Wednesday, 4 November 2026: 9 new papers, 9 cross-lists and 9 replacements in Optics (physics.optics), in arXiv's order, with full abstracts. The same papers as arXiv's own listing, set as a newspaper; every title links to arXiv.
We derive the ray propagation of circularly polarized light in an inhomogeneous, isotropic and lossless medium, including the first correction to geometrical optics, the optical Magnus (spin Hall) effect. Starting from Maxwell's equations and the eikonal expansion to first order in $1/k_0$, we show that the polarization is parallel transported along the ray, identify the Berry connection and curvature of the helicity states, and obtain a ray action in which the Berry phase appears as a correction to the optical path length. This action leads to a Hamiltonian in which the refractive index is evaluated at a position shifted by the Berry connection, $\mathcal H_\sigma(\mathbf r,\mathbf p)=|\mathbf p|-n(\mathbf r+\mathbf A_\sigma(\mathbf p)/k_0) \simeq|\mathbf p|-n(\mathbf r)-k_0^{-1}\boldsymbol\nabla n\cdot\mathbf A_\sigma(\mathbf p)$. In terms of the physical position, the equations of motion contain the Berry curvature as an anomalous velocity. We give the equivalent non-canonical bracket form, the gauge transformation, and the equivalent time-parametrized formulation with the Hamiltonian $\hbar\omega=\hbar c|\mathbf k|/n$, and we state the approximations involved. The resulting ray equations are compared with analytic and numerical results. A planar interface, the helical ray in a cylindrical medium and the exact conservation of angular momentum provide checks. The formulation is set up for numerical propagation of rays in a medium with a spatially varying refractive index.
Nonlinear metasurfaces offer a compact platform for infrared-to-visible conversion and imaging, yet resonant enhancement is often accompanied by restricted angular acceptance, limiting the range of transverse spatial frequencies that can interact efficiently with the nonlinear modes. Here, we address this challenge using a one-dimensional photonic moir\'e superlattice supporting multiple quasi-flat bands in the near-infrared. These moir\'e modes combine strong near-field enhancement with broad momentum-space accessibility, enabling efficient third-harmonic generation (THG) while maintaining stable excitation over a large angular range. Experimentally, we observe strong THG emission near $467~\mathrm{nm}$ under broadband excitation from $ 1180$ to $ 2000~\mathrm {nm} $. We further employ the nonlinear moir\'e superlattice for infrared imaging by converting an infrared target into visible through the THG process. By encoding image information into displacement-dependent THG signals and decoding their spatial variations, we experimentally resolve weak features and internal textures with improved contrast compared with THG imaging using nonlinear films or conventional nonlinear metasurfaces. These results establish photonic moir\'e superlattices as a versatile platform combining efficient frequency conversion, broad angular acceptance, and mode-sensitive spatial encoding for high-contrast, label-free nonlinear imaging with enhanced feature discrimination.
Broadband and simultaneously accessible multi-tone millimeter-wave (mmWave) and terahertz (THz) signals are attractive for multi-band wireless communication and spectroscopy applications, yet their generation remains constrained by electronic bandwidth limitations and inefficient optical frequency conversion. Here, we demonstrate reconfigurable photonic generation of multi-tone mmWave/THz signals with comb spacings of 100, 200, 300, and 400 GHz, using a strongly asymmetric dual-ring silicon-nitride (SiN) microresonator and a broadband spiral plasmonic photomixer. A localized avoided-mode crossing (AMX) introduced by an auxiliary ring supports a fixed optical parametric oscillation (OPO) mode pair over an extended pump-detuning range, consistent with passive compensation of detuning-dependent phase mismatch. This enables a maximum pump-to-generated-line conversion efficiency of 67.3 % with 25.06 dB of pump depletion across different pump wavelength selections. Direct photomixing of these optical states produces simultaneous multi-frequency tones spanning 100 to 900 GHz without intermediate optical amplification. The generated carriers are further characterized through down-converted spectra, frequency-noise power spectral density (PSD), and Allan deviation measurements, yielding a \b{eta}-separation linewidth of 26 kHz at an observation time of 0.1 s. Optical modulation offers high radiated power levels exceeding 2 mW at 100 GHz at a duty cycle of 1 %, while mitigating thermal breakdown in the photomixer. High optical conversion efficiency, reconfigurable and selectable comb spacings, and broadband multi-tone generation on a chip-scale platform provide a compact route toward reconfigurable multi-band mmWave/THz sources for wireless communication, spectroscopy, and sensing.
Effective transition conditions that replace a thin polarizable layer by a zero-thickness sheet are commonly applied in electromagnetic models of two-dimensional (2D) materials and metasurfaces. These conditions usually take the arithmetic mean of the fields on either side as the polarizing field at the boundary, and characterize the layer by the $0^\mathrm{th}$ moment of its susceptibility, but both choices are typically imposed rather than derived. We derive transition conditions for a finite-thickness, in-plane homogeneous, in-plane isotropic layer with a general nonlocal susceptibility, using a single-scale asymptotic expansion in $\eta = l\max(k_0,q)$, where $l$ is the layer thickness, $k_0$ the free-space wavenumber and $q$ the in-plane wavenumber. The sheet limit must be taken after resumming the out-of-plane depolarizing field, as this ensures that the resulting sheet susceptibility is independent of the material thickness. The arithmetic mean then emerges as the leading-order polarizing field for general susceptibility profiles. Asymmetries in the layer profile modify the effective susceptibility appearing in the transition conditions, rather than altering the arithmetic-mean field prescription. If the spatial profile is not even in both out-of-plane coordinates, in-plane and out-of-plane response hybridize, turning the diagonal volumetric susceptibility into a $q$-dependent effective tensor surface susceptibility with off-diagonal components. Mirror symmetry alone does not remove this hybridization. Comparison with exact solutions for local and separable layers shows that the derived conditions remain accurate to $O(\eta)$ across propagating and evanescent excitations, whereas the standard sheet conditions fail once hybridization becomes appreciable.
Multimode fiber (MMF) imaging is highly sensitive to mechanical perturbations, which alter modal coupling and disrupt the speckle-to-target mapping. Here, we propose a reference-beam-assisted method for image reconstruction across multiple fiber configurations. An independent reference beam propagates through the same MMF as the imaging signal without target modulation, providing target-independent transmission-state information through its speckle patterns without requiring a separate configuration-sensing structure. A neural network jointly processes the signal and reference speckles to reconstruct target images. Experiments with MNIST and Fashion-MNIST across 10 fiber configurations demonstrate improved reconstruction quality, faster training convergence, and more accurate configuration identification. Further evaluation across 40 configurations confirms the reconstruction benefit of reference-beam assistance over a larger set of perturbed fiber states. These results provide a foundation for developing perturbation-adaptive MMF imaging systems for flexible endoscopy and minimally invasive optical diagnostics.
Near-zero-index (NZI) media have been explored for controlling spontaneous emission, but their impact on stimulated emission and lasing properties remains unexplored within their different classes, i.e., epsilon-near-zero (ENZ), mu-near-zero (MNZ), or epsilon-and-mu-near-zero (EMNZ). Here, we show theoretically that lasing emission depends on the class of NZI medium and can be controlled through its impedance. The major difference, as compared to conventional laser media, arises from the incorporation of dispersive electric and magnetic properties in the derivation of the Maxwell Boch equations. In this framework, the analysis of laser rate equation reveals that the ENZ class, with high impedance, can reduce the lasing threshold through enhanced electric-field interaction. Numerical simulations further show that the NZI class also governs the spectral response: whereas ENZ and MNZ classes suffer from multimode emission, the EMNZ class preserves single-mode emission.
In single-pixel (ghost) imaging, known illumination patterns probe an object and a detector with no spatial resolution returns one number per pattern. Each measurement costs acquisition time and photons and, for ionizing probes, contributes to delivered dose. Motivated by automated radiographic defect screening, where a class label suffices, we introduce Learning Illumination Patterns for Single-Pixel Classification (LIPS-PC). The patterns are learned end-to-end while the measurement operator remains one fixed linear matrix, identical for every input. We parameterize it as a residual correction to a two-dimensional discrete cosine transform (2D-DCT), $D=D_{\mathrm{base}}+\Delta$, and jointly train the correction and a compact classifier using cross-entropy, without a reconstruction objective. This yields a task-driven linear embedding through optimization. We evaluate LIPS-PC numerically on MNIST and Fashion-MNIST under a controlled protocol, then on RIAWELC, a real industrial X-ray weld-radiograph dataset. Across five budgets, $M\in{8,16,32,64,128}$, corresponding to sampling ratios from below 1% to 12.5%, the learned operator outperforms the best tested fixed transform by 9-15 percentage points at $M=8$ on all three datasets. Its eight-measurement accuracy requires roughly 16-32 measurements with the best fixed transform, indicating a $2\times$ to $4\times$ saving at comparable accuracy. The advantage generally shrinks as $M$ increases, with a small reversal on RIAWELC at higher budgets. These results show that learning what to measure is especially valuable when measurements are scarce.
Photonic pseudospins enable selective excitation and routing of light, but perturbations that mix opposite pseudospins can induce backscattering in helical edge channels. Here, we experimentally demonstrate pseudospin-locked chiral edge transport in gyromagnetic photonic crystals, combining pseudospin selectivity with nonreciprocal unidirectional propagation. By jointly engineering lattice deformation and magnetic bias, we control dipolar-quadrupolar band inversions and construct interfaces supporting pseudospin-selective chiral channels. Microwave near-field and transmission measurements reveal a single chiral edge branch that is selectively excited by only one source handedness and exhibits pronounced nonreciprocal transport. Extending this mechanism to multidomain structures, we combine two pseudospin channels into a common waveguide, separate them into distinct outputs, and route them through intersecting interfaces. These configurations realize a nonreciprocal photonic pseudospin multiplexer and demultiplexer, together with a waveguide crossing exhibiting average measured crosstalk of -14.1 and -15.8 dB for its two input channels. Our results present an experimental route to integrating pseudospin-selective channel control with chiral edge transport, providing a foundation for nonreciprocal photonic networks that use pseudospin to organize and route multiple wave channels.
Nanostructured semiconductors provide an actively tunable platform for light-matter interactions across a broad spectral range. As feature sizes shrink to the deep nanoscale (~10 nm scale), nonclassical carrier dynamics and surface plasmonic responses become prominent, creating substantial challenges for a rigorous and efficient theoretical treatment. Here we develop a generalized Lorentz framework for quantum surface plasmonics in nanostructured semiconductors with arbitrary energymomentum dispersion relations. The framework enables efficient quantitative treatment of carrier nonlocality, carrier surface accumulation and depletion, and nonlocal damping in three-dimensional semiconductor nanostructures, under electrical tuning and across a wide range of doping densities. Building on this framework, we further establish a rigorous quasinormal-mode analysis that provides transparent physical insight into complex surface plasmonic responses of multiscale semiconductor structures. We demonstrate electrically controlled deep-nanoscale carrier accumulation and depletion in a semiconductor nanoantenna, leading to pronounced plasmonic-response modulation that can be interpreted in terms of eigenmode shifts, creation, and annihilation. By quantifying and linking complex nonclassical carrier dynamics with plasmonic eigenmodes, our work establishes a comprehensive quantitative framework for active semiconductor quantum surface plasmonics and will foster the convergence of electronics and photonics at the deep nanoscale.
Standard analyses of quantum phase sensing often neglect the overhead of generating squeezed-light by treating the pump as an undepleted classical resource. Realistically, the pump is a quantum object that undergoes significant depletion and entangles with the produced squeezed light, raising the question of how the crossover from a classical to a quantum pump limits the quantum advantage in sensing. Contrary to prior analyses, we show that quantum advantage persists despite pump quantization, but depends critically on the nonlinear $(\chi^{(2)})$ source dynamics. In particular, we demonstrate that Heisenberg scaling emerges in the regime of strong pump depletion, rather than maximum signal squeezing. At maximum squeezing, the quantum Fisher information (QFI) for interferometric sensing of an unknown phase scales as $N^{3/2}$, where $N$ is the pump photon number, surpassing the shot-noise limit but establishing that squeezing itself is insufficient to attain Heisenberg scaling of $N^2$. At maximal pump depletion, where the signal flux is maximized, the QFI scales as $N^2$. In this regime, standard photon counting is suboptimal, while a joint nonlinear measurement of the entire pump-signal system is shown to achieve Heisenberg scaling. Our work shows that pump quantization changes both the optimal operating point and the measurement strategies required to preserve a realistic quantum advantage in phase estimation and, more broadly, many-body quantum dynamics.
Strong coupling between collective excitations provides a route to controlling quantum states of matter, yet its evolution under thermal fluctuations remains poorly understood. In the van der Waals antiferromagnet FePS3, strong magnon-phonon coupling is established at low temperatures. At the same time, recent terahertz experiments revealed the counterintuitive stabilization of a long-lived metastable magnetic state with an exceedingly long lifetime of 5 msec at the antiferromagnetic transition, not at the base temperature. This was attributed to a strongly coupled spin-lattice scenario, challenging the conventional wisdom that increased magnetic and thermal fluctuations near TN should be detrimental to the coherent control of a metastable phase. Thus it requires an independent examination for correct understanding. Using high-field Raman spectroscopy, we systematically investigate the temperature evolution of magnon-phonon coupling in FePS3. We observe a pronounced avoided crossing between magnon and phonon modes across a wide range of temperatures and find that the coupling strength remains remarkably robust with increasing temperatures. The zero-field magnon-phonon detuning decreases progressively with warming, driving the system toward resonance and thereby enhancing their hybridization near the transition temperature. This anomalous thermal evolution provides the crucial key to understanding the unusual terahertz-driven states recently reported in FePS3. More broadly, our results highlight van der Waals magnets as a promising platform for strong coupling among collective excitations and for controlling magnetic states through light-matter interactions.
The superposition principle at the heart of optics states that electromagnetic fields do not interact in vacuum. This principle is not universal, instead electromagnetic fields couple weakly via quantum vacuum fluctuations. Striking consequences are light-by-light scattering and vacuum birefringence. Experimental results from colliding X-ray free-electron and optical high-intensity lasers combined with dark-field detection and polarimetry for background suppression are reported. This approach yields the most stringent bound for light-by-light scattering in the low-energy regime achieved to date -- improving upon previous experimental limits by ten orders of magnitude -- and paves the way for precision measurements.
Atomic arrays provide a cooperative platform for light-mediated interactions in nonlocal metasurfaces which can operate in the quantum regime without nanofabricated optical elements or intrinsic material absorption. We present a minimal atomic implementation to harness wavefront engineering and create a near-ideal intrinsic atomic Huygens' surface. A simple bilayer square lattice with a two-atom unit cell, not limited to small lattice spacings, provides near-unity reflectionless transmission and a full $2\pi$ phase control of transmitted light without the localized site-dependent level shifts of previous proposals. This simplification is achieved through nonlocal radiative excitations that provide the familiar electric-dipole response with even radiation parity as well as an odd-parity radiation channel whose leading multipolar interpretation combines magnetic-dipole and electric-quadrupole contributions in atoms exhibiting only electric dipole transitions. This eliminates the need to simultaneously excite pure electric-dipole and magnetic-dipole responses with exactly matched radiation. We derive a two-mode model for this response and show that it accurately captures full coupled-dipole simulations near the optimal lattice spacings. Because of the wide applicability of coupled-dipole models, the relevance of our microscopic analysis extends beyond atomic physics to nanostructured metasurfaces. Moreover, the same platform also connects directly to established subradiance-protected single-photon storage protocols, suggesting a route toward combining wavefront engineering, storage, and retrieval in a simple atomic bilayer.
Topological insulators can host chiral, disorder-robust edge states that dissipate energy when coupled to an external bath. We show that nonlinear interactions enable all-optical control over edge-state dissipation in a laser-written Floquet topological insulator periodically coupled to a photonic bath. Although moderate nonlinearity enhances dissipation due to mode mixing, increasing the nonlinear strength to an optimal value dramatically suppresses dissipation by forming highly localized traveling edge solitons. These results establish engineered system-bath coupling and nonlinear interactions as a route to suppress dissipation of chiral topological edge transport.
Spectral bandwidth and frequency mismatch between quantum light emitters and quantum components is a common problem in the realization of hybrid quantum information networks. Here we propose a simultaneous solution to both of these problems by using a free-space cavity with an embedded nonlinear crystal that, through frequency conversion, facilitates perfect coupling into a cavity mode. The slow leakage of the cavity effectively stretches the photon pulse, resulting in a much longer photon pulse at a different wavelength. We show that the amount of stretching is strictly limited by the finesse and length of the cavity. Including realistic internal losses in the cavity, we show that it is possible to stretch a photon by at least two orders of magnitude with high efficiency. The proposed setup can thus constitute a valuable addition to the toolbox of quantum information technologies.
We investigate the synchronization of coupled photonic Bose-Einstein condensates in dye-filled microcavity arrays, subject to a potential gradient. We show that in the absence of noise the desynchronisation threshold is the result of a dynamical instability. For a two-site system, we analytically derive the maximal potential difference that allows for a synchronized solution as a function of the system parameters and find that it crucially depends on the thermalization rate. For systems with $N$ sites, we find a $N^{-3}$ power law scaling for the critical potential difference. When quantum fluctuations are included, stochastic phase slips broaden the sharp transitions, yet the phase velocity and first order spatial coherence still reliably identify the instability threshold.
We investigate the interaction of dissipative Kerr solitons in two nearly identical coherently driven, coupled microresonators in the presence of third-order dispersion (TOD). Using an asymptotic reduction of the coupled Lugiato--Lefever equations, we derive effective interaction equations governing the soliton separation and collective drift for weak intercavity coupling. TOD breaks the parity symmetry of the soliton profile and qualitatively modifies the effective interaction, leading to asymmetric interaction forces and multiple stable and unstable soliton clusters. The existence and stability of these clusters are controlled by the relative phase of the driving fields and the group-velocity mismatch between the resonators. In particular, an appropriate value of the group-velocity mismatch can give rise to multiple stable bound states, whereas deviations from this value trigger successive saddle-node bifurcations, ultimately leading to periodic soliton collisions. The analytical predictions are confirmed by numerical simulations of the full coupled system.
When a language model answers, we can usually only estimate why. We build a transformer in which every step is a Born measurement - attention $|\langle q|k\rangle|^4$ normalised over each row, a measured product in place of the feed-forward nonlinearity, and a Born readout. With its denominators held at their values for the input, each class probability is a polynomial in the word amplitudes, so its integrated-gradient evidence is computed by a finite quadrature with no quadrature error: the words' contributions sum to the class probability above its no-information share, to $10^{-11}$ or better. On e-SNLI natural language inference, with about 98k parameters outside the embedding, the model reaches $76.98 \pm 0.17$% test accuracy, level with a softmax transformer of the same size ($76.62 \pm 0.36$%). Deleting the word its evidence ranks first changes the answer in 45% of test pairs, out of the 53% any single deletion can change, against 32% for the softmax model explained the same way and 40% by its attention x gradient. Attention can look more like the words annotators highlighted, yet accounts for the answer less well: deleting the word it ranks first changes the Born Transformer's answer in 32% of pairs. The same weights compile exactly to optoelectronic optics and to qubits, and a coherent attention layer becomes one circuit equal to the model (852 CNOTs on 23 qubits). On IBM's ibm_kingston, reading only the attention overlaps by Hadamard tests, the device gave the model's answers on two short test pairs and kept or changed them under deletion as the model does; the evidence itself did not yet survive the device, and we work out the shots and calibration that measuring it on hardware would need.