´Plasmonic fibres´ and applications in liquid-level sensors

Solution-processible fabrication of plasmonic nanostructures onto the sidewall of fibres without length limitation is demonstrated, which can be directly used in simple liquid-level sensors based on fibre-coupled particle plasmon resonance. This sensor device actually consists of a bare fibre coated with randomly distributed gold nano-island structures on the sidewall, which is defined as the `plasmonic fibre` in this context. In the fabrication process, colloidal gold nanoparticles are first coated on the sidewall of a bare fibre before being annealed at a temperature of 500°C. Particle plasmon resonance of the gold nano-island structures shows sensitive response to the change of the level of the liquid surrounding the fibre owing to the change in the environmental refractive index. The fibre does not only deliver the incident and signal light, but also mediates the interaction between the light and the gold nanostructures. This flexible sensor device based on the simple fabrication techniques enables a pure optical approach for the remote monitoring of the levels of special liquids with a sub-centimetre resolution, which may include flammable, toxic and non-transparent liquids that are not suitable to be detected using conventional techniques.