski, Logan G. Wright, Tatsuhiro Onodera, Peter L. McMahon

Applied Physics · Cross-listed from cs.LG

BranchIP: Learning Adaptive Equivariant Computation for Interatomic Potentials

Laura Zichi, Gil Harari, Chuin Wei Tan et al. (+6 more)

Equivariant machine learning interatomic potentials (MLIPs) have revolutionized atomistic modeling, but accurate treatment of complex materials and molecular systems demands expensive models. This limits simulation length- and time-scales, with tensor products a key computational bottleneck. The recent emergence of foundation-scale MLIPs further exacerbates this challenge. We present Branch Interatomic Potential (BranchIP), a single-model framework for learned adaptive tensor product computation, trained with a novel distillation loss. In our experiments on two systems of physical interest, a heterogeneous catalysis system and a proton-conducting solid acid electrolyte, BranchIP accelerates MLIPs across model sizes by up to $2.4\times$ while reducing memory usage by up to $2.6\times$. This is achieved while maintaining physical fidelity. Furthermore, the learned adaptive computation provides model interpretability by revealing which interactions demand deeper computation and showing how computational depth relates to chemical complexity and dynamics.
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All authors Laura Zichi, Gil Harari, Chuin Wei Tan, Marc L. Descoteaux, Albert Zhu, Menghang Wang, Yoel Zimmermann, H.T. Kung, Boris Kozinsky

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

Benchmarking average atom potentials derived from atomic cluster expansions

Deepak Somani, Lorenzo Piersante and Anirudh Raju Natarajan

Average atom (A-atom) potentials provide a mean-field description of a chemically disordered alloy and are used to predict the properties of solid solutions without short-range order. Such potentials are usually averaged from an existing interatomic potential and are therefore only as accurate as the parent model. Accurate interatomic potentials are themselves difficult to parameterize and can require large training datasets. Here we benchmark a recently developed formalism that computes an exact A-atom potential directly from a linear atomic cluster expansion (ACE). We first fit a linear ACE to Fe-W data generated with an embedded atom method (EAM) potential. The resulting A-atom potential reproduces the properties of the disordered phase computed from an explicit random supercell and from a conventional A-atom potential averaged from the same EAM potential. We then fit a linear ACE to energies and forces computed from electronic structure calculations for about 1500 small Mo-Nb structures with an average of 7 atoms per structure. The A-atom potential derived from this ACE reproduces the DFT elastic constants, lattice parameter, mixing enthalpy, and Bain path of special quasirandom structures. Of the three chemical site bases, only the occupation basis also reproduces the DFT surface energies of the alloy. For Mo-Nb, the A-atom potential also predicts that the ideal solution entropy outweighs the destabilizing vibrational contribution to the finite-temperature free energy of the disordered phase. These benchmarks show that the properties of disordered alloys can be recovered from small training datasets when the chemical site basis is chosen carefully.
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Applied Physics · Cross-listed from cond-mat.mes-hall

Inertial Dynamics of a Skymeron

Mona Bhukta, Duc Minh Tran, Kilian Leutner et al. (+19 more)

Topological spin textures in antiferromagnets inherit the compensated magnetic order of the host material, resulting for skyrmions in the suppression of the skyrmion Hall effect and enabling ultrafast dynamics that make them attractive for low-power spintronic devices. In intrinsic antiferromagnets, the two sublattices are identical, but synthetic antiferromagnets offer additional control mechanisms by enabling independent tuning of the properties of two ferromagnetic layers. Analogous to a ferrimagnet, the total magnetic moment can be adjusted by compensating the two-layer moments. Here, we demonstrate that tuning the effective magnetic anisotropy difference between the two layers drives a spin-flop transition where one layer reorients and thus stabilizes an orthogonal configuration with one layer oriented along the out-of-plane direction and the other in-plane. In this transition, an antiferromagnetic skyrmion undergoes a homotopic reconfiguration into a complex spin texture comprising a skyrmion coupled to an in-plane bimeron. Element-specific X-ray microscopy resolves this texture layer by layer: one hosts an out-of-plane skyrmion coupled to an in-plane bimeron in the other. We refer to this previously unexplored three-dimensional spin texture as a skymeron. By using time-resolved pump-probe X-ray microscopy, we discover unique polarity ,dependent dynamics during current pulses: a short post-pulse inertia-like continuation of the motion, and a slower return towards the pinned initial state. Micromagnetic simulations reveal that the inertia-like propagation originates from the finite-time relaxation of reorientation of the skymeron. Our results establish layer-selective anisotropy engineering as a route to uncharted composite spin textures with internal dynamical degrees of freedom, not possible in conventional antiferromagnets with identical sublattices.
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All authors Mona Bhukta, Duc Minh Tran, Kilian Leutner, Takaaki Dohi, Nikolai S. Kiselev, Filipp N. Rybakov, Olle Eriksson, Fabian Kammerbauer, Sebastian Wintz, Markus Weigand, Hendrik Ohldag, Maria-Andromachi Syskaki, Robin Tietgen, Edoardo Mangini, Sabrina Kerber, Thibaud Denneulin, Joseph Vimal Vas, Rafal E. Dunin-Borkowski, Bastian Pfau, Oleg A. Tretiakov, Robert Frömter, Mathias Kläui

Applied Physics

Determination of van der Waals thin film thickness via characteristic X-ray photon yield

Guoshun Luo, Shichao Zhao, Xinyu Liu et al. (+7 more)

Characteristic X ray generation is essential for producing X ray sources and for investigating light matter interactions. Here, we demonstrate that characteristic X ray generation can also be employed to determine the thickness of van der Waals thin films in a non destructive manner. Specifically, we establish a sixth order polynomial model that relates film thickness to the characteristic X ray yield. The model can both predict the X ray yield of graphite films at arbitrary thicknesses and infer the film thickness from a corresponding characteristic X ray yield. We validate the model by comparing its predicted X ray yields for graphite films with those obtained from Monte Carlo simulations. Additionally, we experimentally validated the reliability and feasibility of the proposed method for extracting graphite film thickness via X ray photon yield characterization. Our results provide a non destructive approach for determining the thickness of van der Waals thin films.
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All authors Guoshun Luo, Shichao Zhao, Xinyu Liu, Suguo Chen, Yue Wang, Ping Zhang, Zhigang Lu, Shaomeng Wang, Sunchao Huang, Yubin Gong

Applied Physics · Cross-listed from physics.optics

A Mid-Infrared Absorber Inspired by the Wing-Scale Photonic Structure of Troides magellanus

Amandine Marchand, Kevin Delmote, Olivier Deparis et al. (+1 more)

Growing environmental and energy-related challenges have intensified the search for technologies capable of improving energy recovery and resource efficiency. Bioinspired photonic structures offer promising opportunities for the development of efficient infrared absorbers by exploiting optical mechanisms found in natural systems. In this work, we present a bioinspired approach for the design and optimization of a broadband mid-infrared absorber based on the highly absorbing wing scales of the Troides magellanus Magellan birdwing butterfly. The characteristic geometry of the biological structure is rescaled and adapted to shift its optical response from the visible to the infrared spectral range. Numerical simulations are used to investigate the influence of the main geometrical parameters on the spectral absorptance and to identify an optimized configuration. The optimized structure achieves an absorption enhancement of 391% compared with a flat steel reference. The improved optical response is attributed to the optimized high-aspect-ratio geometry, which enhances multiple scattering and light trapping, thereby increasing the effective light-matter interaction area. These results demonstrate the potential of bioinspired geometric scaling as an effective strategy for engineering efficient infrared absorbers and tailoring their spectral response for thermal radiation management and infrared photonic applications.
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All authors Amandine Marchand, Kevin Delmote, Olivier Deparis, Sébastien R. Mouchet

Applied Physics

Correlation Imaging via Hybrid Entanglement between Microwave Photons and Surface Acoustic Wave Phonons

Yu-Yuan Chen, Ling-An Wu and Yu-xi Liu

Establishing effective spatial correlations between distinct quantum fields is essential for multi-physics quantum sensing.~We here propose a correlation imaging approach based on the hybrid entanglement between microwave photons and microwave surface acoustic wave phonons generated via a superconducting quantum circuit.~At matched microwave frequencies, the five order-of-magnitude wavelength disparity between photons and phonons leads to a unique quantum entanglement resource, which allows the optimal field for probing an object to be distinct from that used for readout.~This resource enables two imaging modalities with giant spatial scaling: a $10^{-8}-10^{-6}$-fold demagnification of macroscopic object profiles onto microscopic phononic chips, and conversely, a $10^{4}-10^{6}$-fold magnification of microscopic object profiles into macroscopic photonic readout devices. By interfacing free-space long-wavelength microwaves with on-chip short-wavelength surface acoustic waves, our correlation imaging approach provides a general framework for cross-scale quantum sensing.
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