For decades, engineers have faced an uncomfortable trade-off in meeting that demand. Mechanical scanning systems, built around gimbaled mirrors and rotating assemblies, can cover very large angles, but they carry the penalties of size, weight, power consumption, and moving parts that wear out and drift out of alignment. Chip-based optical phased arrays, by contrast, eliminate almost all mechanical motion by electronically controlling the phase of light emitted from many tiny antennas. Yet most phased arrays scan widely in only one direction. Extending them to true two-dimensional operation typically requires densely packed arrays and large numbers of individually controlled elements, which drives up electrical complexity, fabrication difficulty, and energy consumption. The result has been a persistent gap between what free-space optical systems need and what chip-scale technology has been able to deliver.

A research team led by Professor Juejun Hu’s group at the Massachusetts Institute of Technology, working with the Singapore-MIT Alliance for Research and Technology and collaborators, has now demonstrated a hybrid photonic platform that closes much of that gap. Reported in the journal Opto-Electronic Advances, the platform achieves two-dimensional beam steering across a measured field of view of 161 degrees while maintaining nearly diffraction-limited beam quality across a broad angular range. The work was made available online on August 23, 2026, in the Early View section of the journal, and it represents a distinctly different architectural philosophy from the phased-array mainstream.

The key insight behind the new platform comes from an unexpected direction: optical projection. Rather than controlling a dense array of optical antennas with thousands of independently tuned phase shifters, the system uses a silicon photonic integrated circuit as a light-routing engine. The circuit directs light through an on-chip network of switches to a selected output waveguide. At that output, a freeform micro-optical reflector turns the guided light upward, out of the chip plane, and shapes it into a clean free-space beam. In effect, the chip behaves like a projector that selects which of several pre-defined optical paths the light will follow, rather than an antenna array that synthesizes a direction wavefront by wavefront.

The second stage of the architecture is where the wide-angle steering happens. After leaving the reflector, the beam passes through a metasurface—a flat optical element patterned with nanoscale structures that impose precise, position-dependent changes on the light. According to Professor Hu, the position of the selected reflector determines the direction in which the metasurface sends the beam, so choosing a different optical path on the chip directly produces a different steering direction. This division of labor is the heart of the design. The photonic integrated circuit handles the discrete selection of output paths with low complexity, while the metasurface performs the heavy optical work of redirecting and correcting the beam across an ultrawide angular range.

Building the platform required weaving together several fabrication traditions that rarely meet on a single substrate. The team combined analytical optical design, numerical modeling, established semiconductor manufacturing, three-dimensional microprinting, and nanofabrication. The photonic integrated circuit itself was produced using a standard silicon photonics foundry process, meaning the light-routing backbone could in principle be manufactured with the same mature industrial tooling that produces commercial silicon photonic chips. The freeform reflectors, which must efficiently transform the guided optical mode traveling inside a waveguide into a Gaussian-like free-space beam, were designed for that conversion task and then printed directly onto the chip in three dimensions, a technique that allows curved micro-optical geometries impossible with conventional planar lithography.

The metasurface followed a similarly disciplined design pipeline. Its pattern was first established through an analytical design framework, giving the team a physically grounded starting point, and was then refined using optical simulations specifically aimed at reducing aberrations at large steering angles. Aberration control is critical here: when a beam strikes a flat optic far off its nominal axis, simple lenses and gratings tend to smear the focus and distort the wavefront. By optimizing the nanoscale pattern against these off-axis effects, the researchers ensured that beams steered toward the edges of the field of view would emerge nearly as clean as those near the center. After careful assembly and alignment of the chip, reflectors, and metasurface, the complete system was tested at telecommunications wavelengths, the near-infrared band used by fiber networks and many free-space optical links.

The measured results are what make the demonstration notable. The system steered a beam in two dimensions across a 161-degree field of view—an extraordinarily wide cone for any chip-scale device—while keeping the output nearly diffraction-limited, meaning the beam spread was close to the fundamental physical limit set by the aperture rather than degraded by optical imperfections. Because steering is achieved by switching among discrete reflector positions rather than continuously phasing an antenna array, the control electronics remain comparatively simple, avoiding the dense wiring and high power draw that plague two-dimensional phased arrays. The trade-off is that the platform steers among a set of discrete directions, but the researchers see that as a starting point rather than a ceiling.

The architecture, in fact, offers several clear paths toward even better performance. Compact piezoelectric translation could add fine or continuous steering between the discrete directions provided by the current reflector array, blending the simplicity of the switching approach with the smooth coverage of a scanner. Lower-loss photonic foundry processes, improved assembly alignment, and direction-specific metasurface optimization could each raise the overall optical efficiency, putting more of the laser’s power into the steered beam. Increasing the effective aperture, or pairing the platform with beam-expanding optics, would narrow the output beam further, which is essential for long-distance links where divergence over kilometers of free space determines whether enough light arrives at the receiver.

Professor Hu notes that these advances will build on the demonstrated wide field of view and high beam quality, moving the technology toward scalable optical projectors for communications, sensing, and other applications requiring agile, wide-angle control of light. If the hybrid approach matures along those lines, the implications reach across the free-space photonics landscape: inter-satellite optical links that steer across the sky from a chip smaller than a coin, airborne LiDAR with no moving gimbal, and point-to-point Li-Fi communication that can retarget a data beam instantly. A patent based on the technology has been filed by 2Pi Inc., signaling that the researchers see a commercial future for a platform that turns beam steering from a mechanical problem into a manufacturing one.

Subject of Research: Chip-scale hybrid photonic integrated circuit and metasurface platform for ultrawide-angle two-dimensional optical beam steering

Article Title: New chip-scale optical platform enables high-quality ultrawide beam steering

Article References: New chip-scale optical platform enables high-quality ultrawide beam steering. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: beam steering, photonics, metasurface, photonic integrated circuit, silicon photonics, free-space optics, LiDAR, optical communications, micro-optics, nanofabrication, Li-Fi, optical phased arrays

The research, carried out by Xiaofei Cui of TWI Limited together with Akram Zitoun and Sadik Omairey of the Brunel Composites Centre at Brunel University London, examines a photonic-integrated sensor system developed under the European Union-funded SEER project. The sensor is designed to be embedded directly within composite tooling, where it monitors refractive index, temperature, and pressure in real time. These three parameters are the vital signs of composite production: refractive index reveals how the resin is flowing and curing, temperature governs the chemistry of the cure cycle, and pressure indicates whether the part is being consolidated properly. Because the sensor sits inside the tool surface, it can detect anomalies as they happen rather than after the part has been removed and inspected, offering the prospect of fewer scrapped components and more repeatable processes.

At the heart of the device is a photonic-integrated circuit, or PIC, fabricated using silicon-on-insulator technology. The prototype chips were produced through a multi-project wafer run, starting from a 725-micrometre silicon substrate topped with a 2-micrometre buried oxide layer. Two types of silicon waveguides, each 220 nanometres thick, guide light through the chip: a strip waveguide and a rib waveguide with a 70-nanometre etched slab. A 520-nanometre low-temperature oxide layer and a thin silicon nitride cap complete the stack. Light travelling through these waveguides interacts with its surroundings, so changes in the refractive index of the resin, or in temperature and pressure at the sensor surface, shift the optical signal in ways that can be read out with high precision. The chip is then post-processed, with reactive ion etching or wet etching used to create the refractive index sensing features and anisotropic etching or substrate bonding used to form the pressure-sensing membrane.

What makes the study unusual is that the authors did not stop at demonstrating that the sensor works. They built a combined life cycle costing and life cycle assessment framework to evaluate the device from prototype fabrication through to industrial scale-up. The economic side used a hybrid bottom-up and activity-based costing model, in which the total cost of production is the sum of the costs of every individual manufacturing activity, each broken down into labour, materials, energy, and equipment. The environmental side was modelled in GaBi software in accordance with the ISO 14040 and ISO 14044 standards, taking a single sensor as the functional unit and covering raw material supply, manufacturing, distribution, use, and end-of-life pathways including reuse and landfill.

The cost findings are striking. Across the prototype stage, labour was the dominant expenditure in both recurring and non-recurring costs, dwarfing the contributions of materials, energy, and equipment. At the process level, assembly dominated the recurring costs, substantially exceeding PIC fabrication, testing and validation, and coating. This is not surprising when the assembly sequence is examined in detail: the PIC with its micro-lens must be manually inserted into a metal adapter, a separate lens mounted on a ceramic ferrule holding the optical fibre must be aligned using a six-degree-of-freedom positioner, and each joint must be secured with epoxy and cured. These delicate optical alignment steps are precisely the operations that photonic packaging engineers identify as the bottleneck in moving integrated photonics from the laboratory to volume production.

To understand how costs might evolve as production scales up, the team applied learning curve analysis, a well-established manufacturing economics technique that captures how labour hours per unit fall as workers gain experience and processes mature. The analysis assumed learning rates of 80, 85, and 87 per cent, drawn from the literature, and revealed a pattern familiar from aircraft and semiconductor manufacturing. Early gains are dramatic: at low volumes, suitable investment can improve productivity by around one per cent per unit. But the improvements saturate quickly. After the 191st unit, the potential gain drops to just 0.1 per cent, and after the 365th unit it falls to 0.05 per cent. Beyond that endpoint, further investment in training and tooling will not pay for itself, a conclusion with direct consequences for how manufacturers should time their automation investments.

The sensitivity analysis added a geopolitical dimension to the economics. Labour accounted for 77 per cent of total recurring costs in the scale-up scenario, so regional wage differences matter enormously. Applying Eurostat data from 2022, which showed average hourly labour costs ranging from €8.2 in Bulgaria to €50.7 in Luxembourg and €55.6 in Norway, the researchers found that in high-wage countries labour represents more than 80 per cent of total production costs, while in lower-wage regions it falls to around 40 per cent. For a technology whose commercial viability hinges on labour-intensive assembly, decisions about where to manufacture and how much to automate could determine whether embedded photonic sensing reaches the factory floor at all.

The environmental assessment told a complementary story. PIC fabrication and post-processing emerged as the most energy-intensive stages, together accounting for more than 63 per cent of the total energy required to manufacture and use the sensor, a burden attributed to the energy-hungry clean-room operations and the complexity of photonic device fabrication. The same fabrication stage also returned the highest global warming potential and the highest ozone depletion rate in the life cycle impact results, partly because prototype production was performed manually and involved multiple research iterations. At the other extreme, sensor testing and validation consumed less than two per cent of the total energy, using small, low-emission equipment. Downstream processes such as coating, packaging, and integration contributed minimally to the overall footprint.

Perhaps the most valuable insight of the study is that the economic and environmental hotspots largely coincide. The labour-intensive assembly operations that dominate recurring costs and the PIC fabrication and post-processing stages that dominate energy demand and emissions are the same targets that automation, wafer-level batch processing, and improved first-time-right manufacturing would address. The authors recommend focusing automation specifically on the manual insertion, alignment, epoxy dispensing, and packaging steps; prioritising reductions in repeated prototype iterations during chip fabrication; comparing etching routes not only technically but in terms of energy demand and yield; and concentrating investment in productivity improvements before the learning curve saturates. They also caution against over-investing in environmental improvements to testing and validation, which is simply too small a contributor to matter.

The broader significance extends beyond one sensor design. As industries under pressure to decarbonise turn to lightweight composites, and as smart manufacturing promises to squeeze waste and defects out of production, embedded sensing is likely to become standard practice. But this study demonstrates that the sustainability case for such technologies cannot be assumed; it must be quantified. By showing that cost and carbon concentrate in the same early manufacturing stages, and that learning curves, labour rates, and automation strategies jointly determine commercial viability, the researchers have provided a reusable framework for evaluating emerging photonic sensing technologies as they make the difficult transition from research prototype to industrial product. For composite manufacturers weighing whether to embed intelligence in their tooling, the answer, it turns out, depends as much on economics and energy as on optics.

Subject of Research: Environmental and economic life cycle assessment of an embedded photonic sensor system for real-time composite manufacturing monitoring

Article Title: Environmental and economic assessment of an embedded photonic sensor system for composite manufacturing monitoring

Article References: Cui, X., Zitoun, A., & Omairey, S. (2026). Environmental and economic assessment of an embedded photonic sensor system for composite manufacturing monitoring. Clean Technologies and Environmental Policy, 28(10), Article 265. https://doi.org/10.1007/s10098-026-03614-5

Image Credits: AI Generated

DOI: 10.1007/s10098-026-03614-5

Keywords: composite manufacturing, photonic sensors, photonic integrated circuits, life cycle assessment, life cycle costing, learning curve, industrial automation, sustainability, silicon photonics, process monitoring, labour costs, clean technology

The inspiration for the work came from an unexpected corner of chemistry. In molecular topological insulators built on metal–organic frameworks, the individual molecular building blocks can rotate within the lattice, and those rotations measurably alter optical and mechanical behavior. What remained largely unexplored, however, was how such rotational freedom affects topology itself. The Tsinghua group saw an opportunity: if molecular rotation could be transplanted into a photonic crystal, it would provide a clean, controllable knob for studying how disorder of a very specific geometric kind interacts with topological states. Rather than treating disorder as a nuisance to be minimized, the researchers made it a central variable of the design.

The resulting architecture is elegantly simple to describe. In a valley photonic molecular crystal, optical molecules, each a small cluster of silicon structures engineered to support valley-hall-like physics, sit on fixed and periodic lattice sites, exactly as atoms do in a conventional molecular crystal. The twist is that each molecule retains a randomly distributed orientation. The lattice positions stay ordered while the rotational degrees of freedom are scrambled. This separation is what makes the platform so useful. It isolates rotational disorder from positional disorder, allowing the team to ask a precise question: how much random rotation can a valley-topological system absorb before its protected edge states fail?

Answering that question required abandoning the standard toolkit. Band theory and topological invariants such as Chern numbers are defined in momentum space, which only exists when the structure is periodic. Random rotations destroy that periodicity, so the conventional characterization of valley topology becomes inapplicable. The researchers responded by constructing an effective Hamiltonian approach that captures the disordered system through an averaged Dirac mass and an effective valley Chern number. In parallel, they employed a spectral localizer analysis, a real-space method that certifies topology without any appeal to translational symmetry. The agreement between these two frameworks gave the team confidence that the edge states they observed were genuinely topological rather than accidental.

Before fabricating anything, the group worked through the periodic case first. They showed that molecular orientation itself determines the topological properties of an ordered valley photonic molecular crystal, establishing a baseline from which disorder could be introduced. This step matters because it reframes orientation as an additional design dimension. In ordinary photonic crystals, the topology is fixed by the geometry of the unit cell; here, the orientation of each molecule becomes a degree of freedom that can, in principle, be programmed. The effective Hamiltonian then extends this picture to the disordered regime, where orientations vary from site to site.

The experimental validation took place on silicon chips, the workhorse platform of integrated photonics. Simulations showed light propagating along a Z-shaped interface even under substantial rotational disorder, tracing a path that a conventional waveguide would struggle to maintain in the presence of such geometric scrambling. Measured transmission spectra backed this up, retaining the high-transmission peaks that signal the presence of topological edge states. Crucially, these signatures persisted as the disorder range was pushed to 60 degrees, a level of global rotational randomness that would catastrophically degrade transport in non-topological structures of comparable design.

The study did not stop at confirming robustness. The researchers also examined the competition between topological edge states and Anderson localization, the phenomenon by which strong disorder traps waves and halts transport altogether. This regime, where protection and localization fight for dominance, is among the most actively debated areas in disordered topological matter. By tuning the rotational disorder range, the platform provides a way to watch that competition unfold in a controlled laboratory setting, with the added advantage that photonic systems allow direct imaging of the propagating fields.

One of the most consequential aspects of the work is its generality. The framework developed by the Tsinghua team is not tied to the specific molecular geometry used in their devices; it applies to all molecules with threefold rotational symmetry, the C3 symmetry class that underlies valley physics. That means the effective Hamiltonian and real-space characterization methods can be carried over to any C3-symmetric building block, whether photonic, electronic, or mechanical. The authors suggest the approach could inspire analogous studies in condensed matter systems, acoustic lattices, and cold-atom platforms, where controlled rotational disorder is equally within experimental reach.

The practical implications reach toward programmable topological photonics. Because molecular orientation acts as an additional handle on topological properties, future devices could conceivably encode synthetic gauge fields, including pseudo-electric and pseudo-magnetic fields, by patterning the rotational landscape rather than the physical geometry. Such programmability has been a long-sought goal in the field, since it would allow a single fabricated chip to realize many different topological phases. The Tsinghua demonstration, published under DOI 10.1007/s44519-026-00017-3 by the Chinese Society for Optical Engineering, lays the conceptual and experimental groundwork for that ambition on a silicon platform compatible with existing fabrication infrastructure.

Beyond the immediate results, the study signals a broader shift in how disordered topological systems can be characterized. The combination of an averaged Dirac mass, an effective valley Chern number, and spectral localizer analysis forms a framework that works entirely without structural periodicity, addressing a long-standing gap between the theory of topological phases and the messy reality of fabricated devices. As integrated photonic circuits grow more complex and fabrication tolerances become a genuine constraint, tools that certify topological protection in the presence of real-world disorder are likely to become indispensable. The valley photonic molecular crystal, born from a chemical analogy and validated on a silicon chip, may prove to be one of the templates that the next generation of robust and programmable optical platforms is built upon.

Subject of Research: Topological robustness of valley photonic molecular crystals under rotational disorder on a silicon platform

Article Title: Valley photonic molecular crystals

Article References: Valley photonic molecular crystals. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: topological photonics, valley Hall effect, photonic crystals, rotational disorder, silicon photonics, edge states, Chern number, Anderson localization, molecular crystals, spectral localizer, integrated photonics, Tsinghua University

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

To understand why this matters, it helps to consider what OCT actually does. The technique works a bit like ultrasound, except it uses near-infrared light instead of sound. A broadband or rapidly swept laser illuminates the sample, and an interferometer compares the weak light reflected from different depths within the sample against a reference path. By analyzing the interference pattern, the system reconstructs a depth profile with micrometer-scale axial resolution. Sweeping the beam across the surface builds up cross-sectional images, and stacking those slices yields full three-dimensional volumes. Because the method is entirely contactless and relies on low-power light rather than ionizing radiation, it can inspect delicate, soft, or transparent materials without deforming or damaging them, a property that becomes invaluable when the object under test is a photoresist layer that a physical probe would smear.

The catch has always been the hardware. A laboratory OCT system typically sprawls across an optical table: a bulky swept laser, discrete fiber couplers, a reference arm with moving parts or modulators, balanced photodetectors, and a maze of alignment-critical connections. That complexity translates into cost, fragility, and maintenance overhead, which explains why OCT has penetrated hospitals far more successfully than production lines. Photonic integrated circuits offer a way out, promising to fold the interferometer, the detectors, and eventually the light source onto chips that can be fabricated by the thousands in semiconductor foundries. But the field has struggled with two persistent problems: choosing a material platform that foundries can manufacture reliably at low optical loss, and designing a flexible sample arm so the chip-bound engine can still reach real-world objects.

The Shanghai Jiao Tong team, led by corresponding author Xingchen Ji, attacked both problems simultaneously. Their chip integrates low-loss silicon nitride interferometers with germanium photodiodes on a commercially fabricated silicon-based platform, combining the distinct strengths of multiple photonic integration technologies in a single device. Silicon nitride is prized in photonics for its extremely low propagation loss and broad spectral transparency, making it ideal for the interferometric heart of an OCT engine. Germanium, meanwhile, absorbs near-infrared light efficiently and serves as the natural photodetector material on silicon. The challenge is that these waveguide platforms have different geometries and optical modes, so the researchers developed an interlayer coupling structure to transfer light between the silicon nitride and silicon layers with a coupling loss of less than 0.15 decibels, a figure low enough to preserve the interference contrast on which OCT sensitivity depends.

The result is a remarkably dense consolidation of optical function. The interferometer and photodetectors occupy a footprint of just 0.9 by 0.4 millimeters, yet the design deliberately keeps the sample arm external, connected through fiber. That architectural choice preserves flexibility: the same chip-based engine can drive different probes depending on the inspection task. Ji emphasized the intent behind the design, saying the aim was to create a compact OCT module adaptable to practical industrial inspections. Integrating the interferometer and photodetectors on-chip reduces the number of discrete optical components, he noted, while the fiber-connected sample arm significantly expands the system’s operational range. In other words, the chip handles the precision optics, and the fiber handles the reach.

Performance figures from the experimental testing suggest the miniaturization did not come at the expense of capability. The system achieved 87 decibels of sensitivity, a measure of its ability to detect weakly reflected light from deep within a sample, along with a 3.42-millimeter sensitivity roll-off range, meaning usable signal persists over a respectable imaging depth. The team resolved internal structures within a smartphone camera module, demonstrating that the chip can inspect the kind of densely packaged, multi-layer consumer electronics that are notoriously difficult to verify non-destructively. Thickness measurements of a silica wafer, compared against established contact-based methods, showed absolute errors below 2 micrometers, while measurements of soft SU-8 photoresist blocks yielded relative errors under 0.12 percent. For semiconductor and microfabrication workflows, where film thicknesses are specified to sub-micrometer tolerances, that level of agreement with reference techniques is the difference between a laboratory curiosity and a production tool.

Perhaps the most striking demonstration involved a probe barely wider than a human hair is thick. By connecting an angle-polished ball-lens fiber probe with a diameter of just 450 micrometers, the researchers captured three-dimensional images of threads hidden inside a metal optical post and of internal layers within a roll of plastic tape. The ball lens focuses the emitted light to a tight spot while the angled polish suppresses back-reflections that would otherwise swamp the weak signal from the sample. This plug-and-play attachment effectively turns the chip-based engine into a miniature endoscope for machines, capable of inspecting bores, threads, and sealed assemblies that no conventional microscope objective can physically access.

Hang Su, the study’s first author and a doctoral student at Shanghai Jiao Tong University, highlighted why contactless operation matters so much in practice. Contactless measurement is invaluable when physical contact could deform a soft surface, Su explained, and the fiber probe allows the team to examine structures that are inaccessible to conventional microscopes. Describing the platform as a plug-and-play OCT module, Su said it paves the way for the next generation of chip-scale OCT engines. That framing captures the broader vision: rather than building a monolithic instrument for one task, the researchers envision a standardized photonic core to which different probes and sample arms can be attached as easily as swapping a lens on a camera.

The commercial implications extend beyond the demonstrations themselves. Because the chip was fabricated on a commercially available silicon photonics platform, the design is compatible with foundry-scale production, which is the prerequisite for driving down OCT system costs to levels that mid-sized manufacturers could justify. The researchers note that complementary technologies, including embedded III-V semiconductor lasers on silicon-on-insulator wafers and chip-based silicon nitride tunable delay lines, could be incorporated to realize a fully integrated, miniaturized OCT system in which even the swept light source lives on the chip. The system described in the paper already integrates a III-V semiconductor light source, silicon nitride interferometers, silicon-germanium photodetectors, and the ball-lens microprobe, drawing on the advantages of each material platform. A Chinese provisional patent application has been filed regarding the technology, signaling the team’s intent to move from demonstration toward deployment.

If that transition succeeds, the implications could reach well beyond metrology labs. Inline, non-destructive inspection of coatings, adhesives, semiconductor packages, medical device surfaces, and additive-manufactured parts could shift from sampling-based quality checks to continuous, three-dimensional verification. The same chip-scale engines could find uses in machine vision systems that need depth information at micrometer resolution, or in compact sensors embedded directly into production equipment. What the Shanghai Jiao Tong team has shown is that the optical complexity that once confined OCT to the optical table can be compressed onto a chip the size of a printed period, without sacrificing the sensitivity or accuracy that made the technique worth shrinking in the first place. The factory floor, long the harder market for optical coherence tomography, may finally be within reach.

Subject of Research: Chip-based photonic integrated circuit for swept-source optical coherence tomography in industrial inspection

Article Title: Compact photonic chip enables three-dimensional OCT inspection for industrial samples

Article References: Compact photonic chip enables three-dimensional OCT inspection for industrial samples. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: optical coherence tomography, photonic integrated circuits, silicon photonics, silicon nitride, industrial inspection, non-destructive testing, swept-source laser, ball-lens fiber probe, 3D imaging, metrology, germanium photodiodes, Shanghai Jiao Tong University

The heart of the device is a two-dimensional photonic crystal, a periodic lattice of air holes etched into a thin silicon slab that manipulates the flow of light in much the same way that a semiconductor crystal manipulates electrons. Because the refractive index alternates periodically between silicon and air, the structure exhibits a photonic band gap, a range of wavelengths that cannot propagate through the crystal. By introducing defects into this lattice, engineers can trap light in tiny resonant cavities whose transmission spectrum contains extremely sharp peaks. When something changes the optical properties of the surrounding material, such as mechanical stress altering the refractive index of silicon, those peaks shift in a measurable and repeatable way, turning a purely optical effect into a sensitive physical measurement.

Earlier work by the same community showed that a single hexagonal ring resonator in a photonic crystal suffers from relatively low sensitivity and a modest quality factor, the figure of merit that describes how long photons circulate inside a resonator before leaking away. A high quality factor translates directly into narrow resonance linewidths, and narrow linewidths make it possible to detect very small wavelength shifts. The new design therefore couples two hexagonal ring resonators in series, an arrangement that dramatically improves optical confinement and extends the effective photon lifetime within the structure. The result is a resonance that is both sharper and more responsive to external perturbations than anything a single ring can deliver.

The physical architecture of the sensor is a study in miniaturisation. A silicon substrate 450 micrometres thick and 14 by 14 micrometres in lateral size supports a three-micrometre silicon dioxide layer, on top of which sits the 220-nanometre-thick photonic crystal slab. The active sensing region measures 12 by 12 micrometres and contains an array of 40 by 34 air holes arranged in a hexagonal lattice, with a hole radius of 136 nanometres and a lattice constant of 420 nanometres. The researchers chose the hexagonal lattice deliberately, citing studies showing that it delivers wider photonic band gaps and better performance metrics than triangular or square alternatives. The ratio of hole radius to lattice constant falls between 0.3 and 0.4, a range chosen to keep the design realistic for existing fabrication processes.

Beneath the photonic crystal lies a cavity that leaves the diaphragm free to deform downward when pressure is applied. The team computed the photonic band structure of the defect-free lattice using the plane wave expansion method, finding a band gap spanning wavelengths from 1.3506 to 1.9425 micrometres, comfortably encompassing the 1550-nanometre telecommunications band where the sensor is designed to operate. When the input port is excited with a broadband source, a prominent resonance peak appears at 1.56674 micrometres with a full width at half maximum of just 0.03 nanometres, corresponding to a quality factor of 52,224. Field distribution maps at the resonant and off-resonant conditions confirm that light is efficiently coupled into the double-ring structure only on resonance, the behaviour required for reliable wavelength-shift readout.

The opto-mechanical side of the sensor was analysed with the finite element method, while the optical behaviour was simulated with the finite-difference time-domain technique, a combination that captures both the structural deformation under load and the resulting electromagnetic response. When pressure is applied to the diaphragm as a boundary load, the membrane deflects downward and surface stress rises. Stress changes the refractive index of silicon through the photoelastic effect, governed by a stress coefficient of 1.56 times ten to the minus eleven, and this refractive index change shifts the resonance wavelength. The stress distribution across the diaphragm is not uniform: it oscillates between high and low values as the edge alternates between air holes and solid silicon, reaching maxima at the midpoints of the diaphragm edges and a minimum at the centre.

The numerical results, tabulated across the full 0 to 12 megapascal operating range, show a beautifully linear response. At one megapascal the resonance sits at 1.57842 micrometres; by twelve megapascal it has moved to 1.7104 micrometres. Intriguingly, the quality factor does not degrade as pressure increases but actually climbs, from 52,224 at zero load to 57,013 at the top of the range, because the resonance wavelength itself increases while the linewidth remains fixed at 0.03 nanometres. The researchers also calculated the burst pressure of the diaphragm using the fracture stress of silicon, arriving at a maximum safe operating pressure of approximately twelve megapascals, which conveniently defines the sensor’s full measurement span.

How does this compare with the existing literature? The authors assembled a comparison table of photonic crystal pressure sensors that makes the advance strikingly clear. A two-dimensional photonic crystal Mach-Zehnder interferometer reported in 2018 managed a sensitivity of just 0.02237 nanometres per megapascal, while a 2023 ring resonator design reached only 0.003. Even the strongest earlier entries from the same research group, a photonic crystal microring resonator diaphragm sensor with a sensitivity of 1.37 nanometres per megapascal and a quality factor of 26,180, are surpassed by nearly an order of magnitude in sensitivity and doubled in quality factor. The new device also achieves a minimum detectable pressure of 0.00253 megapascal, roughly seventeen times finer than the earlier diaphragm sensor’s 0.0437 megapascal.

The significance of the work extends beyond the raw numbers. Because the entire sensor is built on a silicon-on-insulator platform using standard photonic crystal geometries, it is inherently compatible with the same semiconductor manufacturing infrastructure that produces computer chips, opening a plausible route to mass production of optical pressure sensors that are immune to electromagnetic interference and capable of operating in harsh environments. The researchers suggest the design could serve applications ranging from industrial process monitoring to biomedical instrumentation, wherever small pressures must be measured with high precision on a chip-scale footprint.

There remain, of course, the usual caveats that separate simulation from product. The reported performance comes from finite element and finite-difference time-domain modelling rather than from a fabricated and tested device, and real-world fabrication tolerances, surface roughness and coupling losses will inevitably erode some of the simulated quality factor. Nevertheless, the design parameters were deliberately constrained to fabrication-friendly values, and the underlying physics of serially coupled resonators is well established experimentally. If the laboratory results survive contact with a cleanroom, serially coupled hexagonal photonic crystal resonators may well become a standard building block for the next generation of chip-scale optical pressure sensors, proving once again that in photonics, as in so much of engineering, the cleverest tricks often come from coupling simple things together in the right way.

Subject of Research: Photonic crystal ring resonator pressure sensor design and simulation

Article Title: Highly sensitive and high Q-factor photonic crystals serially coupled hexagonal resonator for pressure sensing

Article References: Basavaprasad, Kolli, V. R., P.R., Y., K.B., S. K., Ramesh, M., Manivannan, G. S., & Talabattula, S. (2026). Highly sensitive and high Q-factor photonic crystals serially coupled hexagonal resonator for pressure sensing. Results in Optics, 25, Article 101167. https://doi.org/10.1016/j.rio.2026.101167

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101167

Keywords: photonic crystals, pressure sensor, hexagonal ring resonator, silicon-on-insulator, quality factor, FDTD, finite element method, optical sensing, photonic band gap, microelectromechanical systems, wavelength shift, silicon photonics

The work, carried out by Abhishek Kumar Pandey, Deepak Jain, and Catherine Baskiotis, exploits spontaneous four-wave mixing, a nonlinear optical process in which two pump photons at the same frequency are annihilated to create a correlated pair of photons at two very different frequencies. Energy conservation forces the sum of the two new frequencies to equal twice the pump frequency, while momentum conservation, known as the phase-matching condition, dictates which frequency combinations can actually occur. In a waveguide, the phase-matching condition can be engineered by shaping the geometry of the silicon core, allowing the generated signal and idler photons to be tuned with remarkable precision across a wide spectral range.

What makes the new designs remarkable is the sheer distance between the two photons. The flagship design, targeted at atmospheric quantum key distribution, produces a signal photon at 3.905 micrometres, deep inside a mid-infrared atmospheric transparency window, while its idler partner lands at 1.541 micrometres in the telecom C-band. That separation of roughly 2,364 nanometres is dramatically larger than the record of about 1,125 nanometres theoretically reported in earlier all-solid silicon waveguide studies. Crucially, the design achieves this in the true quantum regime, where at most one photon pair is produced per pump pulse, rather than relying on the high pump powers used in classical demonstrations.

This distinction matters. Previous experiments with air-clad silicon waveguides demonstrated large signal-idler separations of about 2,058 nanometres, but only by using pump peak powers around 20 watts to achieve phase matching. In the quantum regime, pump power must be kept low to suppress multi-pair emission, and with those earlier designs the phase-matched separation collapsed to 1,780 nanometres, placing the mid-infrared photon outside any atmospheric transmission window. The new all-solid designs sidestep this problem by building the phase matching into the waveguide geometry itself, so that it holds even at the milliwatt pump levels required for single-pair emission.

The choice of an all-solid, fully oxide-clad structure is itself strategic. Air-clad waveguides suffer from significant scattering losses caused by sidewall roughness, because the optical mode interacts strongly with the etched surfaces. By surrounding the silicon core with fused silica cladding and a buried oxide layer, the proposed waveguides reduce the modal overlap with etched sidewalls, mitigating fabrication-induced scattering. The team’s calculations show that even at the longest signal wavelength, where silica is nominally opaque, the majority of the optical power remains confined to the silicon core, keeping total material attenuation over the 2-centimetre waveguide to acceptable levels.

To give their predictions quantitative teeth, the researchers adopted a model for the probability of photon-pair generation per pulse and validated it against published experimental data from photon-pair generation in photonic crystal fibre. By modelling the fibre as a simple step-index structure and feeding in the experimental parameters, they computed photon-pair rates that closely matched the measured values across a range of pump powers, outperforming the original theoretical treatment of those experiments. Because the model takes as inputs only the modal properties, the nonlinear index, and the operating conditions, the authors argue that the validation is generic and applies equally to the silicon waveguide geometries, whose modal fields they computed independently using full-vectorial finite element simulations.

Each of the three proposed waveguides is tailored to a specific application. Two designs generate signal photons at 3.265 and 3.461 micrometres, which coincide with absorption bands of methane and nitrogen dioxide respectively, the fingerprint region where these gases reveal their presence. Paired with telecom-band idlers, these sources could power schemes for sensing with undetected light, in which the mid-infrared photon probes the gas while only the telecom photon is ever detected, allowing the entire measurement to be performed with cheap, mature silicon-compatible detectors. The third design, optimised for atmospheric quantum key distribution, places the signal photon at 3.905 micrometres, where atmospheric transmission is dramatically better than at the 1.55-micrometre wavelength used in current daylight quantum communication experiments.

The shift to the mid-infrared could be transformative for free-space quantum communication. At 3 to 4 micrometres, the atmosphere exhibits reduced Rayleigh scattering, lower solar background noise, and greater resistance to weather-related variations, all of which translate into more reliable quantum links and an improved ability to operate in daylight. Shrinking the photon-pair source onto a chip also addresses a critical constraint for satellite payloads, where every gram of mass and cubic centimetre of volume counts. Silicon-on-insulator fabrication is cheap, scalable, and compatible with existing CMOS foundries, making the prospect of flight-qualified quantum sources considerably more realistic than with bulk nonlinear crystals.

The team also addressed the practical obstacles that have plagued mid-infrared quantum optics in silicon. Spontaneous Raman scattering, which can contaminate photon-pair spectra, is predicted to be negligible because the generated photons lie more than 330 Raman linewidths away from the silicon Raman gain peak, placing the Raman gain at the signal frequencies at the level of one part in a million. Two-photon absorption, the bane of silicon nonlinear optics at telecom wavelengths, falls off sharply for pump wavelengths above 1.9 micrometres, and the chosen pump wavelengths between 2.1 and 2.21 micrometres sit in a sweet spot combining strong Kerr nonlinearity with suppressed carrier generation. Numerical tolerance studies further showed that small fabrication imperfections of plus or minus 10 nanometres in the core dimensions cause only slight wavelength drifts that can be compensated by tuning the pump laser.

The predicted operating points are within reach of existing technology. Each design targets a probability of photon-pair generation per pulse of about 0.05, a value considered a practical compromise between source brightness and multi-pair suppression, and achieves this with peak pump powers between 9.2 and 32.2 milliwatts from 5-picosecond pulses. Such pump powers are already delivered by mature thulium and holmium-doped solid-state lasers around 2 micrometres, while integrated 2-micrometre laser sources continue to advance toward the required output levels. With mid-infrared single-photon detectors, including superconducting nanowire devices and frequency-upconversion schemes, progressing rapidly, the authors argue that their chip-scale sources could soon move from simulation to experiment, opening a path toward quantum communication and sensing that spans from the fingerprint region of the molecules to the heart of the telecom infrastructure.

Subject of Research: Silicon waveguide design for quantum-correlated photon-pair generation bridging mid-infrared and telecom bands via spontaneous four-wave mixing

Article Title: Quantitative study of silicon waveguides for the generation of quantum correlated photon pairs bridging mid-infrared and telecom bands

Article References: Pandey, A. K., Jain, D., & Baskiotis, C. (2026). Quantitative study of silicon waveguides for the generation of quantum correlated photon pairs bridging mid-infrared and telecom bands. Results in Optics, 25, Article 101158. https://doi.org/10.1016/j.rio.2026.101158

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101158

Keywords: silicon photonics, quantum optics, photon pairs, four-wave mixing, mid-infrared, telecom C-band, quantum key distribution, gas sensing, silicon-on-insulator, phase matching, Raman scattering, integrated photonics

The core idea is to sidestep lattice matching altogether. Traditional heteroepitaxy, in which one crystalline material is grown directly on another, runs into severe trouble when the two crystal lattices differ significantly. Strain builds up, dislocations nucleate, and the resulting film quality degrades the very optical properties that made the material attractive in the first place. Instead of growing these functional crystals on photonic substrates, the researchers grow them elsewhere, on compatible native substrates, and then release them as free-standing single-crystalline nanomembranes using advanced epitaxial growth and layer lift-off techniques. These ultrathin membranes, which retain the near-perfect crystal quality of their parent substrate, are then placed onto silicon and silicon nitride photonic chips using what the team calls photonic van der Waals integration, a bonding approach in which weak intermolecular forces hold the membrane in place without demanding any crystallographic registry with the underlying chip.

The versatility of this framework is demonstrated through several flagship devices. The first is an ultraefficient electro-optic modulator built by transferring thin films of barium titanate, a ferroelectric perovskite oxide, onto silicon chips. Barium titanate possesses one of the largest Pockels coefficients of any known material, meaning its refractive index changes dramatically in response to an applied electric field, but exploiting that property on a chip requires careful control of the crystal orientation. The team ensured well-defined crystallographic alignment of the transferred membranes and measured a Pockels coefficient r42 exceeding 1,290 picometers per volt, together with a 3-decibel electro-optic bandwidth above 23 gigahertz. These figures place the modulators among the most efficient ever demonstrated, suggesting that barium titanate could challenge lithium niobate, the current workhorse of high-performance electro-optics, in future integrated photonic circuits.

Modulators are only half the story, however, because practical photonic circuits also need optical isolators, devices that allow light to travel in one direction while blocking it in the other. Isolators protect lasers from back-reflections that would otherwise destabilize them, and they are indispensable in everything from telecom transmitters to quantum photonic systems. On-chip isolation has historically been difficult because the magneto-optic materials with strong Faraday effects, such as yttrium iron garnet, are notoriously hard to integrate with standard platforms. The researchers instead transferred single-crystalline cobalt ferrite nanomembranes into silicon microring resonators, achieving a Faraday rotation coefficient of 33,800 degrees per centimeter, an exceptionally large value that enables ultracompact non-reciprocal devices. By combining the long light-matter interaction path of a resonant cavity with the enormous Faraday response of high-quality cobalt ferrite, the team demonstrated efficient optical isolation in a footprint far smaller than conventional approaches allow.

The third demonstration showcases perhaps the most conceptually striking capability of the technique: stitching different single crystals side by side on a single photonic template. The researchers laterally combined gallium arsenide and gallium nitride single crystals on top of silicon nitride photonics, creating a detector landscape that spans the spectral range from the ultraviolet to the near-infrared. Gallium arsenide, a classic III-V semiconductor, absorbs efficiently in the near-infrared, while gallium nitride, with its wide bandgap, covers the ultraviolet portion of the spectrum. Placing both materials on the same chip, each precisely positioned over the appropriate waveguide structures, effectively gives a photonic circuit multiple eyes tuned to different wavelengths. This kind of spatially programmed material assembly would be essentially impossible to achieve by direct epitaxial growth, where the differing lattice constants and growth chemistries of the two semiconductors would normally force separate fabrication runs or compromise the crystal quality of one or both films.

Taking the concept one step further, the team constructed vertical heterostructures by stacking cobalt ferrite on barium titanate, producing ring resonators that perform electro-optic and magneto-optic modulation simultaneously. In such a device, an applied electric field modulates the phase of light through the Pockels effect in the barium titanate layer, while the magnetic ordering of the cobalt ferrite layer imposes non-reciprocal behavior on the same optical mode. The ability to coalesce multiple functional materials into arbitrary vertical and lateral arrangements points toward photonic circuits in which each region of the chip is dressed with exactly the material functionality it needs, much as modern electronic chips combine transistors, capacitors, and interconnects within a single architecture.

The enabling technology behind all of these demonstrations is the ability to produce and transfer single-crystalline membranes with atomic precision. The team drew on techniques including remote epitaxy, in which a monolayer of graphene between the growth substrate and the growing film allows the film to inherit the substrate’s crystal orientation while remaining weakly attached, and other 2D-materials-based layer transfer methods developed over the past decade. Once grown, the membranes are released and transferred with processes designed to avoid cracks, wrinkles, and contamination, preserving the single-crystal quality that underpins the exceptional electro-optic and magneto-optic coefficients measured in the devices. The authors and their collaborators have previously shown that such approaches can be scaled to wafer dimensions, an essential prerequisite for any manufacturing-relevant technology.

The significance of this work lies less in any single record-breaking device than in the generality of the framework. Previous heterogeneous integration strategies have typically been bespoke, optimized for one material on one platform, with each new combination requiring a fresh engineering campaign. By contrast, photonic van der Waals integration of free-standing nanomembranes is largely agnostic to the specific materials involved, provided they can be grown epitaxially and lifted off. That means ferroelectric oxides, magnetic oxides, III-V semiconductors, and wide-bandgap materials can all be brought to bear on the same silicon and silicon nitride infrastructure that the semiconductor industry already knows how to fabricate at scale. The benchmarking presented in the paper situates these van der Waals-integrated devices favorably against comparable devices made by conventional means, in terms of efficiency, footprint, and bandwidth.

Looking ahead, the researchers suggest that this approach opens new opportunities for advanced hetero-integrated optoelectronic applications and beyond. Data centers continue to demand ever-greater bandwidth and energy efficiency, pushing modulators toward lower drive voltages and higher speeds. Emerging photonic quantum computing platforms require low-loss circuits combined with a rich toolbox of optical nonlinearities and non-reciprocal elements. Compact LiDAR, metrology, and sensing systems would all benefit from chips that can detect across broad spectral ranges without external optics. If single-crystalline nanomembrane integration can be married to existing foundry processes, the photonic equivalent of the materials toolbox that transformed electronics may finally be at hand, letting chip designers choose the best material for every function rather than the best material that happens to grow on the substrate.

Subject of Research: Heterogeneous photonic integration of free-standing single-crystalline functional nanomembranes onto silicon and silicon nitride photonic platforms.

Article Title: Heterogeneous photonic integration of single-crystalline nanomembranes

Article References: Meng, Y., Mao, W., Xu, Z., Jia, D., Lin, M., Seo, J., Kim, B., Zhang, X., Park, E., Lee, S., Kim, J., Han, S., Moon, J.-Y., Xu, W., Zhang, Q., He, X., Chen, M., Nam, S. H., Hu, J., … Bae, S.-H. (2026). Heterogeneous photonic integration of single-crystalline nanomembranes. Nature, 657(8132), 638-645. https://doi.org/10.1038/s41586-026-11000-w

Image Credits: AI Generated

DOI: 10.1038/s41586-026-11000-w

Keywords: photonic integration, nanomembranes, van der Waals integration, barium titanate, cobalt ferrite, electro-optic modulator, optical isolator, silicon photonics, remote epitaxy, gallium arsenide, gallium nitride, photodetectors

The research, carried out by Chia-Ching Li, Sih-Yuan Wang, Sheng-Chun Weng, Qi-Lin Yang and Ting-Han Pei, focuses on a structure that is deliberately simple to fabricate: a silicon dioxide substrate perforated with a hexagonal array of holes, each filled with silicon. Photonic crystals of this kind control light the way semiconductor crystals control electrons, using periodic modulation of refractive index to open forbidden frequency ranges, called photonic band gaps, where light simply cannot propagate. The hexagonal lattice was chosen over a square one because its higher structural symmetry consistently supports wider band gaps and offers greater design flexibility, a conclusion the authors confirmed through direct comparison. The material pairing matters too. Pure silicon dioxide photonic crystals, whether square or hexagonal, struggle to produce clear band gaps at all, so embedding high-index silicon into the low-index silica background provides the dielectric contrast needed to trap and steer light while remaining fully compatible with chip manufacturing.

What distinguishes this work from earlier thermal studies is the rigor of the optical input data. Rather than treating refractive index as a constant or assuming it varies linearly with temperature, the team coupled experimentally based, wavelength- and temperature-dependent refractive-index models directly into their band-structure calculations. For silicon dioxide, they employed two independent descriptions: the semi-empirical Ghosh model, which explicitly accounts for thermal expansion and the temperature-induced shift of the material’s excitonic band gap, and a modified Sellmeier equation whose fitting parameters vary with temperature. The two models agreed closely, predicting a silica refractive index of about 1.44415 at 314 kelvin rising to 1.44873 at 754 kelvin at the telecommunications wavelength of 1.55 micrometers, with differences between the models so small they are practically negligible. For silicon, the researchers adopted the classic experimentally grounded model published by H. H. Li in 1980, valid from 1.2 to 14 micrometers and from 100 to 750 kelvin, which captures the distinctly nonlinear behavior of silicon’s refractive index with temperature.

The computational engine behind the study is the plane wave expansion method, a workhorse technique more than three decades old that exploits the periodicity of photonic crystals by expanding the electromagnetic fields and the inverse dielectric function into Fourier series. Substituting these expansions into Maxwell’s equations converts the problem of finding allowed light frequencies into a matrix eigenvalue problem in reciprocal space. The authors built their own MATLAB implementation, verified it against worked examples in Kazuaki Sakoda’s standard textbook, and pushed convergence hard, testing truncation sizes from 41 by 41 up to 91 by 91 plane waves. The Bloch wave vector was scanned along the high-symmetry path from M through Gamma to K in the first Brillouin zone, and band gaps were confirmed wherever the minimum frequency of an upper band exceeded the maximum frequency of the band below it. All calculations were performed in the transverse-electric polarization mode at the 1.55 micrometer telecommunications wavelength, deep within silicon’s transparency window, where band-to-band absorption is absent and free-carrier absorption remains negligible even at 600 kelvin.

The results reveal a photonic crystal with a rich thermal personality. Eight band gaps initially emerged below a normalized frequency of 1.1, though one of them, Gap 6, proved so vanishingly narrow that it was excluded from quantitative analysis as a likely numerical artifact. Of the seven reliable gaps that remained, the lower-order ones behaved like thermal bedrock. The first gap, between the first and second bands, held a normalized width of roughly 0.022 with almost no change from 200 to 600 kelvin. The second, the widest of all at a normalized width near 0.035, was similarly indifferent to heating. In stark contrast, the higher-order gaps widened noticeably as temperature climbed. The seventh gap grew by more than 50 percent, from a normalized width of 0.006 to 0.0095, while the eighth expanded by more than 25 percent, from about 0.018 to 0.023. The third, fourth and fifth gaps showed gentler but measurable widening. This split behavior arises because different photonic bands distribute their electric fields differently between the silicon and silica regions, so each gap responds uniquely to the changing refractive-index contrast.

Perhaps the most practically valuable finding concerns the mid-gap positions. Across the entire 400-kelvin temperature span, the centers of all identified gaps shifted only slightly and gradually, meaning the frequencies at which the crystal blocks light stay roughly put even as the gaps themselves breathe. That stability is a gift for engineers, but the study also quantifies how much tolerance designers actually have. For the eighth gap, the team found that its lower edge at 200 kelvin nearly coincides with its upper edge at 600 kelvin, a warning that devices targeting this gap must be designed with care. Operating at the gap center at 250 kelvin, corresponding to a normalized frequency of about 1.007, yields a lattice constant of 1.5609 micrometers at 1.55 micrometers, with a fabrication tolerance of only plus or minus 9.3 nanometers. The second gap is far more forgiving: centered at a normalized frequency of 0.37, it permits a lattice constant of 0.5735 micrometers with a tolerance of plus or minus 27.1 nanometers, roughly three times looser.

The authors also confronted the physical effects they chose to set aside, and the reasoning holds up under scrutiny. Thermal expansion of the silica substrate changes the hole radius by only about 0.02 percent over the full 200-to-600-kelvin range, a variation too small to matter. Differential expansion between silicon and silica does generate compressive stress in the silicon-filled holes, and strain is known to alter silicon’s refractive index by as much as 1.4 percent under extreme conditions, but the estimated internal stress in this geometry falls far below the gigapascal-scale strains required for such shifts, especially compared with the 1.8 percent refractive-index change silicon undergoes purely from heating across 300 kelvin. The photoelastic contribution is therefore safely negligible. The main limitations the authors acknowledge are the boundaries of their material models, which exclude wavelengths shorter than 1.2 micrometers or longer than 14 micrometers, temperatures below 200 or above 600 kelvin, and the absence of direct experimental verification, though every refractive-index input rests on experimentally fitted data.

Placed alongside recent literature, the study carves out a clear niche. Earlier work on diamond-lattice silicon photonic crystals used linearized refractive-index formulas and reported a temperature sensitivity of about 0.0518 nanometers per kelvin at the first band-gap center, hinting at applications in optical temperature sensing and biosensing chips. Other groups have demonstrated thermally tunable silicon valley photonic crystal rings, mapped heat spread from thermo-optic devices, and built photonic crystal temperature sensors from defect-mode resonance shifts. By contrast, this work spans a broader temperature range, covers multiple band gaps simultaneously, and applies nonlinear, experimentally validated refractive-index functions to a structure directly relevant to silicon photonic waveguides. The practical upshot is a design map: lower-order gaps for thermally robust passive components such as filters and waveguides that must survive harsh environments unchanged, and higher-order gaps for thermally tunable or temperature-sensing applications where deliberate sensitivity is the point. As silicon photonics pushes into hotter, harsher and more ambitious territory, knowing exactly where the light is allowed to travel, and how much that map shifts with heat, may prove as important as the circuits themselves.

Subject of Research: Thermal effects on the photonic band gaps of a two-dimensional SiO2-Si hexagonal photonic crystal for silicon photonics

Article Title: The thermal effects on the photonic band gaps of the two-dimensional SiO 2 -Si hexagonal photonic crystal

Article References: Li, C.-C., Wang, S.-Y., Weng, S.-C., Yang, Q.-L., & Pei, T.-H. (2026). The thermal effects on the photonic band gaps of the two-dimensional SiO2-Si hexagonal photonic crystal. Results in Optics, 25, Article 101146. https://doi.org/10.1016/j.rio.2026.101146

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101146

Keywords: silicon photonics, photonic crystals, photonic band gaps, thermo-optic effect, plane wave expansion method, silicon dioxide, refractive index, hexagonal lattice, telecommunication wavelength, thermal stability, waveguide design, high-temperature applications