The new material, described in the Journal of Materials Science, is a core–shell–satellite nanocomposite with the tongue-twisting formula Fe3O4@Au@Ag. Each component of that structure does a specific job. At the center sits a magnetite (Fe3O4) core, made by a standard hydrothermal synthesis, which provides the magnetic handle that makes the whole catalyst recyclable. Wrapped around that core is a shell of gold, grown through a seeded-growth process so that the precious metal coats the magnetic particle uniformly. Finally, the surface is decorated with small silver nanoparticles, deposited in amounts that the researchers can tune by adjusting the dosage of silver nitrate during synthesis.

The reason this three-layer architecture works so well lies in a phenomenon called localized surface plasmon resonance, or LSPR. When light strikes a metallic nanoparticle much smaller than the wavelength of that light, the conduction electrons in the metal oscillate collectively at a characteristic frequency. At that resonant frequency, the nanoparticle becomes an extraordinarily efficient absorber and scatterer of light, and the electromagnetic field near its surface is amplified by orders of magnitude. Gold nanoparticles resonate in the visible range, but the exact wavelength depends on the particle’s size, shape, and chemical environment. By adding silver satellites to a gold shell, the team could tune the resonance and dramatically broaden and strengthen the material’s absorption of visible light, which is the energy source that drives the catalytic destruction of pollutants.

But the silver decoration does something even more important than simply tuning the color of the particles. When a moderate amount of silver sits on the gold surface, the plasmons of the two metals couple together, producing electromagnetic hot spots in the nanoscale gaps between them where the local electric field becomes intensely concentrated. These hot spots accelerate the photocatalytic reactions that break down organic molecules, and they also promote efficient separation of the charge carriers, the energized electrons and holes that do the actual chemical work of shredding pollutant molecules into harmless fragments. The researchers confirmed this picture using finite-difference time-domain simulations, a computational electrodynamics technique that models how electromagnetic fields propagate through complex nanostructures, and the simulated field enhancements matched the experimentally observed catalytic behavior.

To measure performance, the team used the photodegradation of methyl orange, a common azo dye that serves as a standard test compound in photocatalysis research. The best-performing composite destroyed more than 95 percent of the dye within four minutes of irradiation. Just as importantly for any real-world application, the material did not quit after one use. After six cycles of degradation, recovery, and reuse, the catalyst still retained efficiency above 80 percent, a durability figure that many photocatalysts struggle to approach and that speaks to the robustness of the layered structure.

The kinetics of the reaction also revealed a subtle and instructive dependence on how much silver was deposited. The researchers tracked two parameters: the characteristic induction time, denoted t0, which is the delay before degradation reaches full speed, and the apparent reaction constant, which measures how quickly the dye molecules are destroyed once the reaction is running. As the silver nitrate dosage increased during synthesis, the induction time first declined and then rose again, while the reaction constant followed exactly the opposite trajectory, rising and then falling. The sweet spot corresponded to a moderate silver loading, where Au–Ag plasmonic coupling is strongest.

The explanation for this non-monotonic behavior is elegantly physical. A moderate amount of silver creates strong plasmonic coupling between the silver satellites and the gold shell, generating the intense hot spots and charge-separation efficiency that supercharge the catalysis. But when too much silver is deposited, the silver particles grow and merge until they fully encapsulate the gold surface, burying it under an inert metallic blanket. With the gold no longer exposed, the interfacial electromagnetic interactions between the two metals weaken, the hot spots fade, and catalytic performance declines. More is not better; there is a precise structural optimum, and the ability to tune silver loading via the silver nitrate dosage gives chemists direct control over where on that curve their catalyst lands.

The magnetic core is not a minor convenience feature; it addresses one of the biggest practical obstacles to deploying nanoparticles in water treatment. Nanoparticle catalysts offer enormous surface area, but recovering them from treated water is difficult and expensive, and catalyst particles that escape into the environment become pollutants themselves. The Fe3O4 core solves this problem: the composite exhibited a saturation magnetization of 71.6 emu/g, a figure high enough that a simple external magnet pulls the particles out of suspension rapidly and completely. The catalyst can be collected, rinsed, and redeployed, turning a single-use nanomaterial into a genuinely recyclable one and keeping the nanomaterial out of the discharge stream.

The work also highlights a broader trend in materials chemistry: the deliberate engineering of multi-component nanostructures in which each element plays a distinct role, and where the interactions between components, rather than any single component alone, produce the desired property. Previous studies have explored magnetic plasmonic systems in various forms, including Fe3O4@TiO2@Au composites for visible-light degradation and Schottky-type Fe3O4–Au heterostructures, and silver has long been known to enhance the performance of gold in electrocatalysis. The Hangzhou Dianzi team’s contribution is the systematic demonstration that the degree of Ag coverage on a Au shell acts as a tuning knob for plasmonic coupling, and the direct computational corroboration that coupling-induced field enhancement underlies the catalytic gains.

The research was led by Weiwei Que and Song Fu of the College of Materials and Environmental Engineering, together with Jie Liu and Tian-Song Deng of the School of Electronics and Information Engineering, with support from the Zhejiang Provincial Natural Science Foundation and the National Natural Science Foundation of China. The authors suggest that the structural modulation strategy, growing a magnetic core, a plasmonic shell, and a tunable satellite layer, offers a general recipe for building recyclable, high-performance plasmonic photocatalysts. If the approach extends beyond methyl orange to the full zoo of industrial dyes and organic contaminants, and if it can be scaled from beakers to treatment tanks, magnetic plasmonic nanocomposites of this kind could give wastewater engineers something they rarely get: a catalyst that works in minutes, survives repeated use, and comes home when called.

Subject of Research: Recyclable plasmonic Fe3O4@Au@Ag magnetic nanocomposites for photocatalytic degradation of organic dye pollutants in wastewater

Article Title: Enhanced photocatalytic degradation of organic pollutants by plasmonic Au/Ag-decorated magnetic nanocomposites

Article References: Que, W., Fu, S., Liu, J., & Deng, T.-S. (2026). Enhanced photocatalytic degradation of organic pollutants by plasmonic Au/Ag-decorated magnetic nanocomposites. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13949-5

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13949-5

Keywords: photocatalysis, plasmonics, nanocomposites, Fe3O4, gold nanoparticles, silver nanoparticles, methyl orange, wastewater treatment, LSPR, magnetic recovery, hot spots, FDTD simulation

The technique at the heart of the work is known as a light-induced self-written waveguide, or LISW. The concept dates back to the 1990s and relies on a deceptively simple property of photocurable resin: the material hardens preferentially where the optical field is most intense. When curing light shines out of the facet of an optical fiber or waveguide into a droplet of liquid resin, the resin solidifies along the beam path, forming a thin solid waveguide that extends outward from the facet. If two opposing waveguides face each other across a gap of resin, both emit curing light, and the two self-written guides grow toward each other until they meet. Crucially, if the two waveguides are laterally offset from one another, the resulting structure does not form a straight bridge but an elegant S-shaped curve that bends to connect the two misaligned cores. The waveguide essentially forgives the assembly error by reshaping itself around it.

Earlier demonstrations of LISW connections had already shown respectable alignment tolerances of roughly plus or minus 4 micrometers for standard single-mode waveguides with core diameters of 8 to 9 micrometers. Researchers also knew that widening the gap between the waveguides helped: at a gap of 500 micrometers, tolerances near 10 micrometers had been reported, because a longer connection gives the S-bend more room to curve gently. But stretching the gap creates its own difficulties. A longer waveguide means a longer propagation path and therefore more accumulated loss, and the curing light itself weakens as it travels through the resin through scattering and absorption. Beyond a certain length, the light may simply be too faint to keep polymerizing the resin, and the self-writing process stalls before a usable guide forms. The Japanese team set out to find whether a large alignment tolerance could be achieved without paying the price of a long gap.

Their starting point was geometry. Using an analytical expression relating the lateral offset, the gap, and the bending radius of an S-bend built purely from circular arcs, they showed that for a fixed gap, the maximum tolerable offset depends on how tightly the waveguide can bend before radiating light away. A waveguide that tolerates a smaller bending radius can absorb a larger misalignment at the same gap. That is precisely where the refractive-index contrast, denoted by the Greek letter delta, enters the picture. A high-delta waveguide, in which the core index differs strongly from the cladding index, confines light far more tightly and can sustain sharp curves with little radiation loss. In other words, the team identified a materials-based route to alignment tolerance: instead of stretching the gap to relax the bend, make the waveguide itself bend-tolerant by boosting its index contrast.

To test this idea quantitatively, the researchers ran two-dimensional finite-difference time-domain simulations using the open-source Meep software. They modeled an S-bent LISW connecting two waveguides with 3-micrometer cores and a delta of 2.2 percent, values chosen to resemble the spot-size converters found at the edges of silicon photonic chips. Sweeping the gap, waveguide width, and delta across a physically realistic range, they found a clear trend: larger gaps and higher index contrasts both reduced coupling loss at a fixed lateral offset of 8 micrometers. Just as importantly, at a delta of 1.3 percent or above, the coupling loss became nearly independent of the waveguide width, meaning the connection would be forgiving of fabrication variations as well as assembly errors. A delta of 1.3 percent is well within reach of commercial photocurable resins, whose index rises during photopolymerization as the material’s molecular structure and density change.

The experimental demonstration used a commercial acrylic resin with a refractive index of 1.49 after curing and 1.47 before, at a wavelength of 1.31 micrometers, corresponding to a delta of about 1.3 percent. The team connected two high-numerical-aperture fibers, each with a mode field diameter of only 3.4 micrometers and an estimated core diameter near 3 micrometers, a deliberately punishing test case since small cores make alignment exquisitely sensitive. Across a gap of just 250 micrometers, the bare fiber-to-fiber coupling loss was a dismal 19.4 decibels, dominated by light diffracting away into the gap. After dispensing resin and irradiating both fibers simultaneously with 405-nanometer laser diodes at minus 15 dBm for 2.5 seconds, the loss collapsed to 0.7 decibels. The self-written bridge had effectively erased the gap.

The headline result came when the researchers deliberately misaligned the fibers. By fitting their loss measurements with a quadratic curve and finding where the excess loss reached 1 decibel, they extracted a 1-dB alignment tolerance of approximately plus or minus 7.4 micrometers, achieved at the modest gap of 250 micrometers. Microscope images confirmed that an S-bent-like waveguide had indeed formed between the offset cores, curving smoothly from one fiber to the other. For small-core devices such as silicon photonic chips and high-delta multicore fibers, whose tiny mode field diameters normally impose brutal alignment demands, a self-forming connector that shrugs off several micrometers of offset at a short gap is a genuinely attractive proposition.

The study is also candid about where theory and experiment diverge. The simulations predicted losses below 1 decibel even at an 8-micrometer offset, yet the measured tolerance was somewhat tighter. The authors attribute the discrepancy to the simplified model: the simulation assumed a uniform refractive index and constant width along the LISW, while the real structure likely varies in both, and its trajectory may mix straight and bent sections that shrink the effective bending radius. The two-dimensional model itself may also shift absolute loss values. Characterizing the actual structure is hard, since the waveguide is minuscule and removing all uncured resin without disturbing it is difficult. The residual 0.7-decibel loss appears to stem mostly from waveguide-related factors rather than resin absorption; transmission measurements through bulk-cured resin samples 200 to 400 micrometers thick differed by less than 0.1 decibel, and three-dimensional simulations of mode-field mismatch at the two fiber interfaces suggest a contribution of roughly 0.4 to 0.5 decibels that higher delta could further reduce.

Gap-dependence experiments added a further caution. At a fixed 8-micrometer offset, losses fell as the gap grew from small values toward about 250 micrometers, matching the simulations, but beyond 300 micrometers the loss climbed again, contrary to the model. At 350 micrometers the loss spiked sharply, driven by severe waveguide defects caused by insufficient curing power; raising the curing power from minus 15 to minus 12 dBm partially rescued the connection, though the loss still exceeded that at 250 micrometers. The physical picture is a tug-of-war: a larger gap gives diverging writing beams more room to overlap geometrically across a given offset, but it also dilutes their intensity, and past a threshold the light can no longer sustain polymerization. The maximum writable offset is therefore set jointly by the numerical aperture, gap, optical power, and resin photosensitivity, meaning that simply lengthening the gap is not a reliable recipe for both low loss and high tolerance.

The takeaway is a new design rule for the interconnects that future AI data centers will depend on. Rather than fighting for ever-finer alignment or stretching gaps until curing light fades, engineers can raise the index contrast of the self-written waveguide itself, letting tight optical confinement buy bending tolerance at short gaps. The work remains an early-stage demonstration on fiber pairs rather than full chip assemblies, and the authors note that optimizing formation conditions and resin parameters could push losses lower still. But the vision is compelling: optical connectors that assemble themselves out of liquid resin, guided by the very light they will one day carry, quietly bending around the imperfections of the machines that place them.

Subject of Research: High-refractive-index-contrast light-induced self-written waveguides for alignment-tolerant optical coupling

Article Title: High-alignment-tolerance optical coupling based on high-Δ light-induced self-written waveguides

Article References: Saito, Y., Shikama, K., & Sugihara, O. (2026). High-alignment-tolerance optical coupling based on high-Δ light-induced self-written waveguides. Results in Optics, Article 101179. https://doi.org/10.1016/j.rio.2026.101179

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101179

Keywords: light-induced self-written waveguides, optical coupling, alignment tolerance, silicon photonics, multicore fiber, photocurable resin, refractive-index contrast, FDTD simulation, optical interconnects, mode field diameter, S-bend waveguide, fiber-to-chip coupling

Top-emitting OLEDs are the workhorses of high-resolution active-matrix displays because their light exits away from the substrate, decoupling the emitting aperture from the transistor backplane beneath. They also lend themselves naturally to a Fabry–Perot microcavity, formed between a reflective bottom electrode and a semitransparent top cathode. The cavity narrows the electroluminescence spectrum and concentrates emission toward the forward direction, boosting frontal efficiency and color purity. But the same physics that sharpens the spectrum also makes extraction design unforgiving: much of the generated light ends up confined in waveguide modes traveling laterally through the organic stack and in surface plasmon polaritons hugging the metal electrodes. Fixing the air-side interface alone cannot recover those internal losses.

Conventional internal outcoupling structures, such as wrinkles, corrugations, and strongly curved microlens textures, scatter trapped modes back toward useful directions, but they carry a hidden cost. When the underlying texture is steep, the organic layers deposited on top fail to follow it conformally, so the local optical thickness of the cavity varies across the device and the resonance condition drifts from point to point. Stronger scattering then does not translate into a better device, because the cavity-defined forward emission benefit is simultaneously eroded. The key insight of the new work is that the geometry of an internal extraction layer must be judged not just by how much light it scatters but by how gently it does so.

The researchers call their solution a gentle internal corrugation layer, or GICL. Atomic force microscopy over a 50 by 50 micrometer area revealed a peak-to-valley height of about 1.12 micrometers with root-mean-square roughness of 144.5 nanometers, but the defining feature is the slope: representative surface angles of only 2.1 to 6.9 degrees along one axis and 1.8 to 3.5 degrees along the other, spread over lateral spans of nearly 9 micrometers. Cross-sectional electron microscopy showed the entire multilayer stack following this soft undulation without discontinuities. The contrast with reference structures is instructive. A steep imprinted random microlens array reached surface angles of 25.3 degrees with a peak-to-valley height of 7.87 micrometers, while one wrinkle reference achieved low roughness yet still exhibited angles above 53 degrees, demonstrating that roughness amplitude alone says little about geometric severity.

Finite-difference time-domain simulations confirmed that gentler is genuinely better. Among the compared structures, GICL yielded the highest relative total simulated power, outperforming both wrinkle references and the steep microlens array, even though the latter produced far stronger local field perturbation. Ray tracing sharpened the picture: GICL directed 77.3 percent of input power into the forward collection region, versus 43.5 percent for the microlens array, which instead diverted light into wide-angle, internally reflected, and backward trajectories. Under identical conditions, the calculated light extraction efficiency rose from 37.6 percent for the planar microcavity device to 74.1 percent with GICL, an increase of nearly a factor of two. An intermediate-index clear resin with a refractive index of 1.59 was applied at the outer surface to soften the air interface and let the redistributed light actually escape.

The emissive system matters as much as the geometry. In a microcavity, only the spectral portion of the emitter that overlaps the resonance band contributes efficiently to the enhanced external output, so a broad emitter wastes much of its spectrum on off-resonant wavelengths. The team used a phosphor-assisted thermally activated delayed fluorescence sensitization scheme, in which a TADF host and a phosphorescent assistant funnel excitation energy to a terminal multi-resonance TADF emitter called tCzphB-Fl, producing green emission with a full width at half maximum of only about 19 to 22 nanometers. When the microcavity was formed, the narrowband system’s peak shifted just 2 nanometers and its FWHM narrowed from 22 to 19 nanometers, whereas a conventional phosphorescent comparator shifted 13 nanometers and narrowed from 65 to 25 nanometers, evidence that most of the narrowband emission already sat inside the resonance band.

Device results bore out the combined strategy. All GICL devices shared the same 2.25-volt turn-on voltage and essentially unchanged current density, ruling out electrical effects, and the forward spectra showed only minor changes, ruling out spectral reshaping. Under matched resin conditions, adding GICL raised the external quantum efficiency from 34.8 to 69.2 percent, current efficiency from 302.7 to 378.4 candela per ampere, and power efficiency from 199.3 to 396.7 lumens per watt, with maximum luminance reaching nearly 120,000 candela per square meter. Across five samples the efficiencies averaged 68.2 plus or minus 0.7 percent, indicating good reproducibility. The angular color shift at 60 degrees off-axis also fell from 0.0197 to 0.0086 in CIE 1976 coordinates, and the luminance distribution became more Lambertian, both valuable for display uniformity.

Polarization-resolved measurements identified where the recovered light had been hiding. Because waveguide modes are predominantly transverse-electric like and surface plasmon polaritons are transverse-magnetic like, separating the s- and p-polarized emission provides a mode-sensitive diagnostic. Both polarized components, and the non-polarized total, increased across the viewing-angle range in the corrugated device, confirming genuine extraction rather than mere redistribution between channels. Quantitative modal analysis showed the outcoupled fraction rising from 29.7 to 38.6 percent, with the waveguide fraction dropping from 40.0 to 32.0 percent and the plasmon-related fraction changing only modestly. The roughly 30 percent simulated gain in outcoupled power matched the approximately 25 percent uncorrected device-level increase closely, lending quantitative coherence to the extraction interpretation.

Notably, the same corrugation helped the broad phosphorescent system far less, lifting its current efficiency by only about 15.7 percent compared with 25.0 percent for the narrowband system, a difference the authors attribute to spectral overlap with the cavity resonance rather than to bandwidth alone, since the two emissive chemistries differ in other respects as well. The gains did come with a small price: efficiency roll-off at high brightness was moderately larger in the corrugated devices, possibly reflecting local current crowding on the textured surface, and operational lifetime at 1,000 candela per square meter was slightly shorter, though still comparable at about 101 hours to 95 percent of initial luminance. The broader lesson stands: in microcavity OLEDs, internal texture geometry, outer-interface index matching, and emitter-resonance alignment must be engineered together, and gentleness, it turns out, extracts more light than force.

Subject of Research: Cavity-compatible internal light extraction in top-emitting microcavity OLEDs using a gentle corrugation layer and resonance-matched narrowband emission

Article Title: Cavity‐Compatible Light Extraction in Top‐Emitting Microcavity OLEDs via a Gentle Internal Corrugation Layer and Resonance‐Matched Narrowband Emission

Article References: Kim, Y. R., Nimbalkar, A., & Suh, M. C. (2026). Cavity‐Compatible Light Extraction in Top‐Emitting Microcavity OLEDs via a Gentle Internal Corrugation Layer and Resonance‐Matched Narrowband Emission. Advanced Science, Article e78013. https://doi.org/10.1002/advs.78013

Image Credits: AI Generated

DOI: 10.1002/advs.78013

Keywords: OLED, microcavity, light extraction, external quantum efficiency, corrugation, TADF, narrowband emission, waveguide modes, surface plasmon polariton, display technology, FDTD simulation, phosphorescence

The technique at the heart of modern overlay control is diffraction-based overlay, or DBO. Instead of imaging the wafer, a DBO tool shines light onto a specially designed grating target and measures how the light diffracts into positive and negative first orders. In the ideal case, the difference between the two diffraction signals is strictly proportional to the overlay shift between layers. This approach has largely displaced older image-based metrology because it offers superior precision and better resilience to tool-induced shifts. But the clean proportionality assumption breaks down in the real world. Processes such as chemical mechanical polishing and plasma etching leave their fingerprints on the grating targets: sidewalls that lean at different angles on the left and right, rounded corners, and imbalanced critical dimensions. These geometric imperfections generate spurious asymmetry signals that can masquerade as genuine overlay shifts, producing what metrologists call process-induced shift errors that quietly corrupt the measurement.

Hsieh and Yang frame this problem in an illuminating way: process-induced asymmetry is fundamentally an optical leakage problem. Structural perturbations of the grating generate diffraction asymmetry that has nothing to do with overlay, and this nuisance signal leaks into the measurement channel that is supposed to carry only overlay information. Their proposed remedy is to stop reducing the diffraction data to a single scalar asymmetry value at one wavelength, and instead capture the full broadband spectral response of the target. By recording the positive and negative first-order diffraction efficiencies for both TE and TM polarizations at 61 wavelengths spanning 400 to 700 nanometers, each measurement becomes a 244-dimensional vector. The physical premise is that wavelength-dependent diffraction acts as a fingerprint of the grating’s state, one rich enough to distinguish true overlay from the spectral distortions introduced by imperfect processing.

To test this premise rigorously, the researchers built a simulation framework based on the finite-difference time-domain method, using a nonuniform mesh with steps as fine as 0.25 nanometers and, in the critical asymmetric target region, 0.10 nanometers. They designed a hierarchy of three evaluation phases of increasing structural complexity. Phase I isolates a single nuisance mechanism: asymmetric sidewall angles, with the left and right sidewall angles of the bottom grating independently randomized between 80 and 90 degrees. Phase II adds top-corner rounding and global film-thickness variation on top of the sidewall asymmetry. Phase III moves to an advanced PolyFin spacer architecture, mimicking the multilayer dielectric environment of modern gate structures, with overlay plus eight independently varied nuisance variables including left and right poly critical dimensions, sidewall angles, spacer corner radii, and two thickness variations. The datasets grew from 932 samples in Phase I to 2,394 in Phase III, each sample generated by a separate full electromagnetic simulation.

On top of these simulated spectra, the authors posed the inverse problem: given only the raw 244-feature spectrum, with no geometry metadata, estimate the overlay. Crucially, they did not bet everything on deep learning. They compared linear regression, Ridge regression, RBF support vector regression, XGBoost, and a four-hidden-layer deep neural network, all trained on identical raw inputs with identical data partitions and validation-only model selection. This design deliberately separates two questions that are often conflated: how much overlay information does the broadband optical response actually contain, and how much does the choice of estimator matter? The results delivered a surprising verdict. In the noise-free Phase I dataset, plain linear regression achieved a root-mean-square error of just 0.0015 nanometers, meaning overlay was almost perfectly linearly decodable from the broadband spectrum. In the most complex Phase III scenario, linear regression and Ridge still reached 0.229 and 0.230 nanometers respectively, beating the DNN’s 0.374 nanometers and far outperforming XGBoost at 1.564 nanometers.

The contrast with conventional narrowband approaches was stark. A fixed single-wavelength reference at 550 nanometers collapsed to errors of roughly 8 to 9 nanometers under compound process imperfections, and even the best single wavelength managed only 5.2 nanometers in the PolyFin case. Multi-wavelength inputs changed the picture entirely, and a Ridge regression on paired-target broadband features reached 0.299 nanometers in Phase III. The authors are careful about what this does and does not prove. The deep network is a competitive nonlinear estimator, but the study explicitly does not establish universal DNN superiority; rather, it shows that broadband spectral observability, rather than model complexity alone, drives much of the performance. No inverse model, however expressive, can recover overlay information that is absent or locally confounded in the measured channels.

The team backed the headline numbers with an unusually thorough battery of robustness analyses. Empirical spectral diagnostics showed that the wavelength carrying the strongest overlay association shifts with the target: 485 nanometers in Phase I, 590 in Phase II, and 690 in Phase III. At individual wavelengths, the overlay response can become nearly parallel to a nuisance response, creating local ambiguity, but across the full 244-feature space the largest global overlay-geometry alignment in Phase III was a moderate cosine of 0.339, and in Phase I the overlay and sidewall-asymmetry responses were nearly orthogonal. Singular-value analysis revealed that the spectra are highly redundant, with only 4, 5, and 14 components explaining 95 percent of the training variance across the three phases, though the very large condition numbers warn that unregularized inversion can amplify small perturbations dramatically.

That warning proved prophetic in the noise stress tests. When trained on noise-free data and then confronted with test spectra perturbed by independent relative noise, the models diverged sharply. Unregularized linear regression, which had appeared almost magical on clean data, exploded to errors of tens of nanometers at just 1 percent noise, while Ridge and the DNN degraded far more gracefully; the Phase III DNN moved only from 0.374 to 0.406 nanometers at 1 percent noise. A blocked holdout test, in which the 20 percent of samples closest to the boundary of the simulated process space were withheld entirely, showed all models degrading by 35 to 68 percent relative to in-domain performance, a reminder that interpolation within a known process window does not automatically transfer to unseen conditions. Bootstrap resampling and repeated data partitions confirmed that the Ridge-versus-DNN difference was statistically robust for the fixed test set, while wavelength ablations showed that five evenly spaced combined-polarization wavelengths already deliver sub-nanometer accuracy, though matching the full-spectrum result to within 10 percent required all 61 labels.

The authors are candid that this is a simulation-based study with defined limits: the two-dimensional model excludes finite mark size, line-edge roughness, wafer-scale nonuniformity, and tool drift, and the noise model is a generic sensitivity test rather than a calibrated instrument specification. The sub-nanometer errors therefore establish feasibility within the evaluated simulations, not validated production-line accuracy. Yet the practical pathway they sketch is concrete. They propose a hybrid digital-twin workflow in which a convergence-verified FDTD library pretrains an inverse estimator, a deliberately selected set of measured calibration data adapts it to the real tool, active learning targets high-uncertainty regions, and continuous monitoring of residuals and spectral distance triggers fallback to reference metrology when drift or out-of-distribution conditions appear. They also point out that target pitch shapes which wavelengths carry overlay information, opening the door to co-designing grating targets and measurement bandwidths.

For an industry where every fraction of a nanometer of edge-placement error translates directly into yield and performance, the message of this work is quietly radical: the information needed to defeat process-induced overlay errors may already be present in the full spectrum of light bouncing off the wafer, waiting to be read properly. The finding that simple, well-regularized linear models can rival deep networks when given rich broadband inputs is a useful corrective to the assumption that harder problems always demand bigger models. As chipmakers push toward gate-all-around transistors and beyond, the combination of full-spectrum diffraction fingerprints, physics-guided simulation, and disciplined statistical validation may prove to be the microscope that keeps the nanoscale world in focus.

Subject of Research: Broadband diffraction-based overlay metrology for semiconductor lithography under process-induced structural asymmetry

Article Title: Broadband spectral metrology for robust diffraction-based overlay estimation under process-induced asymmetry

Article References: Hsieh, H.-C., & Yang, C.-S. (2026). Broadband spectral metrology for robust diffraction-based overlay estimation under process-induced asymmetry. Results in Optics, 25, Article 101156. https://doi.org/10.1016/j.rio.2026.101156

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101156

Keywords: overlay metrology, diffraction-based overlay, semiconductor manufacturing, broadband spectroscopy, FDTD simulation, process-induced shift, deep learning, ridge regression, lithography, nanometrology, digital twin, PolyFin spacer

The terahertz regime, which sits between microwaves and infrared light on the electromagnetic spectrum, has long promised transformative applications because it is non-ionizing, interacts distinctively with molecular and biological systems, and offers enormous bandwidth. Yet building compact, efficient, and broadband functional components in this range remains a formidable engineering challenge. Metamaterial absorbers, which use carefully engineered resonant structures to capture electromagnetic energy at subwavelength scales, have emerged as leading candidates. Conventional metallic versions, however, tend to suffer from limited bandwidth, weak tunability, and high sensitivity to geometric variations, particularly when multiple resonant modes are needed to cover a wide spectrum.

Graphene offers a way around these constraints. The atomically thin carbon sheet supports strongly confined surface plasmon polaritons, and its optical response can be actively controlled by adjusting its Fermi level through electrostatic gating or chemical doping. This means the resonance frequency and absorption characteristics of a graphene device can be modulated reversibly without changing its physical geometry. Earlier graphene-based terahertz absorbers relied on multilayer stacks, arrays of ribbons of different widths, or hybrid integration with phase-change materials such as vanadium dioxide. While effective, these approaches often demand complex fabrication, precise alignment, or dual tuning mechanisms, and thermally activated phase-change designs introduce challenges related to switching speed and power consumption.

The new design, developed by Hamzeh Sam Daliri, Hamed Saghaei, Hadi Dehbovid, and Reza Yousefi, consists of a three-layer metal-dielectric-graphene architecture. At the top sits a patterned graphene metasurface: a central multi-tip element inspired by the maple leaf, flanked by two rectangular graphene strips, together evoking the Canadian flag. The central pattern spans approximately 2.4 by 1.0 micrometers, with a patterned graphene area of about 0.84 square micrometers, while the lateral strips measure 1.25 by 2.75 micrometers. Beneath the graphene lies a dielectric spacer 6 micrometers thick with a refractive index of 1.4, and at the bottom a continuous gold film 1 micrometer thick acts as a back-reflector that suppresses transmission entirely.

The novelty lies not in the motif itself but in its functional plasmonic topology. The multi-tip central element and the lateral strips introduce multiple characteristic current-path lengths, sharp edges, and high-curvature regions within a single patterned plane. These features excite several interacting plasmonic modes whose controlled spectral overlap produces broadband absorption without vertically stacked resonators or hybrid material systems. The researchers traced the design’s evolution step by step: a single central resonator produced a narrow response, adding the two lateral strips broadened it by introducing additional current paths, and replacing the simple resonator with the multi-tip maple-leaf pattern further enhanced bandwidth through localized charge accumulation at its edges and tips.

The team simulated the structure using a three-dimensional finite-difference time-domain method, launching a broadband plane wave from 0.5 to 10 terahertz and applying periodic boundary conditions to model an infinite metasurface. Because the gold backplane eliminates transmission, absorptance reduces simply to one minus reflectance. Graphene was modeled as an atomically thin conductive sheet using the Kubo formalism, with an effective conductivity scaling factor representing multiple electronically decoupled sheets. An equivalent-circuit interpretation, in which each current path behaves as a lossy series RLC branch coupled to a grounded dielectric transmission-line section, explains why near-unity absorption occurs when the input impedance of the structure approaches that of free space, allowing incident energy to be fully dissipated in the graphene’s finite resistance.

A central finding of the parametric study is that absorption depends non-monotonically on graphene conductivity. For a single graphene layer, raising the chemical potential from 0.2 to 0.6 electronvolts boosted peak absorption from about 62 percent to 98 percent at 5.2 terahertz, close to a critical-coupling condition where impedance is optimally matched to free space. Pushing further to 0.8 electronvolts caused overcoupling, impedance mismatch, and a drop to roughly 85 percent. Similarly, increasing the effective number of graphene layers helped at low Fermi energies but degraded performance at higher ones: at 0.1 electronvolts, three- and four-layer structures achieved broadband absorption near 97 percent, whereas at 0.4 electronvolts and above, one- or two-layer configurations were clearly superior, with thicker stacks suffering detuning and destructive interference.

The dielectric spacer and lateral geometry proved equally consequential. Spacer thicknesses between 6 and 10 micrometers generally yielded absorption above 90 percent, while thinner cavities could not support well-developed resonant modes and thicker ones entered a regime of mode saturation and phase accumulation. For one optimized configuration with four effective graphene layers and a chemical potential of 0.6 electronvolts, an 8-micrometer spacer produced absorption approaching 100 percent across 4.8 to 8.2 terahertz. Low-index dielectrics such as polymethylpentene, PTFE, TOPAS, and PDMS emerged as suitable spacer candidates, and increasing the refractive index generally redshifted resonances while reducing peak absorption. Strip width sweeps showed that widths near 1.25 to 1.5 micrometers maximize plasmonic confinement at low-to-moderate chemical potentials, while narrower strips fail to sustain strong resonances.

Electric-field maps confirmed the physical picture: the strongest field confinement and energy concentration occurred in the 4 to 6 terahertz range, with pronounced hotspots along the edges, tips, and high-curvature regions of the maple-leaf pattern, precisely where charge accumulates and plasmonic modes localize. The authors emphasize that the high absorption should not be confused with the roughly 2.3 percent single-pass absorption of isolated monolayer graphene; here, plasmonic patterning, cavity enhancement, a reflective ground plane, and impedance matching combine to multiply light-matter interaction dramatically. Compared with prior broadband absorbers, the design meets a stricter 95 percent absorption criterion over a 4-terahertz interval within a single patterned plane, avoiding both stacked resonator architectures and thermally activated phase-change materials.

The researchers position the device as a performance-complexity compromise rather than an absolute bandwidth record, and its practical significance lies in the transferable design guidelines it establishes: electronic conductivity, resonator geometry, and cavity parameters must be jointly optimized, since excessive values of any single parameter transform the absorber into a reflector. With electrical tunability achieved through gate voltages that set the graphene chemical potential, and with compatibility with planar fabrication processes, the flag-inspired metasurface points toward reconfigurable terahertz systems for sensing, imaging, filtering, electromagnetic shielding, and future wireless communication networks.

Subject of Research: Electrically tunable broadband terahertz absorption using a Canada-flag-inspired patterned graphene metasurface

Article Title: Tunable broadband terahertz absorber based on a Canada-flag-inspired graphene metasurface

Article References: Sam Daliri, H., Saghaei, H., Dehbovid, H., & Yousefi, R. (2026). Tunable broadband terahertz absorber based on a Canada-flag-inspired graphene metasurface. Results in Optics, 25, Article 101157. https://doi.org/10.1016/j.rio.2026.101157

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101157

Keywords: terahertz absorber, graphene metasurface, surface plasmon polaritons, broadband absorption, electrical tunability, impedance matching, FDTD simulation, metamaterial, plasmonic resonance, terahertz sensing, dielectric spacer, Kubo formalism

The motivation behind the work is the growing mismatch between the relentless growth of global data and the physical limits of electronic memory. Conventional silicon-based architectures, despite decades of refinement, are approaching hard boundaries in scalability, speed, and power density. All-optical memories, which encode, store, and retrieve information entirely within the optical domain, promise high transfer rates, low latency, and favorable energy efficiency. Yet each competing approach carries drawbacks: silicon-on-insulator platforms suffer from high losses, localized surface plasmon resonance structures are limited by ohmic losses and fabrication complexity, photonic crystals demand extreme fabrication precision, and Kerr nonlinear materials require high optical intensities and are sensitive to noise. Phase-change materials stand apart because they retain data even after the light source is removed, thanks to their non-volatile nature and high optical contrast between structural states.

The heart of the new device is a dumbbell-shaped resonator: a rectangular cavity flanked by two semi-disk resonators, coupled to MIM input and output waveguides carved in silver with air serving as the dielectric. Surface plasmon polaritons, hybrid waves of light and collective electron oscillations, are confined to the metal-dielectric interfaces and squeezed into regions far smaller than the wavelength of light. The team modeled the optical response of silver with the Drude formalism and validated the design using two-dimensional finite-difference time-domain (FDTD) simulations with a 3-nanometer mesh and perfectly matched layer boundaries. As a passive filter, the structure exhibits a Gaussian transmission peak at 1210 nanometers with a peak transmittance of 91 percent, a full width at half maximum of 56 nanometers, and a quality factor of about 21.

Geometry proved to be the decisive lever in tuning performance. Through systematic parametric sweep simulations, the researchers found that the radius of the semi-disk resonators dominates the resonance wavelength, shifting it by 2.22 nanometers for every nanometer of radius change, while the width of the rectangular cavity induces a blueshift at a sensitivity of minus 0.69 nanometers per nanometer, and the cavity length contributes a modest redshift of 0.3 nanometers per nanometer. The coupling gap between the waveguides and the resonator, varied from 0 to 18 nanometers, showed that transmission degrades progressively as the gap widens, because the evanescent field that transfers energy between the waveguide and the cavity decays exponentially with distance. Notably, the optimal configuration is a zero-gap, monolithic design, which sidesteps the notoriously difficult requirement of maintaining sub-10-nanometer alignment during fabrication.

Transforming this filter into memory required integrating a strip of GST, measuring 500 by 190 nanometers, into the central resonator and introducing a second optical pathway. A control signal, delivered from above the chip by a laser, programs the memory: a SET pulse of 50 milliwatts lasting 150 nanoseconds, roughly 7.5 nanojoules, gently heats the material above its crystallization temperature of about 450 kelvin, converting it from the amorphous to the crystalline state. A RESET pulse of 110 milliwatts for 20 nanoseconds, about 2.2 nanojoules, melts the material at roughly 880 kelvin before it rapidly quenches back into the amorphous phase. Crucially, this programming is spatially and functionally decoupled from the readout, which uses a low-power probe signal sent through the MIM waveguide, too weak to disturb the stored phase state.

The readout mechanism exploits the dramatic optical difference between the two phases of GST. In the amorphous state, the refractive index is 3.94 with low absorption, and the device transmits a resonance peak at 1912 nanometers with 95 percent transmittance, corresponding to logic state 1. In the crystalline state, both the real and imaginary refractive indices rise sharply to 6.11 plus 0.83i, absorption soars, and transmission drops to essentially zero, registering logic state 0. The contrast between the states reaches 95 percent, the extinction ratio reaches 25 decibels, and the insertion loss is a remarkably low 0.19 decibels in the amorphous state versus 25.2 decibels in the crystalline state. Because the phase persists without power, the bit is genuinely non-volatile, classifying the device within the PRAM and NVRAM families of memory.

The team went beyond static spectra and performed transient femtosecond-scale analysis using a square excitation pulse. In the amorphous state, the output rises to 95 percent of the input within about 200 femtoseconds after a 7-femtosecond propagation delay, with rise and fall times of 82 and 84 femtoseconds respectively, before ohmic losses drain the signal by roughly 1100 femtoseconds. In the crystalline state, the output settles at only about 4 percent, an OFF regime with minimal leakage. A reference simulation without the GST-loaded resonator confirmed that the MIM waveguides themselves preserve signal integrity with negligible distortion, meaning any temporal modification originates from the light-matter interaction inside the resonator. The researchers note that while these read dynamics reflect the electromagnetic impulse response rather than the slower phase-change kinetics, existing experimental literature supports GST switching frequencies between 1 and 10 megahertz with endurance up to one million cycles.

Perhaps the most forward-looking aspect of the design is its neuromorphic potential. The optical transmittance of the resonator is governed by the complex refractive index of the GST layer, and GST is well documented to undergo fractional crystallization, passing through intermediate states between fully amorphous and fully crystalline. Each intermediate state yields a distinct transmission level, which can be mapped onto an analog synaptic weight. The high-transmission amorphous state corresponds to a potentiated synapse, while the low-transmission crystalline state represents a depressed one. The authors frame the device as a scalable building block for all-optical neuromorphic synapses, potentially enabling spike-timing-dependent plasticity through tailored pulse-programming sequences in future circuits, thereby merging logic and memory on a single photonic platform in a departure from conventional von Neumann architectures.

Practical integration also received attention. The device is compatible with back-end-of-line CMOS processes, relying on standard techniques such as electron-beam evaporation, sputtering, electron-beam lithography, and reactive ion etching. The monolithic zero-gap resonant cavity can be patterned as a single continuous unit, eliminating fragile alignment steps, and sensitivity analysis confirms that performance peaks precisely at this configuration. Ultra-thin passivation layers of silicon dioxide or aluminum oxide can shield the silver from oxidation without compromising plasmonic confinement, while the high thermal conductivity of the silver substrate acts as a passive heat sink that, combined with nanosecond control pulses, suppresses thermal crosstalk. The authors acknowledge that the design deliberately prioritizes high transmission over an ultra-high quality factor, because the active GST layer introduces losses during switching and a strong baseline signal is essential for reliably distinguishing the two logic states. Compared against a broad field of plasmonic and photonic-crystal memories built on GST and Kerr materials, the proposed cell posts the highest transmission contrast in its class while remaining among the most compact, suggesting that light-written, power-free memory at the nanoscale may be moving from concept toward chip-ready reality.

Subject of Research: A non-volatile plasmonic memory based on an MIM waveguide and GST phase-change material for binary logic and neuromorphic computing

Article Title: Design of a non-volatile Plasmonic Memory Based on MIM waveguide and phase-change Materia for binary logic and neuromorphic integration

Article References: Kehtarmanesh, M., Keshavarzi, P., & Danaie, M. (2026). Design of a non-volatile Plasmonic Memory Based on MIM waveguide and phase-change Materia for binary logic and neuromorphic integration. Results in Optics, 25, Article 101141. https://doi.org/10.1016/j.rio.2026.101141

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101141

Keywords: plasmonics, phase-change material, GST, non-volatile memory, MIM waveguide, optical memory, neuromorphic computing, photonic integrated circuits, FDTD simulation, optical contrast, synaptic weight, CMOS compatibility

The architecture is deceptively simple in outline. The device follows a nip configuration, beginning with a transparent zinc oxide cathode 100 nanometers thick, followed by a 60-nanometer cadmium sulfide electron transport layer, a 500-nanometer active layer of the semiconducting polymer blend PTB7:PC71BM, an 80-nanometer zinc tungstate hole transport layer, and a gold back contact. What distinguishes the design is what happens inside the electron transport layer: instead of a flat film of CdS, the researchers patterned it into four parallel rectangular ridges, each 450 nanometers tall and 26 nanometers wide, with a grating period of 273.2 nanometers. These gratings penetrate the polymer absorber and act as continuous electron highways, shortening the distance carriers must travel before collection while simultaneously increasing the optical path length of light bouncing through the active layer.

Into the spaces between these gratings, the team embedded a double layer of gold nanospheres with radii ranging from 50 to 105 nanometers. The choice of gold is not arbitrary. When illuminated, the conduction electrons in each nanoparticle oscillate coherently at the particle surface — a phenomenon known as localized surface plasmon resonance, or LSPR. This resonance produces intense local electromagnetic fields and strong far-field scattering, both of which can funnel otherwise lost photons back into the absorber. Isolated gold nanospheres typically resonate around 520 nanometers, deep in the visible range, but the researchers showed that the strong near-field coupling between the two nanoparticle layers, combined with the high refractive index environment of the surrounding CdS grating, shifts and broadens the resonance into the near-infrared region between roughly 650 and 1100 nanometers — precisely where the PTB7:PC71BM polymer is weakest at absorbing on its own.

The bimodal spacing of the nanoparticles proved critical. Inter-particle distances vary from less than 70 nanometers up to 110 nanometers, deliberately combining strong near-field plasmonic coupling in closely packed regions with efficient far-field scattering from more widely separated particles. According to Mie scattering theory, the amount of light scattered by a nanoparticle scales with its size, which is why the team swept radii from 50 to 105 nanometers in their optimization. Their tailored parametric algorithm — chosen over machine learning approaches because the complex electromagnetic interactions of the plasmonic-grating hybrid exceed the predictive reach of models trained on conventional flat devices — identified configurations that produce peak absorption in the 730 to 860 nanometer band and sustained absorption out to 1000 nanometers. Finite-difference time-domain simulations revealed local field enhancements of up to 25 times at the nanoparticle surfaces, contributing an additional 6.77 milliamperes per square centimeter to the short-circuit current density.

Plasmonic nanoparticles are not without drawbacks, and the researchers confronted them directly. Metal nanoparticles can introduce parasitic absorption and serve as recombination centers, and previous experimental studies have reported performance losses of 15 to 30 percent from such effects. In the proposed architecture, however, the strategic placement of the double-layer nanoparticles between the CdS gratings, along with the sub-wavelength 273.2-nanometer grating period, suppresses parasitic losses to less than 2 percent. The gratings also screen trapped charges at the gold–polymer interface, reducing the interface recombination velocity from 100,000 to 1,000 centimeters per second, while the modeled surface trap density of 10^12 per square centimeter produces only a negligible 0.02-volt drop in open-circuit voltage. Because the grating period is sub-wavelength and gold coverage is discontinuous, propagating surface plasmon polaritons are not excited, keeping the enhancement firmly in the localized, beneficial regime.

The electrical results are striking. The short-circuit current density rises from 24.91 milliamperes per square centimeter for the grated CdS structure without nanoparticles to 31.68 milliamperes per square centimeter once the gold nanospheres are added. Open-circuit voltage reaches 1527 millivolts, a value the authors attribute to excellent energy level alignment among the grated CdS electron transport layer, the PTB7:PC71BM absorber, and the zinc tungstate hole transport layer, combined with suppressed non-radiative recombination. The conduction band offset of 0.15 electron-volts between CdS and PC71BM facilitates efficient electron injection while blocking hole back-transfer, and the small 0.15 to 0.25 electron-volt offset between the absorber’s HOMO level and the valence band of ZnWO4 provides a strong driving force for hole extraction. The fill factor holds at a remarkable 91 percent for both configurations, and the power conversion efficiency climbs from 34.66 percent to 44.02 percent.

External quantum efficiency measurements from the simulations underscore where the gains come from. In the near-infrared band between 800 and 1070 nanometers, the nanoparticle-enhanced structure reaches an EQE of approximately 62 percent at 930 nanometers, compared with a mere 3 percent for the grated structure without gold. Enhancements also appear across the 300 to 430 nanometer and 530 to 790 nanometer bands, driven by the increased contact area between the CdS gratings and the polymer absorber. Overall absorption in the 850 to 1150 nanometer range improves by more than 50 percent relative to the initial grated design. The team validated their model by comparing predicted parameters against experimental literature values for PTB7:PC71BM devices, acknowledging that the simulated efficiency represents a theoretical ceiling achievable only under ultra-low defect conditions that remain challenging to realize at scale.

Fabrication feasibility was a central consideration throughout the study. The authors propose a hybrid manufacturing route in which the zinc oxide, PTB7:PC71BM, and zinc tungstate layers are deposited by slot-die coating — a scalable, roll-to-roll-compatible technique in which ink forms a controlled meniscus between a slot die and the substrate, governed by the interplay of viscous, gravitational, and shear forces. The CdS grating would be patterned by nanoimprint lithography, while the double-layer gold nanospheres could be realized through controlled electrodeposition or colloidal spin-coating. Recent demonstrations of similarly processed devices reaching 21.87 percent efficiency suggest the pathway to experimental validation is realistic, though the authors caution that annealing temperatures, process tolerances, and yield analysis will require experimental optimization.

Thermal stability remains an open challenge. The mismatch in thermal expansion coefficients among the layers is relatively small, but localized plasmonic heating around the gold nanoparticles and the inherent sensitivity of the organic active layer could induce mechanical stress and long-term degradation under prolonged solar exposure at operating temperatures of 50 to 70 degrees Celsius. The researchers propose thin encapsulation layers of PMMA or glass, buffer layers with matched thermal expansion, and — in future work — replacing the fullerene-based PTB7:PC71BM absorber with more thermally robust non-fullerene acceptors. They also note that the design deliberately avoids machine learning optimization for now, arguing that full-wave electromagnetic modeling combined with drift–diffusion electrical simulation offers physically transparent insight into plasmonic hot-spot distribution and recombination suppression that black-box models cannot yet provide, though machine learning-assisted global optimization is planned for future refinements.

The broader significance of the work lies in its demonstration that photonic and plasmonic strategies, often studied in isolation, can be engineered to reinforce one another. By coupling the diffractive light-trapping of a periodic CdS grating with the resonant field concentration of a bimodal gold nanosphere array, the design extracts useful current from a spectral region that most single-junction cells abandon. Whether the simulated 44.02 percent efficiency can survive contact with real-world fabrication imperfections is the question that will define the next chapter of this research, but the numerical evidence presented offers a concrete, physically grounded roadmap — one that could reshape expectations for what polymer solar cells, with their low-cost solution processing and mechanical flexibility, might ultimately deliver in the global transition away from fossil fuels.

Subject of Research: Plasmonic enhancement of polymer solar cells using gold nanosphere arrays and grated CdS electron transport layers

Article Title: Achieving higher efficiency in solar panels using gold nanosphere arrays in structure of Grated CdS and advanced interfacial layers

Article References: Kianjo, S., & Nooramin, A. (2026). Achieving higher efficiency in solar panels using gold nanosphere arrays in structure of Grated CdS and advanced interfacial layers. Results in Optics, 25, Article 101140. https://doi.org/10.1016/j.rio.2026.101140

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101140

Keywords: solar cells, gold nanoparticles, plasmonics, LSPR, CdS grating, PTB7:PC71BM, FDTD simulation, near-infrared absorption, power conversion efficiency, zinc tungstate, slot-die coating, nanoimprint lithography