Cyano-Engineered Polymer Beats Glass With Record Infrared Refractive Index

For decades, the optical world has been ruled by a quiet division of labor. When engineers needed materials that bend light strongly and transmit infrared radiation cleanly, they turned to inorganic compounds: chalcogenide glasses, cadmium sulfide, zinc sulfide, zinc selenide. When they needed lightweight, flexible, cheap and easily processed optics, they turned to organic polymers — but accepted a stubborn penalty, because almost every transparent polymer bends light far less efficiently, with refractive indices typically confined between roughly 1.4 and 1.6, and most polymers become notably opaque across large stretches of the infrared spectrum. A new study published in Nature Photonics now reports an all-organic polymer, designated K153, that shatters that trade-off, achieving a refractive index of approximately 2.53 at 750 nanometers — a figure that exceeds even the inorganic chalcogenide glasses and the II–VI semiconductor optics that have long defined the state of the art.

The achievement matters because the refractive index, denoted n, governs how efficiently a material can bend, focus and manipulate light. A higher index permits thinner lenses, stronger light confinement in waveguides, and higher-contrast interference and diffraction structures. In the near-infrared region, where imaging, sensing, biomedical diagnostics and telecommunications flourish, the difference between a polymer at n = 1.6 and one at n = 2.53 is transformative. Optical components that previously demanded millimeters of dense glass could, in principle, be made from a fraction of the thickness of a flexible plastic film, opening the door to ultrathin infrared cameras, conformable sensors and lightweight wearable optics.

The secret of K153 lies in its chemistry. The researchers, led by Hongbin Chen, Jianrong Guo and Chaobin He of the National University of Singapore, together with collaborators in China and Singapore, designed the polymer around carefully placed cyano groups — the linear carbon–nitrogen substituents known for their strong electronic character. According to the team, the outstanding refractive index arises from two synergistic effects induced by these linear cyano groups: enhanced polarizability, which directly increases how strongly the material’s electrons respond to passing light waves, and strengthened intermolecular interactions that pack the chains into a denser, more optically responsive solid. In simple terms, the cyano groups give the polymer both more responsive electrons and a more tightly organized structure, and both effects push the refractive index upward.

Boosting the index is only half of the problem. The other half is infrared transparency, and here most polymers fail spectacularly. Ordinary organic molecules contain bonds — carbon–hydrogen, carbon–oxygen, nitrogen–hydrogen — that vibrate at frequencies matching infrared light, absorbing it strongly and creating the characteristic absorption bands that make plastics useless for much of the infrared. The team circumvented this by designing symmetric molecular architectures for the polymer’s constructing units. Symmetry suppresses the infrared-active vibration modes responsible for light loss, so the material avoids absorbing the very wavelengths it is meant to transmit. The result is a polymer that combines a record-high refractive index with outstanding transparency in the near-infrared regime, a combination that the authors note lags far behind inorganic analogues in essentially all prior all-organic materials.

The practical demonstrations reported in the study suggest that K153 is not merely a laboratory curiosity. The researchers fabricated thin films of the polymer and showed that they deliver high-quality infrared imaging, meaning the material can serve as a functional optical medium rather than simply a passive coating. The films also exhibited impressive environmental tolerance, surviving conditions that would degrade many organic optical materials, and extraordinary flexibility, bending without cracking or losing optical performance. That mechanical resilience is precisely what inorganic infrared optics cannot offer: chalcogenide glasses and semiconductor windows are brittle, heavy and expensive to shape, whereas a polymer film can be rolled, wrapped around curved surfaces, or integrated into flexible devices.

Two further capabilities distinguish the new material. First, the refractive index of K153 is tunable through chemical doping, giving optical engineers a dial they can turn to match specific design requirements — a flexibility unavailable in fixed-composition glasses. Second, the polymer demonstrates nanoimprintability: it can be molded at the nanoscale using nanoimprint lithography, allowing researchers to stamp fine photonic structures such as gratings, microlens arrays and meta-optical textures directly into the film. Combining high index with nanoscale patterning is the essential recipe for flat optics, and a polymer that supports both processes cheaply could dramatically lower the cost of advanced infrared components.

The broader context explains why this result has generated excitement beyond the immediate field. Inorganic high-index materials — from titanium dioxide nanoparticles to silicon photonics — have fueled a wave of metalens and metasurface research, but their fabrication is demanding and their integration with flexible or biological systems is awkward. Sulfur-containing polymers developed through inverse vulcanization have pushed organic indices higher and improved infrared transparency, yet they typically remain below the inorganic benchmark or sacrifice transparency in key bands. K153, the authors report, surpasses a refractive index of 2.25 — the level associated with chalcogenide glasses and CdS/ZnS/ZnSe optics — while remaining a fully organic, ductile, lightweight polymer.

The molecular design philosophy behind the work may prove as influential as the material itself. Rather than doping a polymer with high-index inorganic particles, which often causes scattering and haze, the team engineered the index into the polymer backbone through targeted cyano substitution, an approach the authors describe as tailored cyano engineering. Because the linear cyano group simultaneously raises polarizability and strengthens intermolecular packing without introducing heavy inorganic domains, the material stays optically homogeneous and transparent. The study, according to its authors, demonstrates that deliberate molecular-structure optimization can deliver a dramatic increase in refractive index, representing a decisive step toward using all-organic polymers for infrared applications previously considered the exclusive territory of inorganic materials.

What comes next could reshape several industries at once. Near-infrared imaging is expanding rapidly, from night vision and autonomous vehicle perception to medical diagnostics that exploit the deep tissue penetration of shortwave infrared light. Flexible, transparent, high-index polymer films could enable conformal infrared lenses, ultralight thermal cameras, wearable NIR sensors and infrared-transparent protective coatings that are currently impractical with brittle glass. Meanwhile, the demonstrated combination of tunability, nanoimprintability and environmental robustness suggests K153-type polymers could become a platform material for photonic integrated circuits, diffractive optics and augmented-reality systems operating in the infrared. If the promise of cyano-engineered organic polymers scales from laboratory films to manufactured components, the rigid, heavy glass lens — an icon of optics for centuries — may finally face a genuinely flexible rival.

Subject of Research: An all-organic cyano-substituted polymer with a large near-infrared refractive index and high infrared transparency for optical imaging applications.

Article Title: All-organic polymer with large refractive index in the near-infrared

Article References: Chen, H., Guo, J., Ma, K., Miao, X., Liu, S., Huang, B., Self, T. J., Vinod, K., Abdelraouf, O. A. M., Long, G., He, T., Meng, L., Zhang, Y., & He, C. (2026). All-organic polymer with large refractive index in the near-infrared. Nature Photonics. https://doi.org/10.1038/s41566-026-02000-3

Image Credits: AI Generated

DOI: 10.1038/s41566-026-02000-3

Keywords: high refractive index polymer, all-organic polymer, near-infrared transparency, cyano substitution, infrared imaging, nanoimprint lithography, optical materials, chalcogenide glass, flexible photonics, chemical doping, molecular design, Nature Photonics