A research team led by SUTD has created nanoscale glass structures with near-perfect reflectance, overturning long-held assumptions about what low-index materials can do in photonics.

For decades, glass has been a reliable workhorse of optical systems, valued for its transparency and stability. But when it comes to manipulating light at the nanoscale, especially for high-performance optical devices, glass has traditionally taken a backseat to higher refractive index materials. Now, a research team led by Professor Joel Yang from the Singapore University of Technology and Design (SUTD) is reshaping this narrative.

With findings published in Science Advances, the team has developed a new method to 3D-print glass structures with nanoscale precision and achieve nearly 100% reflectance in the visible spectrum. This level of performance is rare for low-refractive-index materials like silica, and it opens up a broader role for glass in nanophotonics, including in wearable optics, integrated displays, and sensors.

The researchers' breakthrough is enabled by a new material called Glass-Nano: a photocurable resin made by blending silicon-containing molecules with other light-sensitive organic compounds.

Unlike conventional approaches that rely on silica nanoparticles—often resulting in grainy, low-resolution structures—Glass-Nano cures smoothly and contracts uniformly during heating, transforming into clear, robust glass. When printed using two-photon lithography, these polymer structures shrink during sintering at 650°C, preserving their form while achieving nanoscale features as small as 260 nanometers.

"Instead of starting with silica particles, we worked with silicon-bearing molecules in the resin formulation," explained Prof Yang. "This resin enables us to build up nanostructures with much finer detail and smoother surfaces than was previously possible. We then convert them into glass using our 'print-and-shrink' process without sacrificing fidelity."

The team focused their fabrication on photonic crystals (PhCs)—artificially structured materials featuring repeating patterns that interact with specific wavelengths of light. These structures can reflect light very efficiently, but only if built with extreme regularity and precision. Previous efforts to realize low-index 3D PhCs have consistently fallen short, exhibiting only poor reflectance due to structural irregularities and distortions.

With their new method, the researchers overcame these limitations. By printing more than 20 tightly stacked layers and fine-tuning the design geometry, they achieved a structurally highly uniform, diamond-like photonic crystal that reflects nearly 100% of incident light within a broad range of viewing angles.

"The result was unexpected," shared Dr. Wang Zhang, SUTD Research Fellow and first author of the paper. "Historically, low-index materials like silica were seen as optically weak for this purpose. But our findings show that with enough uniformity and structural control, they can outperform expectations—and even rival high-index materials in reflectance."

Importantly, the team's optical measurements align closely with theoretical simulations of the photonic band structure. The fabricated structures not only match the main expected reflectance peaks but also feature finer spectral details predicted by models.

"Even tiny spectral reflectance features—so small that we originally suspected they might be measurement artifacts—line up well with calculated predictions of standing-wave oscillations," said Associate Professor Thomas Christensen, a co-author of the paper from the Department of Electrical and Photonics Engineering at the Technical University of Denmark.

Preserving the structural shape during the dramatic shrinkage process was no small feat.

"At the macroscale, shrinkage like this would collapse the structure," Dr. Zhang added. "But at the nanoscale, the high surface-to-volume ratio actually helps preserve stability. Our resin formulation, engineered with multiple cross-linkers and a silicon-rich precursor, ensures both the printability and the mechanical robustness needed to survive the heat treatment."

The implications go beyond reflectance. Because the resin formulation and fabrication process are compatible with standard nanoprinting tools, these glass PhCs could be integrated into a variety of devices. The pigment-free structural colors produced by the crystals, for instance, could be used in displays that consume less power. They also provide a model system for exploring future photonic crystal geometries that guide light in novel ways, including helical and robust edge transport in topological systems.

"With the ability to fabricate and control the geometry of not just an entire crystal but individual unit cells within that crystal, demonstrations of waveguides and cavities in 3D photonic crystals at visible and telecom frequencies appear to be achievable, which is a very exciting outlook," says Associate Prof. Christensen.

Looking ahead, the team is broadening the capabilities of the Glass-Nano platform. They are exploring hybrid resins that incorporate light-emitting or nonlinear properties, and investigating faster, large-area printing methods to scale production. In parallel, new geometries are being studied to push the boundaries of light manipulation.

"With the ability to print high-resolution nanostructures in both low- and high-index dielectrics, we're now turning to applications where 3D optical components could reduce transmission losses and enable more efficient photonic systems," said Prof. Yang.