Source: Advanced Optical Technologies, by Hausner, Michal; Feiler, Martin; Ziman, Martin; Podlucký, Ľuboš; Kováčová, Soňa; Kováč, Jaroslav (September 7, 2026). AI-generated summary by biochip.com, published . Not independently reviewed.
Key takeaways
- Slovak University of Technology researchers designed a SiON rib-waveguide SPR sensor using a 20 to 40 nm gold plasmonic layer.
- Simulations of water-IPA mixtures gave 158 nm/RIU average sensitivity, increasing from 80 nm/RIU in water to 350 nm/RIU for IPA.
- The work does not establish performance with fabricated chips, biological targets, or independent experimental validation.
Researchers at the Slovak University of Technology in Bratislava have designed an integrated optical sensor that uses a silicon oxynitride, or SiON, rib waveguide to detect changes in aqueous liquids through surface plasmon resonance. The device is intended for lab-on-a-chip systems, compact instruments that move chemical analysis onto a small photonic chip. Its central advantage is that SiON can be tuned to different refractive indices during fabrication, allowing the optical waveguide to be matched more closely to the liquid being measured. In simulations of water and water-isopropyl alcohol mixtures, the proposed sensor achieved an average sensitivity of 158 nanometers per refractive index unit, with sensitivity rising from 80 nm/RIU in water to 350 nm/RIU for the modeled isopropyl alcohol condition. The team also examined how a 20 to 40 nm gold film changes the sensor's plasmonic resonance, using dimethyl sulfoxide as an analyte. Gold provides the metal surface needed to create surface plasmons, collective electron oscillations that react strongly to changes immediately above the metal. The work points to a fabrication-compatible route for combining robust SiON photonics with label-free chemical and biosensing. The reported results are based on device modeling and optical evaluation over a visible to near-infrared wavelength range of 550 to 915 nm.
A waveguide built for liquid sensing
An optical waveguide is the chip-scale equivalent of an optical fiber: it confines light and guides it along a chosen path. In this design, light travels through a raised SiON strip called a rib waveguide, rather than through a fully etched narrow channel. That geometry can provide a mechanically robust structure while still allowing part of the guided light to extend beyond the core and interact with the surrounding liquid.
That outward-reaching component is known as the evanescent field. A useful analogy is the light spilling under a closed door: most stays in the room, but a small amount reaches outside. In the sensor, this optical spillover samples the refractive index of liquid next to the waveguide and gold layer, creating the basis for measurement without needing fluorescent labels or chemical dyes.
Why SiON is useful on a chip
SiON sits between silicon dioxide and silicon nitride in optical behavior. By changing the oxygen-to-nitrogen ratio during deposition, manufacturers can tune its refractive index from roughly 1.45, near silicon dioxide, toward 2.0, near silicon nitride. This gives designers a practical way to adjust how tightly a waveguide holds light and how much of its optical field reaches an analyte.
The material also fits conventional complementary metal-oxide-semiconductor, or CMOS, manufacturing approaches used across semiconductor fabrication. The team highlights plasma-enhanced chemical vapor deposition, a low-temperature process that can deposit relatively thick, low-stress SiON layers. Compared with high-index silicon-on-insulator platforms, SiON can support larger optical mode fields, which can reduce propagation losses and make coupling light from standard optical fibers easier.
Using plasmons to amplify a small change
Surface plasmon resonance, or SPR, occurs when incoming light couples to coordinated electron motion at the boundary between a metal and a dielectric material such as water. The resulting surface plasmon polariton is highly sensitive to conditions at that boundary. Small changes in a liquid's refractive index can shift the wavelength where this coupling is strongest.
In practice, the sensor reads that shift as a dip in transmitted optical intensity at a particular wavelength. It works much like pushing a child on a swing: when the rhythm matches, the response is strongest. Here, the matching condition depends on the waveguide mode, the thin gold layer, and the liquid above it, so changing the liquid moves the resonance dip.
Optimizing the gold and optical modes
Hausner and colleagues used two-dimensional finite element method simulations to optimize the device geometry and its optical modes. Finite element modeling breaks a complex structure into many small pieces and calculates how electromagnetic fields behave across them. This let the team examine the coupling between the guided waveguide light and the plasmon supported by the gold film before fabrication choices are finalized.
The researchers first tracked the resonance response for gold thicknesses between 20 and 40 nm, with dimethyl sulfoxide, or DMSO, modeled as the analyte at a refractive index of 1.47. Gold is widely used in aqueous plasmonic sensors because it is chemically stable in water and compatible with biological applications. The study evaluated the device through direct edge coupling, where light enters through the side of the chip, across wavelengths from 550 to 915 nm.
Matching the core to the analyte
The most important design lesson is that the SiON core should not be treated as a fixed material with one ideal composition. Instead, its refractive index can be selected with the target liquid in mind. When the core index better matches the analyte index, the modeled structure increases evanescent coupling between the guided optical mode and the gold-supported plasmon.
The team simulated refractive indices of approximately 1.33 to 1.38, corresponding to volumetric mixtures of water and isopropyl alcohol, or IPA. Across that range, the average sensitivity was 158 nm/RIU. Sensitivity was not constant: it increased with analyte refractive index, from 80 nm/RIU for water to 350 nm/RIU for the modeled IPA condition.
Why This Matters
Many useful chemical and biological measurements begin with an exceptionally subtle event: a molecule binds to a surface, a solution changes composition, or a biological sample alters the local optical environment. Label-free SPR sensing can turn that local change into a measurable wavelength shift. Integrating the approach into a SiON waveguide could make it easier to place sensing functions alongside optical routing and fiber connections on a single chip.
The platform could be relevant wherever aqueous samples need compact optical analysis, including chemical monitoring and biosensing. Its value lies less in one fixed sensor specification than in the ability to tune the SiON material itself for a chosen refractive-index range. That design flexibility may help engineers balance sensitivity, optical confinement, fabrication robustness, and coupling efficiency for different applications.
Next steps for the platform
The reported modeling establishes how gold thickness, waveguide geometry, and SiON refractive index can be coordinated to strengthen plasmonic coupling. The next practical challenge is to translate that optimization into fabricated chips and test them with real analytes, including biological targets that create much smaller surface changes than bulk liquid mixtures. If those experiments preserve the predicted response, tunable SiON waveguides could become a useful building block for CMOS-compatible optical sensors designed around the liquids they need to measure.