The detection principle is deceptively simple. In the classic Kretschmann configuration, a prism couples p-polarized light into a thin metal film, and at a precise angle of incidence the light’s in-plane momentum matches that of surface plasmon waves, collective oscillations of electrons rippling along the metal-dielectric interface. When that resonance condition is met, energy drains from the reflected beam, producing a sharp dip in reflectance. Because the resonance angle depends exquisitely on the refractive index of whatever sits just above the metal, the presence of bacteria, which changes the local optical environment, shifts the dip in a measurable way. Even minute refractive index variations produce a noticeable displacement of the resonance minimum, enabling label-free, real-time detection without the fluorescent tags or enzymatic amplification that conventional assays require.

Traditional methods for spotting dangerous bacteria, including microbial culture, polymerase chain reaction, enzyme-linked immunosorbent assays, fluorescence-based techniques, and electrochemical approaches, are accurate but often demand complex instrumentation, skilled personnel, and lengthy sample preparation, which limits their usefulness for rapid, on-site testing. That bottleneck is why the search for optical biosensing platforms has intensified. Among optical techniques, SPR is regarded as one of the most sensitive for chemical and biological detection, and the new work aims to push that sensitivity further by engineering the stack of materials through which the evanescent electromagnetic field must pass before probing the sample.

The authors modeled their six-layer device with the transfer matrix method, an exact analytical technique for electromagnetic wave propagation through layered thin films, under angular interrogation at a wavelength of 633 nanometers. The stack begins with a BK7 prism of refractive index 1.515, chosen for its optical transparency and phase-matching capability, followed by a silver film whose complex refractive index of 0.056266 plus 4.27761i reflects the metal’s low optical loss and its knack for producing a sharp resonance curve. Above the silver sit two dielectric interlayers, one nanometer of TeO2 with a refractive index of 2.2596 and one nanometer of CdGa2S4 with a refractive index of 2.4482, and finally a 0.53-nanometer layer of AlSb with a complex refractive index of 3.8638 plus 0.0035982i, on which the sensing medium rests.

Each layer plays a distinct physical role. The tellurium dioxide both confines the electromagnetic field and shields the silver surface from oxidation, addressing a well-known weakness of silver-based plasmonic sensors, which otherwise outperform many alternatives in field confinement but degrade in air. The CdGa2S4 layer enhances plasmonic coupling and broadens the interface over which the evanescent field can interact with the analyte, lending stability to the resonance. The AlSb nanolayer, with its ultra-high surface area and strong light-matter interaction, increases the effective sensing area available for biomolecular adsorption. Together, the researchers argue, these materials create a synergistic improvement in plasmonic coupling and electromagnetic field localization that no single layer could deliver alone.

Getting the metal thickness right turned out to be critical. The team swept silver film thicknesses from 40 to 60 nanometers in 5-nanometer steps and tracked how the minimum reflectance and resonance angle responded in sensing media of refractive index 1.371 and 1.410. As the film thickened from 40 to 55 nanometers, the minimum reflectance plunged from 0.34442 to just 0.002464, a sign that nearly all the incident energy was being funneled into the surface plasmon waves. At 60 nanometers, the reflectance minimum climbed again because the thicker metal added optical losses and suppressed the evanescent field’s reach into the sample. A 55-nanometer film was therefore selected as the optimum, and it anchors all subsequent performance claims in the study.

With the metal fixed, the authors compared their full stack against four simpler configurations: a conventional BK7/Ag/analyte structure, versions with only AlSb, only TeO2, or only CdGa2S4 added, and the complete proposed design. The conventional structure managed a sensitivity of 185.620 degrees per RIU, while the partial additions lifted sensitivity to roughly 200 to 203.68 degrees per RIU. The full multilayer design reached 269.397 degrees per RIU, an improvement of approximately 45.1, 32.3, 34.4, and 32.3 percent over the four reference structures respectively, and it simultaneously posted the best quality factor at 181.04 per RIU, figure of merit at 180.6 per RIU, and combined sensitivity factor at 179.679 per RIU. In other words, the design improves sensitivity without sacrificing the resonance sharpness that determines real-world detection accuracy, a balance many competing sensors fail to strike.

The team also swapped the 2D layer among black phosphorus, graphene, molybdenum disulfide, and tungsten disulfide to see how each material changes the picture. Black phosphorus came closest to AlSb with a sensitivity of 261.179 degrees per RIU and a quality factor of 178.036 per RIU, while tungsten disulfide actually produced the largest angular shift, at 282.538 degrees per RIU, but its broader resonance curve dragged the quality factor down to 144.152 per RIU and the figure of merit to 134.875 per RIU, meaning the resonance profile becomes too damped for high-quality sensing. AlSb emerged as the best overall performer because it combines high sensitivity with a sharp, well-defined resonance dip, and the authors note that in a 2D-material comparison the abstract reports maximum attained parameters of 285.538 degrees per RIU sensitivity, 144.152 per RIU quality factor, and 134.875 per RIU figure of merit when WS2 configurations are included in the analysis.

Electric field calculations add a biological plausibility check. The evanescent field of the optimized design penetrates 448.43 nanometers into the sensing medium, decaying to roughly 36.8 percent of its maximum intensity at that depth, which is well matched to the scale at which biomolecules, proteins, pesticides, and bacterial cells interact with a functionalized surface. The dielectric interlayers proved to modulate the field confinement in a way that strengthens the interaction between the plasmonic field and the analyte while keeping optical losses in check, so that the slightest refractive index shift in the sample translates into a large deviation of the resonance condition.

The second half of the study is where the machine learning enters, and it may prove to be the paper’s most transferable contribution. The researchers generated an augmented dataset from their numerical simulations, covering silver thickness, refractive index, resonance angle, minimum reflectance, full width at half maximum, quality factor, figure of merit, and combined sensitivity factor, and trained a Random Forest regression model of 250 decision trees with a maximum depth of ten levels to predict sensor sensitivity. Trained on 80 percent of the data and tested on the remaining 20 percent, the model showed excellent agreement between predicted and target values, with residuals clustered symmetrically near zero and learning curves confirming the absence of significant overfitting. Feature importance analysis identified the silver layer thickness as the most influential parameter, with resonance angle and refractive index close behind, a result consistent with the direct electromagnetic optimization. When the trained model was then used to predict sensitivity across the 40-to-60-nanometer thickness range, it independently identified an optimum near 55 nanometers, precisely matching the transfer matrix analysis but at a fraction of the computational cost.

The authors are candid about the work’s limitations. Everything reported so far is numerical; no device has been fabricated or experimentally characterized, and the model does not yet include bacteria-specific surface functionalization or molecular recognition elements, so selective detection of a particular bacterial species would still require a suitable bioreceptor layer. Real biological samples bring nonspecific adsorption, background refractive index fluctuations, and matrix interference from salts, proteins, and cells, while temperature variations, fabrication tolerances, surface roughness, and material absorption could all degrade the idealized response. The team plans to pursue experimental fabrication, bacteria-directed surface chemistry, and validation with real-world bio samples. If those steps succeed, the combination of a carefully engineered TeO2/CdGa2S4/AlSb plasmonic stack with a data-driven optimization loop could offer a template for a new generation of fast, label-free pathogen detectors for clinics, food safety labs, and environmental monitoring, and the Random Forest framework may let future designers find the best sensor geometry in hours of computation rather than weeks of simulation.

Subject of Research: Machine learning-optimized surface plasmon resonance refractive index sensor with TeO2/CdGa2S4/AlSb layers for pathogenic bacteria detection

Article Title: Pathogenic bacteria sensing with tellurium dioxide/cadmium gallium sulfide-assisted surface plasmonic resonance refractive index sensor: a machine learning approach

Article References: Merigala, R., Boddu, S., Vasimalla, Y., Prashanth, M. S., Chebaane, S., Alsaif, N. A. M., Khalifa, S. B., & Karki, B. (2026). Pathogenic bacteria sensing with tellurium dioxide/cadmium gallium sulfide-assisted surface plasmonic resonance refractive index sensor: a machine learning approach. Frontiers in Sensors, 7, Article 1962401. https://doi.org/10.3389/fsens.2026.1962401

Image Credits: AI Generated

DOI: 10.3389/fsens.2026.1962401

Keywords: surface plasmon resonance, pathogenic bacteria, biosensor, machine learning, Random Forest, tellurium dioxide, cadmium gallium sulfide, aluminium antimonide, refractive index sensing, Kretschmann configuration, 2D materials, nanophotonics

The heart of the platform is a periodic array of Y-shaped gold nanocavities, each unit cell consisting of a symmetric cavity with three identical arms arranged at 120 degrees. This geometry was chosen deliberately. Compared with conventional nanohole arrays, Y-shaped cavities concentrate electromagnetic energy at their sharp inner vertices, producing intense localized hotspots where the Raman scattering cross-section of nearby molecules is dramatically amplified. The researchers fabricated the arrays on indium tin oxide coated BK7 glass using electron-beam lithography, spinning a thin ZEP520A resist film, exposing the Y-shaped pattern in a hexagonal lattice with periodicities of 350 and 347.3 nanometers, and then depositing a 50-nanometer-thick gold layer by electron-beam evaporation. Scanning electron microscopy confirmed that the finished structures faithfully reproduced the designed geometry with uniform orientation and periodicity across a 150 by 150 micrometer sensing area.

Optical characterization revealed that the metasurface supports plasmonic resonances near 670 nanometers, arising from coupling between localized surface plasmon modes and propagating plasmon modes within the periodic array. Numerical simulations based on Rigorous Coupled-Wave Analysis predicted two resonances, at approximately 670 and 742 nanometers, in good agreement with the experimental transmission spectra. The small discrepancies are attributable to nanofabrication tolerances, particularly slight rounding of the Y-shaped cavity vertices during resist development, which modifies the effective plasmonic mode volume, and to minor differences in the refractive index of the polymeric adhesion layer. Importantly, although the dominant extinction maximum sits near 670 nanometers, the optical response is broad and multimodal, extending into the near-infrared where it overlaps substantially with the 785-nanometer excitation laser used for SERS. The authors emphasize that maximum far-field extinction does not necessarily coincide with maximum local field enhancement, so efficient Raman amplification can still be achieved through these broader near-infrared modes. The 785-nanometer choice also suppresses fluorescence background, a common nuisance in biological samples, improving signal-to-noise during viral measurements.

To quantify the enhancement mechanism, the team ran finite-difference time-domain simulations of the local electric field at 785 nanometers and estimated the electromagnetic SERS enhancement factor using the standard fourth-power approximation, which accounts for simultaneous amplification of both the excitation field and the Raman-scattered emission. The calculated maximum enhancement was approximately 6.4 × 10³, concentrated at the sharp inner vertices of the Y-shaped cavities. Two physical mechanisms drive this amplification: the lightning rod effect, in which high curvature at the arm tips accumulates charge and boosts the local field, and inter-cavity plasmonic coupling, in which neighboring cavities in the periodic lattice further intensify the field distribution. While higher enhancement factors have been reported elsewhere, including values around 1.3 × 10⁶ for pyramidal nanohole metasurfaces previously developed by the same group, the modest figure here conceals a crucial design advantage: the hotspots are distributed uniformly across the entire metasurface. That homogeneity dramatically reduces spectral variability from point to point, which the authors identify as the key to the tight spectral clustering observed in their statistical analysis.

For the sensing experiments, the team used non-infectious, inactivated samples of human norovirus (HNoV) and murine norovirus (MNV-1), both members of the Caliciviridae family of single-stranded RNA viruses, supplied by the Istituto Zooprofilattico Sperimentale del Mezzogiorno. No viral propagation or handling of infectious material took place during the study. Droplets of virus solution at a concentration of 10⁵ tissue culture infectious dose 50 percent per milliliter were deposited directly onto the plasmonic sensors, incubated for one hour in a humid chamber, rinsed seven times with bidistilled water, and dried under nitrogen before spectral acquisition. SERS measurements employed a custom-built automated Raman platform with a 785-nanometer excitation laser, a motorized precision stage for spatial mapping, and a high-sensitivity spectrometer covering 200 to 2,500 inverse centimeters, with 20 milliwatts of laser power and 10-second integration times per spectrum.

The experimental design was deliberately rigorous. Four independently fabricated metasurfaces were used, two dedicated to human norovirus and two to murine norovirus, with 25 spectra acquired from different spatial locations on each sensor, yielding 100 spectra in total. Measurements were performed in separate sessions by different operators, allowing the team to probe substrate-to-substrate, spatial, temporal, and operator-dependent reproducibility all at once. The resulting spectra showed high consistency and well-defined vibrational features. Both viruses produced fingerprints containing bands associated with disulfide bonds, aromatic amino acids such as phenylalanine, tryptophan, and tyrosine, protein backbone amide modes, and nucleic-acid-related structures, consistent with their close taxonomic relationship. The authors caution that because multiple biomolecular components can contribute to the same spectral region, individual band assignments should be regarded as putative rather than definitive molecular identifications.

Despite the broad similarity, distinctive differences emerged. The human norovirus spectra displayed features that were absent or weaker in the murine dataset, such as a disulfide stretching band near 524 inverse centimeters, an adenine band near 730, and a valine contribution near 970, while the murine spectra showed their own characteristic peaks, including phenylalanine at 605, adenine at 746, histidine at 779, tyrosine at 841, and an amide III band near 1,312 associated with alpha-helical structure. These differences likely reflect subtle variations in protein composition, capsid organization, amino-acid exposure, and nucleic-acid vibrational contributions. The researchers are careful not to over-interpret: given the overlapping nature of Raman signatures in complex biological systems, no single band can be treated as a unique biomarker. What matters is that the reproducible differences carry enough information for multivariate statistics to exploit.

That statistics came in the form of principal component analysis, applied in the R environment after baseline correction and normalization of all spectra. The two-dimensional score plot built from the first two principal components revealed two cleanly separated clusters, one for each virus, with tight grouping within each cluster indicating high spectral reproducibility. Quantitatively, the first principal component accounted for 87.1 percent of the total spectral variance and the second for an additional 11.6 percent, meaning that more than 98 percent of the captured variance was explained by just two dimensions. Analysis of the PCA loading vectors showed that the discrimination arises from distributed contributions across multiple Raman regions rather than from any single dominant feature. Crucially, the clustering pattern persisted across independently fabricated sensors, different spatial positions, separate sessions, and different operators, indicating that the separation is driven by intrinsic spectral differences between the viral capsids rather than by measurement artifacts or substrate variability.

The authors position the work as a proof of concept rather than a fully optimized diagnostic assay, noting that the study was not designed to determine an analytical limit of detection. Even so, the tested concentration of 10⁵ TCID₅₀ per milliliter falls within the range commonly associated with active norovirus infections, making it a clinically relevant benchmark. What sets the platform apart from most previously reported SERS-based viral sensing strategies is its complete freedom from antibodies, aptamers, Raman reporter tags, molecular amplification, or any target-specific functionalization. Earlier approaches, such as a dual SERS nanotag immunoassay for norovirus built on molybdenum trioxide nanocubes, depended heavily on the specificity of antigen-antibody sandwich complexes, with all the reagent costs and preparation complexity that entails. The Y-shaped metasurface instead reads the virus’s own biochemical identity directly.

The research group, which has previously applied similar PCA-assisted plasmonic platforms to discriminate Shiga toxin variants and to detect rotavirus in wastewater with supervised learning, sees a clear path forward. Future studies will evaluate sensor performance in clinically and environmentally relevant matrices, assess interference effects, expand the spectral datasets, and implement supervised machine-learning classifiers such as linear discriminant analysis, support vector machines, and random forests, which the team has already shown can boost classification performance in related SERS applications. If those steps succeed, a chip of gold Y-shaped cavities, a near-infrared laser, and a statistical model could one day offer rapid, label-free identification of closely related viral pathogens in clinics, food safety laboratories, and environmental monitoring networks, no molecular labels required.

Subject of Research: Label-free discrimination of human and murine noroviruses using plasmonic metasurface-enhanced Raman spectroscopy

Article Title: Plasmonic metasurface-enhanced Raman spectroscopy for label-free discrimination of human and murine noroviruses

Article References: Guilcapi, B., Milano, A., D’Avino, A., Sagnelli, D., Rippa, M., Marchesano, V., Fusco, G., & Petti, L. (2026). Plasmonic metasurface-enhanced Raman spectroscopy for label-free discrimination of human and murine noroviruses. Frontiers in Photonics, 7, Article 1864447. https://doi.org/10.3389/fphot.2026.1864447

Image Credits: AI Generated

DOI: 10.3389/fphot.2026.1864447

Keywords: norovirus, SERS, plasmonic metasurface, label-free biosensing, Raman spectroscopy, principal component analysis, nanocavities, viral diagnostics, nanophotonics, Caliciviridae, hotspots, biosensor

The story of optical forces begins more than a century ago, with the pioneering measurements of radiation pressure by Pyotr Lebedev and by Ernest Nichols and Gordon Hull in 1901. The modern era, however, is usually dated to Arthur Ashkin’s work in the 1970s and 1980s, which showed that focused laser beams could trap and steer microscopic dielectric and metallic particles. That line of research produced optical tweezers, tools now ubiquitous in biology and chemistry, and earned a share of the 2018 Nobel Prize in Physics. In the classical picture, light exerts two kinds of forces: a gradient force that draws particles toward regions of high intensity, and a scattering force that pushes them along the beam’s propagation direction through the transfer of photon momentum. Under almost all ordinary circumstances, the net result is pushing.

Over the past two decades, researchers have discovered that the nanoscale world harbors exceptions. Chirality, plasmonic resonances, optical gain, metamaterial architectures, and excitonic effects can all conspire to produce negative optical forces, in which an object is drawn toward the light source rather than pushed away from it. These tractor-beam effects have been demonstrated or predicted for Janus particles, plasmonic nanorods, and engineered metamaterial structures. What distinguishes the new work is the physical mechanism. The pulling force in a carbon nanotube does not come from clever geometry or exotic materials design; it emerges from the spatial dispersion of the nanotube’s conductivity, the fact that the current induced at any given point along the tube depends on the electric field in its neighborhood, not just at that point.

To understand why non-locality matters, it helps to recall how electrons behave in a carbon nanotube. A single-wall nanotube is essentially a rolled-up sheet of graphene, and its low-energy electronic states mimic those of graphene: the energy depends linearly on momentum, with the Fermi velocity playing the role of the speed of light in an effective relativistic description. In graphene, electrons roam freely across a two-dimensional plane. In a nanotube, however, the transverse direction is wrapped up and quantized, so the electron momentum is discrete around the circumference and continuous only along the tube axis. The authors adapted the Kubo formalism of linear response theory, treating the electrons as a pseudo-spin liquid in the collision-free, low-temperature regime, to compute how the axial conductivity depends on both frequency and wavevector. This wavevector dependence is precisely what local models discard.

Mathematically, the non-local conductivity can be expanded in powers of the wavevector, and symmetry ensures that the first correction term vanishes, leaving a second-order contribution. Transforming back to real space, this term becomes a second spatial derivative of the electric field multiplied by a complex non-locality factor. The physical meaning is direct: the current at a point on the tube is the sum of a local Ohmic response and a non-local component that senses the field’s curvature along the axis. The non-locality factor obeys Kramers-Kronig relations and splits into interband and intraband parts, just as the conductivity itself does. The interband component dominates at high frequencies, while the Drude-like intraband component governs the low-frequency response, and the crossover between them sits near the energy where the photon energy equals twice the electrochemical potential.

The geometry of the problem turns out to be just as important as the electronic physics. Most previous treatments modeled nanotubes as infinitely long, one-dimensional objects, which conveniently sidesteps what happens at the ends. Berghaus and colleagues insisted on a finite-length tube, and that choice has profound consequences. Electrons traveling along the tube reflect from the open ends, and these reflections are captured through so-called additional boundary conditions that supplement Maxwell’s equations. The induced current near the terminations behaves like an effective multipole, absent in the local limit, and it is these end-induced multipoles that ultimately reverse the direction of the optical force. The non-locality of the conductivity, in other words, manifests itself entirely through edge effects.

Computing the force itself required careful handling of the Maxwell stress tensor, the mathematical object whose integral over a closed surface yields the total electromagnetic force on an object. For a slender nanotube, the axial force is determined by the off-diagonal components of the tensor, which couple the radial and longitudinal field components. Here the authors encountered a striking result: a purely local conductivity model produces non-integrable singularities in the diagonal tensor elements, rendering the force calculation mathematically ill-defined. Including spatial dispersion removes these singularities and makes the integral convergent. Non-locality, in this sense, is not optional refinement but a prerequisite for a physically consistent theory of optomechanical forces in nanotubes.

To obtain quantitative predictions, the team formulated a Fredholm integral equation of the second kind for the axial surface current, a framework long used in the electrodynamics of nanotube antennas. The kernel of this equation contains a weak singularity, and naive numerical treatment produces unphysical oscillations in the current density near the tube ends, a pathology familiar from classical microwave antenna theory. The authors adapted renormalization techniques from that literature and regularized the singular part analytically using properties of Hankel functions. They then solved the equation numerically in MATLAB and, in parallel, derived an approximate analytical solution from the zeroth-order term of a Neumann series. The two approaches agreed closely, lending confidence to the central prediction: a narrow, sharp peak of negative force appears near the critical frequency where the photon energy matches twice the electrochemical potential of the nanotube’s electrons.

The parameter dependence of the effect is rich and experimentally testable. Pulling occurs for relatively short tubes, roughly 100 to 200 nanometers long, and only in the frequency regime where intraband transitions dominate. As the tube lengthens, the relative importance of the end effects diminishes, the force turns positive, and it settles into a linear dependence on length, exactly the behavior of the classical Ashkin-like pushing regime. The force also depends strongly on the angle of incidence and on the electrochemical potential, which suggests practical applications: measuring the doping level of a nanotube optically, sorting nanotubes by length or radius with light, and developing nanotube-based mechanical sensors. The calculated force magnitudes, in the tens of femtonewtons for incident powers of 50 to 100 milliwatts, fall squarely within the range already measured in optical trapping experiments on nanotubes, so the predicted pulling should be within reach of existing instrumentation.

The theoretical framework is deliberately simple in its baseline configuration, a single achiral, open-ended, metallic nanotube suspended in vacuum and illuminated by a plane wave, but the authors outline natural extensions to tubes in fluids, in waveguides or optical cavities, and in the presence of neighboring structures, all of which could enhance the force. Structured light such as Gaussian or Bessel beams offers another route to strengthening the pull. Limitations remain, including the restriction to achiral tubes and the simplified treatment of nonlinear saturation effects, which the authors show are negligible at the field strengths considered since the saturation intensity of metallic nanotube films exceeds the simulation intensity by nearly two orders of magnitude. Still, the message of the work is clear and likely to resonate across nanophotonics: at the nanoscale, the response of matter to light is fundamentally non-local, and ignoring that fact can make the difference between a beam that pushes and one that pulls. The study’s data and code have been deposited publicly on Zenodo, and the authors explicitly welcome experimental validation of the newly predicted tractor-beam effect.

Subject of Research: Optical pulling forces on carbon nanotubes arising from non-local conductivity

Article Title: Optical pulling force in carbon nanotubes: Manifestation of non-local conductivity

Article References: Berghaus, T., Miloh, T., & Slepyan, G. Y. (2026). Optical pulling force in carbon nanotubes: Manifestation of non-local conductivity. iScience, 29(11), Article 117817. https://doi.org/10.1016/j.isci.2026.117817

Image Credits: AI Generated

DOI: 10.1016/j.isci.2026.117817

Keywords: carbon nanotubes, optical pulling force, non-local conductivity, spatial dispersion, optical tweezers, Maxwell stress tensor, Kubo formalism, tractor beam, nanophotonics, Landau damping, integral equation, edge effects

The central challenge the researchers set out to solve is a familiar one in nanophotonics. Two-dimensional semiconductors such as WSe2 host tightly bound electron-hole pairs called excitons, which possess large oscillator strengths and make these materials attractive for photodetectors, light emitters, valleytronics, and quantum technologies. Yet because the material is only one atom thick, its overlap with free-space light is vanishingly small. Conventional dielectric resonators, including photonic crystals, microring resonators, and distributed Bragg reflector cavities, confine light only to diffraction-limited volumes, which limits how strongly the cavity field can interact with the monolayer. Plasmonic structures can squeeze light into nanoscale gaps, but the ohmic losses in metals reabsorb photons and degrade the coherence that quantum applications demand.

The Purdue-Harvard collaboration turned to photonic inverse design, specifically adjoint-based topology optimization, to escape this trade-off. In this approach, a computer algorithm iteratively reshapes the distribution of silicon and air within a metasurface unit cell to maximize the electric-field intensity at a target point on the surface. Each optimization cycle consists of a forward electromagnetic simulation, an adjoint simulation driven by a virtual dipole at the target location, and a gradient-based update of a continuous material-density field. Gaussian filtering and a progressively sharpened Heaviside projection push the design toward binary, manufacturable patterns, with roughly 200 to 250 iterations required before the figure of merit saturates. Strikingly, regardless of the random starting point, the algorithm consistently converged on a bowtie-like gap at the center of each unit cell, suggesting that this geometry represents a global optimum for field concentration.

The resulting cavities were fabricated from 100 to 200 nanometer-thick silicon layers on sapphire substrates using electron beam lithography and reactive ion etching. Simulations of the ideal designs predicted local field-intensity enhancements exceeding 50-fold relative to the incident field, with an effective mode volume of approximately 30 by 30 by 100 cubic nanometers, corresponding to roughly one 2500th of a cubic wavelength. The optical hotspots sit directly beneath where the WSe2 monolayer would rest, ensuring deterministic coupling between the excitons and the confined cavity mode. Because the cavities are supported on a substrate rather than suspended as membranes, they are mechanically robust and well suited to the scalable integration of van der Waals materials.

Experimentally, the team transferred large hexagonal boron nitride-encapsulated WSe2 monolayers, roughly 40 by 30 micrometers in area, onto arrays of these cavities using a gold-assisted exfoliation and transfer technique. Each flake was positioned so that part of it lay on the cavity array and part on adjacent unpatterned silicon, enabling a direct on-chip comparison between cavity-coupled and reference emission. Raman spectroscopy confirmed that strain induced by the underlying pattern was negligible, with peak shifts below one wavenumber. Photoluminescence maps then revealed bright, reproducible emission from the cavity regions: raw enhancements of approximately 6-fold, 14-fold, and, for the best-performing array, nearly 28-fold were measured across three different cavity designs, with a consistent average of about tenfold. When normalized to the tiny effective hotspot area, this corresponds to a per-area enhancement on the order of one thousand.

Numerical simulations that incorporated realistic material absorption and the actual shapes extracted from scanning electron microscope images helped explain why the measured enhancement fell short of the ideal 50-fold prediction. The small feature sizes and dense patterning amplify proximity effects during electron beam lithography, and etching introduces roughness that scatters light and degrades field localization. Loading the cavity with the WSe2 and boron nitride stack further broadens and shifts the resonance. When all of these imperfections were included, the simulated near-field enhancement dropped to approximately tenfold, in close agreement with the experimental photoluminescence results and the observed lifetime shortening. The simulations also showed that emitters placed in the hotspot radiate highly directional light, with much of the power falling within the collection cone of a standard high-numerical-aperture microscope objective, whereas emitters on flat silicon radiate mostly at grazing angles.

Perhaps the most intriguing findings came from time-resolved photoluminescence measurements. Using femtosecond pulsed excitation, the researchers found that exciton decay on the cavities became strongly non-exponential at all pump powers, while decay on the flat reference regions remained nearly exponential except at the highest powers. Fitting the decay curves with a mixed model combining a stretched exponential and a single exponential revealed a stretching parameter of roughly 0.38 on the cavity, very close to the value of one third expected for two-dimensional dipole-dipole energy transfer between excitons. Off the cavity, the stretching parameter started near 0.8 at low power and decreased as excitation increased. The team also measured an approximately eightfold shortening of the exciton lifetime on the cavity, consistent with Purcell-enhanced radiative decay.

The power dependence of the decay dynamics pointed to cavity-mediated exciton-exciton interactions as a dominant mechanism. A biexcitonic recombination model best described the shortening of the effective decay time with pump power, and the extracted biexcitonic recombination coefficient on the cavity was more than an order of magnitude, about 33 times, larger than off the cavity. By concentrating optical near fields into nanoscale hotspots, the cavities increase the local exciton density and strengthen resonant dipole-dipole interactions, effectively amplifying many-body excitonic phenomena that are normally difficult to access. Saturation measurements reinforced this picture: emission on the cavity saturated at 20 microwatts of pump power, five times lower than the 100 microwatts required off the cavity, indicating substantially enhanced excitonic nonlinearities.

Low-temperature spectroscopy added further richness. Between 5 and 200 kelvin, the cavity-coupled monolayers displayed a well-resolved manifold of excitonic states, including bright excitons, trions, biexcitons, and defect-bound excitons, with noticeably narrower linewidths on the cavity than on the flat substrate. The presence of trions, which normally require deliberate gate-controlled doping, revealed unintentional electron doping introduced during the gold-assisted transfer, at densities estimated between ten to the eleventh and ten to the twelfth per square centimeter. Notably, defect-bound exciton emission, which persisted even to room temperature, exhibited unexpectedly short lifetimes on the cavity, roughly half those of the bright excitons, and about twice shorter on the cavity than off it at every detection wavelength.

The researchers emphasize that the reported raw enhancement values are the most physically meaningful metric for practical applications, even though normalized figures are conventionally used to compare across studies. Compared against previously reported plasmonic enhancements of WSe2 emission, the dielectric platform is competitive while preserving coherence and enabling efficient light extraction. The work establishes topology-optimized silicon-on-sapphire cavities as a versatile, CMOS-compatible alternative to plasmonics for controlling exciton-photon and exciton-exciton interactions, and the authors anticipate that the platform will accelerate the integration of van der Waals semiconductors into scalable quantum photonics, opening routes toward cavity-enabled quantum many-body physics, valley-selective emission, and unprecedented excitonic nonlinearities, with a detailed study of enhanced two-photon nonlinearities planned as a follow-up.

Subject of Research: Topology-optimized dielectric nanocavities enhancing excitonic light emission and exciton-exciton interactions in monolayer WSe2

Article Title: Topology-optimized dielectric cavities for enhanced excitonic light emission from WSe2

Article References: Matthiessen, O., Triplett, B., Yesilyurt, O., Cassara, D., Pagadala, K., Yang, M. M., Llacsahuanga Allcca, A. E., Ather, H., Fruhling, C., Solanki, A. B., Chen, Y. P., Alaeian, H., Kildishev, A. V., Shalaev, V. M., Capasso, F., Boltasseva, A., & Mkhitaryan, V. (2026). Topology-optimized dielectric cavities for enhanced excitonic light emission from WSe2. Light: Science & Applications, 15(1), Article 387. https://doi.org/10.1038/s41377-026-02436-w

Image Credits: AI Generated

DOI: 10.1038/s41377-026-02436-w

Keywords: WSe2, topology optimization, dielectric cavities, nanophotonics, excitons, photoluminescence, Purcell enhancement, two-dimensional semiconductors, inverse design, quantum photonics, silicon photonics, light-matter coupling

The core idea builds on nearly two decades of transformation optics and plasmonic research. In 2006, John Pendry and colleagues showed that carefully engineered materials could steer electromagnetic fields around a region of space, rendering it invisible; a year later, metamaterial cloaks demonstrated the principle experimentally at microwave and optical frequencies. In parallel, Andrea Alù and Nader Engheta demonstrated that plasmonic coatings—thin shells of materials whose electrons oscillate collectively in response to light—could achieve scattering cancellation, making coated objects transparent in selected bands. The new work fuses these two strands: metamaterial-based plasmonic layers are integrated directly with photonic waveguides, the conduits through which quantum information flows, so that the cloak and the communication channel form a single engineered structure rather than an add-on shield.

Technically, the hybrid architecture exploits the complementary physics of its two components. Plasmonic nanostructures confine light to subwavelength volumes, producing extremely intense, tunable resonances whose spectral position can be adjusted through geometry and material choice. This gives the cloak its adaptive shielding capability: at frequencies where an adversary might inject probing light—such as the Trojan-horse attacks in which an eavesdropper sends light into a quantum device and analyzes the returning reflection—the plasmonic layers present a resonance that cancels the scattered field. Photonic waveguides, meanwhile, provide the low-loss, phase-coherent pathways that quantum signals require. By combining plasmonic resonance with photonic interference, the structure can suppress scattering in targeted probing bands while allowing entangled photons to propagate with their quantum coherence preserved.

The quantitative results reported in the study are striking. Analytical modeling and numerical simulations show that the hybrid cloak achieves attenuation exceeding 40 decibels in the targeted probing bands—a reduction in leaked or back-scattered probe light by a factor of more than ten thousand. Crucially for quantum applications, the cloak maintains entanglement visibility above 90 percent, meaning that the correlations between entangled photon pairs survive the shielding with enough fidelity to support quantum key distribution and entanglement-based protocols. This dual performance is the central challenge of the field: a cloak strong enough to hide hardware from an adversary is usually lossy or dispersive enough to destroy the quantum states it is meant to protect, and the hybrid design claims to thread that needle.

The motivation for physical-layer protection comes from a well-documented gap between idealized quantum cryptography and its practical implementations. Theoretical protocols such as BB84 and device-independent schemes derive their security from the no-cloning theorem and the disturbance that measurement inflicts on quantum states, but real devices deviate from their mathematical models. Detectors, modulators, and optical interfaces can behave differently when illuminated by bright injected light, and imperfections create side channels through which information leaks without disturbing the encoded quantum states. A series of landmark papers by Gisin, Scarani, and their colleagues catalogued the security of practical quantum key distribution, and subsequent experimental work demonstrated attacks that exploit precisely these hardware vulnerabilities. Cryptographic patches address some of these weaknesses, but the new study argues that the hardware itself should carry part of the defensive burden.

This is where the cloaking framework departs from conventional countermeasures. Rather than detecting an attack after the fact or randomizing operation parameters to limit what an eavesdropper can learn, the hybrid cloak acts at the level of electromagnetic scattering: it changes what the hardware looks like to incoming light. A probing pulse that strikes the cloaked device encounters a structure whose plasmonic response is engineered to cancel the scattered signature, so the interrogator receives, in principle, nothing useful back. Because the resonance properties of metamaterial layers are tunable, the shield can in principle be reconfigured to cover different probing bands as threats evolve, offering a scalable and reconfigurable defense strategy rather than a fixed one.

The authors position this work within a broader research program on physical-layer quantum security. Three companion papers by Hosseinnezhad and Sabri, also published in 2026, explore phase modulation as a quantum anti-eavesdropping strategy, hybrid cryptosystems that combine entanglement-assisted decryption with physical phase obfuscation, and a trust-free quantum internet architecture built on distributed entanglement and reconfigurable shields. A fourth study in Results Optics examined adaptive, artificial-intelligence-driven metamaterial cloaking against Trojan-horse probing in photonic quantum channels. Taken together, these works sketch a multi-layered security philosophy in which protocol-level cryptography is complemented by engineered materials that actively shape the electromagnetic environment of quantum devices.

The proposed framework is notable for its breadth of applicability. According to the study, the hybrid cloak concept extends across fiber-based, free-space, and satellite communication systems, the three physical settings in which next-generation quantum networks are expected to operate. Fiber networks, which already carry metropolitan quantum key distribution trials, would benefit from integrated cloak-waveguide structures fabricated on photonic chips. Free-space and satellite links, which underpin long-distance entanglement distribution experiments, face different probing geometries and atmospheric conditions, but the tunable resonance of plasmonic layers offers a route to adaptive shielding in those regimes as well. The study draws on a rich literature in quantum plasmonics—including demonstrations of single-photon transistors based on nanoscale surface plasmons and the generation of single optical plasmons in metallic nanowires coupled to quantum dots—which establishes that quantum states of light can interact with plasmonic structures without immediate decoherence.

Significant engineering hurdles remain between simulation and deployment. Plasmonic structures are inherently lossy, since the electron oscillations that produce extreme field confinement also dissipate energy as heat, and the study’s field has long pursued low-loss plasmonic metamaterials to mitigate this. Any cloak inserted into a quantum channel must add negligible loss and dispersion to the single-photon signals, and the 90 percent entanglement visibility reported in simulation will need to be reproduced in fabricated devices operating at real network link budgets. Fabrication tolerances, thermal stability, and the practical bandwidth of the tunable resonances are all open questions that the analytical models and numerical simulations presented here do not fully resolve. The authors describe their results as establishing a foundation for multi-layered quantum security architectures, a framing that acknowledges the work’s status as a design framework rather than a field-tested device.

Even so, the study arrives at a moment when the quantum communication community is actively confronting the reality that physical hardware, not just mathematical protocols, determines security. National quantum network deployments, satellite entanglement experiments, and integrated photonic quantum processors are moving from laboratories into infrastructure, and every new interface between quantum light and the classical world is a potential aperture for attack. A defense strategy that renders quantum hardware electromagnetically invisible to interrogation—while the entangled photons inside continue their work undisturbed—offers an appealing complement to the cryptographic stack. If subsequent experimental work can reproduce the simulated attenuation and entanglement preservation in practical devices, hybrid plasmonic-photonic cloaking could become a standard layer in the defense-in-depth architecture of future quantum networks, joining quantum error correction, decoy states, and device-independent protocols in the ongoing effort to make quantum communication secure not only in theory but in the messy electromagnetic world of real hardware.

Subject of Research: Hybrid plasmonic-photonic cloaking to protect quantum communication channels from side-channel leakage and electromagnetic probing

Article Title: Hybrid plasmonic-photonic cloaking for secure quantum communication

Article References: Afshari-Bavil, M., Hosseinnezhad, A., & Sabri, H. (2026). Hybrid plasmonic-photonic cloaking for secure quantum communication. Quantum Information Processing, 25(10), Article 343. https://doi.org/10.1007/s11128-026-05365-0

Image Credits: AI Generated

DOI: 10.1007/s11128-026-05365-0

Keywords: quantum communication, quantum cryptography, plasmonic cloaking, metamaterials, photonic waveguides, entanglement preservation, side-channel attacks, Trojan-horse attack, quantum key distribution, nanophotonics, surface plasmon resonance, quantum security

The team, led by Andrea Alù, director of the Photonics Initiative at the CUNY ASRC and Distinguished Professor of Physics at the CUNY Graduate Center, calls these self-organizing structures hyperbolic wave attractors. The name is deliberate and evocative: like the attractors familiar from chaos theory, these are paths toward which the wave dynamics are drawn and on which they settle, but here the organizing principle comes from an unusual interplay between geometry and material response rather than from nonlinearity or external driving. According to the researchers, the discovery demonstrates how geometry and material properties can work together to produce wave behavior that is both surprising and useful, opening a route to controlling energy, information, and communication signals in environments where waves would normally be expected to scatter chaotically.

To appreciate why the result is striking, it helps to start with how waves ordinarily behave. In everyday materials, a wave striking a surface reflects in the familiar way taught in introductory physics: the angle at which it arrives matches the angle at which it leaves. In an irregularly shaped room, that simple rule applied over and over again is precisely what generates complexity. A ray of light entering a stadium-shaped cavity, for instance, will visit wildly different regions on successive bounces, and two rays that begin almost identically will diverge exponentially fast. This sensitivity to initial conditions is the hallmark of chaos in the dynamical billiard problem, and for ordinary reflecting walls it has long been regarded as the generic outcome.

Hyperbolic materials rewrite those rules. They are named after the hyperbola because of the peculiar shape they force light waves to take: instead of spreading freely in all directions, waves in these media are confined to narrow, highly defined directions determined by the material’s anisotropic structure. The consequence for reflection is profound. When a wave traveling in such a medium encounters a tilted wall, its outgoing direction can differ dramatically from what ordinary reflection would predict. The reflection is no longer a simple mirror operation; it is a transformation governed by the hyperbolic dispersion of the medium itself. Alù’s team set out to understand what happens when these unusual reflection rules are combined with an irregularly shaped cavity, a question that sits at the intersection of wave physics, geometry, and materials science.

The answer, reported in the study, is the opposite of what intuition suggests. Simon Yves, a postdoctoral researcher in Alù’s lab and a first author of the paper, explained that while a complicated cavity is normally expected to produce complicated, chaotic wave patterns, in this system the unusual propagation and reflection of waves instead create a strong geometric organization, producing well-defined paths that persist across a broad range of wavelengths. In other words, the very ingredients that should amplify disorder conspire to suppress it. The waves inside the cavity progressively organize into closed trajectories, and these paths are stable and scale-invariant, meaning that the underlying geometric structure repeats itself across different spatial scales. That scale invariance is a remarkable property for a wave system, suggesting a deep connection between the reflection dynamics and the mathematical structure of the hyperbolic dispersion surface.

The mechanism behind the organization involves a subtle symmetry breaking. As waves bounce around the cavity, the reflection process breaks mirror symmetry, and with each encounter the wavelengths progressively shrink. This continuous contraction of wavelength helps create the conditions under which the attractors form, drawing the wave energy onto closed loops rather than letting it wander through the chaotic sea of possible paths. The resulting patterns also exhibit a property known as handedness: the organized trajectory inside the cavity rotates either clockwise or counterclockwise, and the attractors’ handedness and stability are both tied to the unusual geometry of wave propagation in the hyperbolic material. Handedness of this kind is a valuable resource in photonics, where chiral wave states underpin technologies ranging from circularly polarized light sources to spin-controlled optical devices.

Demonstrating the effect required an experimental platform in which the team could engineer and observe the wave dynamics directly. The researchers built their demonstration using a mechanical metamaterial, a human-made structure designed to control how waves move through it, and excited vibrations within an oddly shaped cavity carved into that structure. Mechanical metamaterials are an ideal testbed for this kind of physics because their geometry can be fabricated with precision and their vibrational modes can be mapped in detail. Enrico Renzi, a doctoral student in Alù’s lab and co-first author of the study, noted that the most exciting aspect of the results is that they connect a simple geometric idea with a rich set of wave phenomena: the waves can be observed organizing themselves, and that organization can be linked to concrete properties such as stability and handedness, providing a framework for designing wave behavior rather than simply observing it.

The findings also build an unexpected bridge between two distant fields of physics. Oceanographers and fluid dynamicists have long studied internal wave attractors in stratified fluids, where waves traveling through water whose density varies with depth can reflect in unusual ways from underwater slopes. Those reflections can focus energy into closed paths and contribute to wave turbulence in the deep ocean, a phenomenon with consequences for mixing and energy transport on planetary scales. The new study translates a closely related phenomenon into a tabletop, solid-state metamaterial platform, showing how concepts borrowed from geophysical fluid dynamics can be explored in engineered materials and, notably, in simple linear settings that do not require the nonlinearities or turbulence of the oceanic original. That linearity is a practical advantage, because linear systems are easier to model, predict, and engineer.

The framework extends beyond mechanical vibrations. The researchers point out that it can apply to hyperbolic phonon polaritons, hybrid light-matter excitations that arise naturally in two-dimensional anisotropic crystals such as hexagonal boron nitride and molybdenum trioxide. These materials support propagation and reflection properties strikingly similar to those exploited in the metamaterial experiment, and they can confine light to regions far smaller than its free-space wavelength. That confinement makes them attractive for low-loss nanophotonic circuits, enhanced interactions between light and matter, and precise control of infrared light. Hyperbolic wave attractors could therefore inform the design of compact optical chips, devices that separate optical signals, particle-trapping systems, analog wave-computing platforms, and highly sensitive biological sensors, all of which benefit from wave patterns that are robust rather than fragile.

The broader significance of the work lies in its reframing of what disorder means in wave systems. For centuries, the geometry of a container was seen as the enemy of control: the more irregular the space, the more hopeless the task of predicting or steering the waves inside it. This study shows that with the right material, irregularity itself can become a design tool, a knob that engineers can turn to sculpt stable, chiral, scale-invariant wave paths in cavities of arbitrary shape. Alù and his colleagues describe the research as opening a path toward engineering stable and robust wave patterns in systems where waves would normally be expected to rapidly become chaotic, with the goal of harnessing these effects to create new functionalities for photonics, phononics, and wave-based technologies. The research was supported by the Simons Foundation and the National Science Foundation Science and Technology Center New Frontiers of Sound, and the team included scientists affiliated with the CUNY ASRC, Institut Langevin at ESPCI Paris PSL in France, and the University of Amsterdam in the Netherlands. As hyperbolic materials mature from laboratory curiosities into practical platforms, the strange order discovered inside these oddly shaped cavities may well become a foundation for the next generation of wave-control devices.

Subject of Research: Hyperbolic wave attractors in irregularly shaped metamaterial cavities

Article Title: Waves find order in the chaos of an oddly shaped cavity

Article References: Waves find order in the chaos of an oddly shaped cavity. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: hyperbolic materials, wave attractors, metamaterials, dynamical billiards, photonics, chaos, handedness, phonon polaritons, nanophotonics, mechanical metamaterials, wave control, CUNY ASRC

The new work, published in Light: Science & Applications, comes from a collaboration led by Professors Xinliang Zhang and Cheng Zhang of Huazhong University of Science and Technology, working alongside collaborators at the University of Cambridge and Kyung Hee University. Their device belongs to a class of components known as metasurfaces, which are engineered arrays of subwavelength structures that shape the wavefront of light in ways that bulk optics cannot easily achieve. In this case, the team fabricated a dielectric metasurface that acts simultaneously as an imaging lens and as an optical differentiator, a component that computes the spatial derivative of an optical field in real time. The achievement lies in doing both at once, on the same layer of material, without the cascades of lenses and processing stages that earlier designs required.

To understand why this is technically significant, it helps to consider what optical differentiation actually means. In Fourier optics, taking the derivative of an image is equivalent to multiplying its spatial frequency spectrum by a factor proportional to frequency. Conventional implementations of this operation require a pair of lenses to move between real space and Fourier space, with a filter placed at the intermediate focal plane. That architecture works, but it is bulky, alignment-sensitive and difficult to scale. Previous metasurface differentiators inherited some of these burdens, typically needing Fourier transform lens pairs or supplementary imaging optics to deliver a usable result. The new device sidesteps that complexity by engineering the point spread function of the metasurface itself, the characteristic pattern into which a point source of light is spread by the optical system, so that the desired differentiation operation is embedded directly into the imaging process.

The key innovation is spin multiplexing. Light can carry spin angular momentum in two forms, corresponding to left-handed and right-handed circular polarization. By designing the metasurface so that it responds differently to each spin state, the researchers gave a single flat optic two independent operational channels. One polarization channel performs zeroth-order differentiation, which is functionally equivalent to conventional bright-field imaging and preserves the original scene information, while the other channels execute higher-order differentiation that emphasizes high-frequency content, highlighting the regions of a scene where the light field changes rapidly, such as edges, boundaries and fine textures. The team demonstrated two versions of the device, one handling zeroth and first orders and the other handling second and third orders, all within the same single-layer architecture.

The performance figures reported by the team underline the practical maturity of the approach. The metasurface differentiators operated across a broad wavelength range extending from yellow light into the near-infrared, a span that matters for real-world applications where illumination sources vary and biological samples respond differently at different wavelengths. At the same time, the devices maintained fine spatial resolution of up to 228.0 line pairs per millimetre, corresponding to a line width of 2.19 micrometres. That combination of spectral breadth and resolving power in a single flat component is what distinguishes the work from earlier proof-of-concept demonstrations, which often achieved the computing function at the cost of imaging quality or vice versa.

To show that the platform works on real targets rather than only on synthetic test patterns, the researchers imaged two very different kinds of objects. The first was an amplitude-type object, a custom-made binary metallic pattern fabricated on a coverslip, which modulates the intensity of transmitted light. The second was a phase-type object, a transparent diatom cell, which leaves the intensity of light essentially unchanged and instead imprints information in the phase of the transmitted wavefront. Phase objects are notoriously difficult for conventional cameras to capture, because standard detectors register only intensity. The ability of the differentiator platform to extract useful contrast from such samples points toward applications in biological microscopy, where much of the structural information in unstained cells lives in phase rather than amplitude.

The contrast with conventional image processing pipelines is central to the significance of the work. In a standard architecture, an imaging system first captures the intensity of a scene, and the resulting digital images are then processed electronically, with edge detection, sharpening or feature extraction performed by algorithms running on processors. Each step in that chain costs time and energy, and the phase information of the light field is lost at the moment of detection. The all-optical differentiator instead performs the mathematical operation directly on the light field itself, before any detection takes place. The result is instantaneous multi-dimensional acquisition and processing, in which enhanced details emerge in the optical signal without any intermediate digital computation. The researchers also demonstrated that the device remains robust under high-intensity illumination, an important consideration for deployment in demanding imaging environments.

Real-time capability was validated in a living system. The team used the platform to observe live Euglena cells, showing that the metasurface can keep pace with dynamic biological samples rather than being restricted to static targets. This matters because many of the most valuable applications of computational imaging, from tracking motile microorganisms to monitoring cellular responses to stimuli, require the imaging and the computation to happen faster than the scene itself changes. An optical element that computes as it images has no inherent processing latency, so the temporal resolution of the system is set by the optics and the detector rather than by a downstream processor.

“This work provides a practical pathway toward ultracompact all-optical computing devices,” says Professor Cheng Zhang. “By embedding computation directly into the imaging process, we bypass the speed and energy bottlenecks of digital electronics while simultaneously capturing phase information that is inaccessible to conventional cameras.” The statement captures the dual promise of the approach: it addresses the throughput and power constraints that limit electronic processing, and at the same time it recovers information, namely the phase of the optical field, that a standard intensity camera simply discards.

The potential applications extend across several domains that share a common need for fast, parallel processing of optical information. Real-time biological imaging could benefit from a compact optic that highlights cellular boundaries and internal structure without staining or digital post-processing. Material inspection and machine vision systems, which depend heavily on edge and texture detection, could integrate the differentiator directly into their front-end optics, reducing the computational load on downstream electronics. Looking further ahead, the team anticipates that the same point spread function engineering strategy could be extended to implement a broader repertoire of optical computing functions, such as denoising or feature extraction, expanding the toolkit of meta-optics for intelligent imaging and sensing. As artificial intelligence continues to push against the limits of digital hardware, flat optics that compute while they image offer a glimpse of how the front end of machine vision may itself become a computational element.

Subject of Research: Spin-multiplexed point spread function engineering in dielectric metasurfaces for simultaneous optical differentiation and high-resolution imaging

Article Title: Spin-multiplexed point spread function engineering via dielectric metasurface for simultaneous optical differentiation and high-resolution imaging

Article References: Spin-multiplexed point spread function engineering via dielectric metasurface for simultaneous optical differentiation and high-resolution imaging. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: metasurface, all-optical computing, optical differentiation, point spread function, spin multiplexing, high-resolution imaging, dielectric metasurface, machine vision, biological imaging, phase imaging, nanophotonics, Light Science & Applications

Hyperbolic phonon polaritons, often abbreviated HPhPs, are among the most remarkable optical excitations known to materials science. They arise when infrared electromagnetic fields couple strongly to the natural lattice vibrations, or optical phonons, of certain crystals. The resulting hybrid waves behave in ways that ordinary light cannot: they can squeeze electromagnetic energy into volumes thousands of times smaller than the wavelength of the light itself, guiding it coherently along a material’s surface or through its bulk. This extreme confinement makes them attractive candidates for nanoscale light manipulation, high-sensitivity molecular spectroscopy, and the miniaturization of optical circuits to dimensions that rival modern microelectronics.

Hexagonal boron nitride, or hBN, a layered van der Waals crystal famous for its role as a protective encapsulant in two-dimensional electronics, is a particularly powerful host for these waves. Within specific infrared frequency bands, hBN behaves as a hyperbolic medium, meaning that its dielectric response is positive along one crystal axis and negative along another. In this regime, the polariton wavefronts take on hyperbolic shapes, and the waves can propagate with wavelengths dramatically shorter than free-space light, carrying electromagnetic energy along the crystal with remarkable confinement. But harnessing such waves in practical devices requires more than passive propagation; engineers need the ability to switch, steer, and reshape them on demand, and to do so at speeds fast enough to matter for information processing.

That is precisely where the experimental challenge has always lain. Hyperbolic phonon polaritons are strongly dispersive, which means their wavelength changes substantially depending on the frequency of the light that excites them. Femtosecond laser pulses, the only tools capable of resolving dynamics on the relevant timescales, are inherently broadband: a single pulse contains a wide spread of frequencies. When such a pulse strikes a sample, it excites polaritons of many different wavelengths simultaneously. In a real-space image, the interference fringes produced by all these waves overlap and average together, washing out the very propagation patterns scientists want to see. The obvious remedy, narrowing the spectrum of the pulse before it reaches the sample, comes at a cruel price: by the fundamental rules of Fourier optics, a narrower spectrum demands a longer pulse, which destroys the femtosecond time resolution that motivated the experiment in the first place.

A research team led by Kazuki Kamada of the Institute for Molecular Science (IMS) and Osaka Metropolitan University, together with Jun Nishida, Assistant Professor at IMS, and Takashi Kumagai, Associate Professor at IMS, has now found an elegant way around this trade-off. Their solution was to leave the excitation pulse untouched and instead perform the frequency selection after the fact, during detection. By inserting a diffraction grating into the detection path of an ultrafast infrared near-field optical microscope, the team spectrally separated the broadband infrared light scattered from the sharp metallic tip of an atomic force microscope before it reached the detector. Because each frequency component could then be imaged independently, the overlapping interference patterns were cleanly disentangled without ever lengthening the excitation pulse.

The resulting instrument is a genuine tour de force of measurement engineering. It combines nanoscale spatial resolution, inherited from the near-field interaction between the microscope tip and the sample, with a temporal resolution of approximately 150 femtoseconds and a spectral resolution of roughly 10 inverse centimeters. In practical terms, the technique allows researchers to select a single narrow slice of the infrared spectrum, watch the polariton associated with that frequency propagate across the sample in a snapshot lasting only a few hundred atomic vibrations of time, and then repeat the measurement at successive pump-probe delays to assemble a full movie of the wave’s evolution. No previous near-field imaging scheme had brought all three of these capabilities together for strongly dispersive polaritons.

With the new microscope in hand, the team turned it on van der Waals heterostructures composed of tungsten disulfide (WS2) and hexagonal boron nitride. The experimental geometry was a classic pump-probe arrangement: a pulse of visible light generated photoexcited charge carriers, mobile electrons and holes, within the WS2 layer, while a time-delayed infrared near-field pulse probed how those carriers affected the hyperbolic phonon polaritons propagating in the adjacent hBN. Because the two atomically thin materials lie in intimate contact across the van der Waals interface, any change in the optical properties of one should, in principle, be felt by the waves traveling in the other. The question was whether the effect would be fast enough and strong enough to see directly.

The answer was a clear yes, and the details proved richer than a simple on-off switching picture. In heterostructures containing a thin WS2 layer, the researchers observed a transient modulation dominated by a change in the electric-field amplitude of the polaritons, as if the photoexcited carriers were momentarily absorbing or damping the wave. More strikingly, in structures containing a relatively thick WS2 layer, the team resolved a change in the polariton’s wavelength itself. This is a qualitatively different and more powerful form of control: it demonstrates that optical excitation can modify not merely how strongly the wave is present but how it propagates, effectively rewriting the dispersion relation of the polariton on the fly. Electromagnetic simulations confirmed that both observations could be explained by a transient, carrier-induced change in the dielectric response of WS2, which then modifies the HPhPs propagating in the neighboring hBN through the shared interface.

The physical mechanism is worth appreciating in detail. When visible light excites electron-hole pairs in WS2, the free carriers alter the material’s dielectric function, particularly at infrared frequencies relevant to the hBN polaritons. Since the hyperbolic phonon polariton is a hybrid mode whose electromagnetic field extends across both layers of the heterostructure, this sudden change in the WS2 dielectric environment is imprinted directly onto the wave. The carriers then relax and recombine on their own timescale, and the polariton returns to its unperturbed state. The entire cycle, from optical switching to recovery, unfolds within a few hundred femtoseconds, which is exactly the regime the new imaging technique was designed to capture. In effect, the researchers have demonstrated an all-optical gate for nanoscale light, in which one beam of light controls another through the mediation of photoexcited carriers and a van der Waals interface.

The implications extend well beyond this particular material system. The study, published online in Nano Letters on July 27, 2026, establishes a general platform for observing and controlling nanoscale light propagation on femtosecond timescales, and the same imaging strategy should apply to other strongly dispersive polaritons, including surface plasmon polaritons in metals and phonon polaritons in other hyperbolic crystals. As the authors note, the results open new opportunities for ultrafast nanophotonics, a field in which devices might one day modulate, route, and process signals encoded in confined light fields at speeds approaching the fundamental limits set by the optical oscillations themselves. The work was supported by JSPS KAKENHI, the JST FOREST and CREST programs, the MEXT World Premier International Research Center Initiative, and several other Japanese research foundations, and it involved collaborators from the National Institute for Materials Science, the Fritz Haber Institute in Berlin, and Osaka Metropolitan University. For now, the immediate contribution is conceptual and methodological: a way to see, frame by femtosecond frame, how light confined to the nanoscale can be commanded to change its shape, its speed, and its very wavelength, all at the whim of a flash of visible light.

Subject of Research: Ultrafast nano-imaging and optical control of hyperbolic phonon polaritons in van der Waals heterostructures

Article Title: Capturing fleeting changes in “nanoscale light”—femtosecond nano-imaging reveals ultrafast optical control of phonon polariton

Article References: Capturing fleeting changes in “nanoscale light”—femtosecond nano-imaging reveals ultrafast optical control of phonon polariton. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: phonon polaritons, hexagonal boron nitride, tungsten disulfide, femtosecond imaging, near-field microscopy, nanophotonics, van der Waals heterostructures, ultrafast optics, pump-probe spectroscopy, dielectric response, Nano Letters, light-matter coupling

The research, led by physics professor Elizabeth Goldschmidt and published in Nano Letters, describes an integrated nanophotonic platform built on thin-film lithium niobate, one of the leading materials for modern quantum optics experiments. The device dopes this waveguide with erbium atoms and uses a technique called spectral hole burning to arrange those atoms into an atomic frequency comb, a carefully spaced pattern of absorption frequencies that can catch incoming photons and re-emit them after a predetermined delay. According to the team, the device preserved quantum information with high fidelity, stored multiple photons simultaneously, and exceeded storage times of one microsecond, all on a chip that could in principle be manufactured commercially at scale.

To appreciate why a microsecond matters, it helps to consider the scale involved. The storage region on such a chip is roughly a centimeter across, a distance that light crosses in a few tens of trillionths of a second. Holding a photon for a full microsecond therefore means delaying it by a factor of tens of thousands relative to its natural transit time, effectively compressing meters of optical delay line into a device small enough to fit on a fingertip. For quantum chips, where operations must be synchronized and photons must wait their turn in a queue of quantum logic steps, that kind of on-demand delay is not a luxury. It is a prerequisite for building processors and networks that actually work.

There are, broadly speaking, two established routes to delaying light, and each has failed the quantum community in a different way. The first is the brute-force approach of conventional photonics: send the light down a longer path, using coiled optical fibers or arrangements of mirrors to stretch its journey. The trouble is that photons are easily absorbed by matter, and every additional meter of travel increases the odds that a photon simply vanishes. For the long delays relevant to quantum information processing, the survival probability of a photon in a conventional delay line becomes vanishingly small, making the approach hopelessly inefficient. A quantum memory cannot afford to lose the very qubits it is supposed to protect.

The second route is more elegant in principle. Instead of making light travel farther, one can couple photons to atoms whose quantum states are highly coherent, allowing the photonic information to be transferred into an atomic excitation, stored there, and later retrieved. Atomic ensembles can hold quantum states for far longer than a photon can survive in a fiber. But integrating such coherent atoms with scalable nanophotonic platforms has remained largely out of reach, because the exotic materials and cryogenic laboratory conditions that make long atomic coherence possible rarely coexist with the fabrication demands of chip manufacturing. This gap between laboratory demonstrations and manufacturable technology has been one of the defining bottlenecks of the field.

The Illinois team’s platform attempts to close that gap by combining two ingredients that had previously lived in separate worlds. Thin-film lithium niobate offers strong optical confinement, low loss, and wafer-scale integration, which is why it has become a workhorse material for both classical and quantum photonic chips. Erbium atoms, meanwhile, are prized in quantum memory research because their electronic transitions sit in the telecommunications band and can exhibit long coherence times, particularly when embedded in suitable crystalline hosts. By doping erbium into the lithium niobate waveguide itself, the researchers created a structure in which the light and the atoms that store it share the same nanoscale volume, maximizing their interaction without requiring the photon to leave the chip.

The storage mechanism relies on spectral hole burning, a technique in which a tunable laser is used to selectively excite and shelve subsets of atoms according to the precise frequencies they absorb. Rather than letting the erbium ensemble absorb light in a broad, featureless band, the researchers sculpted its absorption spectrum into a comb-like pattern with evenly spaced frequency peaks. When a photon enters the device, its energy is coherently shared among the comb’s teeth, and the resulting quantum interference causes the excitation to rephase after a fixed interval, at which point the photon is re-emitted in its original form. The storage time is set by the spacing of the comb, giving the researchers a knob with which to control how long the light waits.

Priyash Barya, an electrical engineering graduate student and co-first author of the paper, emphasized that no one else has stored light on a chip in a platform with this potential for scalability, describing the approach as the only way of achieving this result on one of the leading platforms for quantum optics and quantum information systems. Daren Chen, a physics graduate student and the paper’s other co-first author, framed the achievement in terms of the field’s central difficulty, noting that long delays remain an outstanding problem in quantum information processing and that the team wanted to demonstrate a promising approach using their nanophotonic platform. The demonstration of storage exceeding one microsecond, with multiple photons held at once, positions the device among the most capable integrated quantum memories reported to date.

What may ultimately matter as much as the performance numbers is the device’s manufacturability. Goldschmidt stressed that this is not a one-off bespoke device, arguing that its promise lies in its simplicity: the team has taken something that could typically only be done in a highly specialized laboratory environment and recast it in a platform that can be made commercially at scale by people who know nothing about quantum optics. That framing points toward a future in which quantum memories are not hand-built instruments but standard components, fabricated in the same facilities that produce telecommunications hardware and integrated into larger quantum systems by engineers rather than specialists.

Challenges remain before the platform can anchor a practical quantum computer or network. Goldschmidt noted that a useful version of the device will need better retrieval efficiency and longer storage times, goals the group plans to pursue by refining the atomic frequency comb and experimenting with erbium isotopes that are more resistant to noise. Her lab is also exploring additional applications for spectrally tailoring atomic ensembles to build other devices for quantum photonics, treating the memory as one component in a broader program of building capabilities on the integrated lithium niobate platform. If those efforts succeed, the humble act of making a photon wait could become one of the quiet foundations of the quantum internet, a piece of engineering so routine that the extraordinary physics inside it is all but invisible.

Subject of Research: On-chip quantum memory for photon storage in erbium-doped thin-film lithium niobate

Article Title: Illinois researchers give quantum light a place to wait

Article References: Illinois researchers give quantum light a place to wait. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: quantum memory, photon storage, thin-film lithium niobate, erbium doping, atomic frequency comb, spectral hole burning, nanophotonics, quantum information, integrated photonics, quantum networks, telecom photons, University of Illinois

The technique at the heart of the work is photoacoustic microscopy, or PAM, a hybrid imaging modality that marries light and sound. A pulsed laser is delivered into tissue, where molecules such as hemoglobin absorb the energy and heat slightly, expanding and launching ultrasonic waves. Because ultrasound scatters far more weakly in tissue than light does, these waves can be detected and traced back to their origin, yielding images with optical contrast but acoustic penetration. Optical-resolution PAM, the variant used here, focuses the laser beam as tightly as diffraction allows, achieving micron-scale lateral resolution that can resolve the microvascular networks feeding the iris, choroid, and cornea. The catch is that this tight focusing produces a very shallow depth of field: structures even a fraction of a millimeter away from the focal plane blur dramatically, forcing researchers into slow, repeated refocusing scans to build a three-dimensional image.

Previous attempts to stretch the depth of field have relied on nondiffracting beams, such as Bessel beams and needle beams, which concentrate light into long, pencil-like axial profiles. These beams do maintain resolution over a longer range, but they carry a punishing penalty: their energy is distributed into prominent side lobes that generate spurious photoacoustic signals, and spreading the laser power over a larger volume weakens the signal at any single depth, degrading the signal-to-noise ratio. The POSTECH team took a fundamentally different route. Rather than smearing a single focus along the axis, they built a metalens that produces two separate, diffraction-limited Gaussian foci, each as strong and clean as a conventional lens focus, and switches between them electrically without moving a single mechanical part.

The metalens itself is a marvel of nanoscale engineering. It consists of silicon nitride nanostructures, each smaller than the wavelength of light, arranged on a flat glass substrate. Each nanostructure, or meta-atom, acts as a tiny waveguide that imparts a precise phase delay to passing light. By combining two phase-control mechanisms, the propagation phase and the geometric, or Pancharatnam-Berry, phase, the designers encoded two entirely independent lens phase profiles into the same surface, one for right-handed circularly polarized light and one for left-handed circularly polarized light. When the incident beam is right-circularly polarized, the metalens focuses at 4.5 millimeters; flip the polarization to left-handed, and the focus jumps to 5.2 millimeters. In water, where the device is designed to operate to minimize photoacoustic signal loss, this translates into two focal planes separated by roughly 0.9 millimeters.

Choosing the right material was critical, because photoacoustic imaging drives the lens with a high-power pulsed laser at 532 nanometers, the wavelength at which hemoglobin absorbs strongly. Any absorption in the lens material would both waste laser energy and generate unwanted photoacoustic signals from the lens itself. The team used plasma-enhanced chemical vapor deposition to grow silicon nitride films, carefully tuning the ratio of silane to nitrogen gas precursors to engineer the film’s bandgap. The optimal recipe, a silane-to-nitrogen ratio of 1.52, yielded a high refractive index of 2.4 with a near-zero extinction coefficient at 532 nanometers, and a wide optical bandgap of 5.17 electronvolts. A full micrometer-thick film of this material absorbs only about two percent of the green laser light, keeping the lens optically quiet while maximizing the focusing efficiency of its meta-atoms, each of which was optimized to act as a half-wave plate with conversion efficiencies exceeding seventy percent.

The electrical switching is performed by a liquid crystal cell placed in front of the metalens. The liquid crystal, a common material called 5CB, rotates its molecules in response to an applied alternating voltage, changing the polarization state of the transmitted light. At 0.87 volts, the output is right-circularly polarized and the metalens focuses at the near plane; at 1.03 volts, it becomes left-circularly polarized and the focus shifts to the far plane. The response time is about 250 milliseconds, fast enough to alternate between modes during a scan, and the optical contrast between the on- and off-state foci reaches 40:1 at the near focus and 20:1 at the far one. Crucially, both focal spots exhibit Strehl ratios close to unity, meaning each behaves as a nearly ideal diffraction-limited Gaussian focus rather than a compromised beam shape. The measured lateral resolutions, 3.1 and 3.7 micrometers for the two modes, match theoretical predictions almost exactly.

To turn this into an imaging system, the researchers mounted the metalens on a custom ring-shaped ultrasound transducer made from an 18-micrometer-thick polyvinylidene fluoride film, with the metalens sitting in a central aperture so that the optical and acoustic axes are perfectly coaxial. Bench tests with resolution targets and layered nylon threads confirmed that switching modes reliably shifted the sharp focal zone by about 0.9 millimeters, extending the usable axial imaging range to roughly 1.2 millimeters, a dramatic improvement over conventional single-focus PAM. Tests in a scattering phantom made of water, gelatin, and intralipid showed that the deeper focus mode genuinely improved contrast and resolution for objects buried about a millimeter deeper, confirming the effect survives realistic tissue-like scattering.

The demonstration that will interest clinicians most came in living animals. The team imaged the eyes of Sprague-Dawley rats, first healthy animals and then a model of corneal neovascularization induced by applying an alkali-soaked filter paper to the cornea, a standard model of chemical burn injury. In the healthy eye, the short-focus mode sharply resolved the root vessels and fine microvasculature of the iris, while the long-focus mode brought deeper structures into view, including the posterior ciliary vessel and pupil-associated vasculature that appeared blurred in the near mode. Fourteen days after the burn, the dual-focus system mapped the newly grown corneal vessels in detail while simultaneously imaging iris vessels as deep as 1.5 millimeters beneath the neovascular layer in one mode and about 2.2 millimeters in the other, a 0.7-millimeter extension in detectable depth. Notably, the images confirmed that the pathological new vessels remained strictly confined to the cornea, leaving the iris vasculature untouched.

The quantitative results underscore why this matters. Vessels imaged out of focus appear artificially fat: iris vessels measured through the near-focus mode showed an apparent average diameter of about 42 micrometers, but when the focus was shifted deeper, the same vessels measured a truer 16 micrometers, comparable to the roughly 18-micrometer diameter of the corneal vessels measured in their own focal plane. Meanwhile, the total area of the neovascular lesion came out nearly identical in both modes, at 5.55 and 5.65 square millimeters, showing that the dual-focus approach yields reliable lesion-scale measurements even as it trades resolution between depths. The 0.9-millimeter focal separation neatly spans the thickness of a typical cornea, around 0.5 to 0.6 millimeters, meaning the clinically relevant corneal layers fall comfortably within the high-resolution regime of the system.

The researchers are candid about the limitations and the road ahead. The current frame rate of about 0.42 frames per second could rise to 20 frames per second with faster optical scanning, though that would demand motion compensation to counter respiration-induced eye movement. Switching modes changes the numerical aperture slightly, which can be corrected by subpixel registration when merging the two image sets, and extending the design to multispectral imaging will require metasurfaces that correct for the wavelength dispersion of silicon nitride. Still, the concept reaches well beyond the eye: the same tunable dual-focus optics could be folded into optical coherence tomography, two-photon microscopy, or metalenses designed around the optical properties of skin and brain tissue. What the POSTECH team has delivered is more than a clever lens. It is a demonstration that flat, electrically reconfigurable optics can solve one of biomedical imaging’s oldest trade-offs, offering a compact, mechanically silent path to high-resolution, high-contrast, volumetric imaging of living tissue.

Subject of Research: An electrically tunable dual-focus metalens integrated into photoacoustic microscopy for depth-extended in vivo imaging of ophthalmic vascular disease

Article Title: Electrically switchable dual-focus metalens for depth-extended in vivo photoacoustic imaging of ophthalmic vascular disease

Article References: Kim, H., Park, E., Park, J., Oh, D. K., Shin, J., Heo, H., Kim, C., & Rho, J. (2026). Electrically switchable dual-focus metalens for depth-extended in vivo photoacoustic imaging of ophthalmic vascular disease. Light: Science & Applications, 15(1), Article 369. https://doi.org/10.1038/s41377-026-02365-8

Image Credits: AI Generated

DOI: 10.1038/s41377-026-02365-8

Keywords: metalens, metasurface, photoacoustic microscopy, silicon nitride, liquid crystal, corneal neovascularization, ophthalmic imaging, depth of field, nanophotonics, in vivo imaging, microvasculature, flat optics