Shape Measurement of Single Gold Nanorods in Water Using Open-access Optical Microcavities

Shape measurement of rod-shaped particles in fluids is an outstanding challenge with applications in characterising synthetic functional nanoparticles and in early warning detection of rod-shaped pathogens in water supplies. However, it is challenging to achieve accurate and real-time measurements at a single particle scale in solution with existing methods. Here we introduce a novel technique to measure the aspect ratio of rod-shaped particles by analysing changes in the polarisation state of a laser beam transmitted through an optical microcavity through which the particle diffuses. The resolution in aspect ratio measurement is found to be around 1%. Our work opens the new possibility of in-situ and single-particle shape measurements, which have promising applications in nanoparticle characterisation, water monitoring, and beyond.