Shape measurement of single nanoparticles in water using optical microcavities

Thesis

Nanoparticles have attracted significant attention with their unique properties and wide applications in diverse fields. Shape, as an important physical property of a nanoparticle, impacts the electrical, optical and chemical properties of a nanoparticle and is worth characterising. In addition, rod-shaped bacteria and viruses in water raise concerns about public health. Therefore, shape measurement can not only be applied in the quality control of nanoparticles but also acts as an early indication of harmful bacteria in water to people. In this work, a novel technique is proposed to perform the shape measurement of rod-shaped particles in water, namely measuring the aspect ratio at a single-particle scale in situ. The perturbation of a rod-shaped particle in water to the resonant mode formed by the optical microcavity is captured to obtain the shape information of the particle. Three Stokes parameters are measured to characterise the polarisation state of the transmitted light through the optical microcavity, deviating from the incident circularly-polarised light. With the theoretical model constructed, the polarisability-related parameters can be extracted from the curves of the Stokes parameters and the aspect ratio can be deduced afterwards. The experimental results of both gold nanorods and rod-shaped bacteria particles are presented along with the theoretical simulations of the diffusion behaviour and the optical field inside the microcavity. Therefore this technique can be a powerful tool in the shape measurement of rod-shaped particles in water with the potential in real-world applications and can be adapted to more complex measurements.

Authors: Yin, Y

• DOI: 10.5287/ora-jb4zn1br5
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Stokes parameters; rod-shaped nanoparticles; optical microcavities
• Subjects: Optics