Light scattering and rheological effects in an optical fibre coupled nanoparticle suspension

Abstract

This study forms the first part of research into enhancing the forward and back scattering of light in an optical fibre using nanoparticles (NPs). This approach has the potential to enhance the sensitivity of optical fibre sensing by increasing the signal-to-noise ratio. The work described in this paper is focused on understanding the scattering of light by a suspension of NPs in refractive index matching liquid. It was noted early in the experimental work that rheological effects related to the viscosity and flow of the liquid affect the scattered light measured and therefore these effects are considered in the analysis. Gold nanoparticles in the tens to hundreds of micrometre size range were selected as the scattering particles based on their optical properties. These are suspended in a refractive index liquid with a similar refractive index to the optical fibre core. Effort was needed to transfer the NPs from their aqueous sodium citrate solution to the paraffin based solution. We investigated two types of interaction with the optical fibre: (i) dropping the NP suspension onto the end of a single-mode optical fibre and (ii) using the NP suspension as an interface between two single-mode optical fibres. It was noted that the surface tension of the liquid, the diameter of the fibre and the spacing between the fibres in case (ii) influence the reflected and transmitted light. In case of excess liquid, droplets flowed down the fibre and interestingly in case (ii) modified the reflected and forward transmitted light as it passed across the fibre interface. Our initial findings are that the influence of the gap between two optical fibres decreased after dropping refractive index liquid into the gap after fibre collimation, which is a beneficial result for understanding the influence of scattered light from a liquid containing NPs. Note, the position between the two fibres can also change due to the weight of the droplet and the fibre ends had to be re-collimated to investigate the influence of the moving droplets. These results will be expanded by additional experiments and modelling of the scattering from the nanoparticales and droplets.

Modelling of light scattering by gold nanoparticles at optical fibre interfaces

“…However, it is not easy to transfer gold NPs to refractive index liquid. In our former work [18], we tried to transfer commercial gold NPs suspension in citrate buffer (Sigma Aldrich) to refractive index liquid (Cargille). The experimental results showed that the transferring process is not efficient enough.…”

“…In practice, manufacturing optical fibres with NPs is not easy in an optical laboratory and a careful design is needed. In order to investigate the behaviours of NPs in fibre optic sensors, we proposed a simple setup to investigate the characteristics of NPs for further development in SHM [18]. It is to drop refractive index (RI) matching liquid between two optical fibre tips and to study the scattering characteristics for different NPs under different experimental parameters.…”

Light scattering by gold nanoparticles cured in optical adhesive at optical fibre interface

“…However, it is not easy to manufacture NP-doped optical fibres to test different designs in the an optical laboratory. To overcome this problem, we used the method of dropping refractive index matching liquid containing gold NPs at the optical fibres end tips to investigate the intensity change of the scattered light from the interfaces in our previous work 9 and intend to build on this relationship in the light scattering between the optical fibre end tips and in the optical fibre itself in order to recover the scattered light signal in the optical fibre containing NPs. Although refractive index liquid can have a good refractive index match with the optical fibre, traditional optical fibre sensors used for SHM are solid sensors rather than liquid sensors.…”