SX AR LWL 2.0 Optical Waveguide Material for Bimodal Plasmonic RI Sensors

With this Resist of the Month, we are exploring new territory for Allresist. For the first time, we are featuring both an experimental material and a new application technology, giving our Allresist community an insight into our latest research findings.

As part of a RUBIN – Regional Entrepreneurial Alliances for Innovation project, a consortium of 13 partners investigated fabrication technologies for photonic circuits. The project was successfully completed in 2025, yielding many interesting results.

One specific application of the developed materials is in bimodal plasmonic refractive index (RI) sensors, developed jointly with AMO and other research partners. The sensor consists of a two-layer SU-8 waveguide with an embedded aluminium strip just 80 nm thick. The fundamental TM mode couples into two plasmonic modes at the upper and lower surfaces of the metal. Only the upper mode responds to changes in the refractive index of the surrounding medium; the lower mode remains largely unaffected at the Al/SU-8 interface. When the two modes couple back into the SU-8 waveguide, they interfere to generate the sensor signal. The exposed sensing surface can be functionalised for different analytes.

For optical isolation, the waveguide is enclosed in an approximately 6 µm thick cladding layer of SX AR LWL 2.0. This layer is selectively opened above the aluminium sensor using i-line photolithography (Step 5 in Fig. 1). The polymer is compatible with SU-8 processing and has a refractive index of approximately 1.42 at a wavelength of 1.55 µm, providing a refractive index contrast of around 0.14 relative to the SU-8 core. The full results were published in Optics Express in 2025 (DOI: 10.1364/OE.553386).

Fig. 1: Fabrication process for bimodal plasmonic RI sensors.

Source: O. Bhalerao et al., “High-sensitivity polymer-based bimodal plasmonic refractive index sensors with polymer cladding,” Optics Express 33(5), 9813–9824 (2025), Fig. 2, doi:10.1364/OE.553386, CC BY 4.0.

With its LWL series, Allresist has developed a fluorinated copolymer whose refractive index can be adjusted by varying the monomer ratio. The LWL polymers are also readily patterned using photolithography and offer refractive indices ranging from approximately 1.4 to 1.55. We invite interested users to work with us to assess the suitability of these specialised resists for their own optical applications. Contact us to discuss your requirements!

These exciting results have inspired us to select this optical waveguide polymer as our Resist of the Month.