Shape Measurement\nof Single Gold Nanorods in Water\nUsing Open-Access Optical Microcavities

Shape measurement of single nanoparticles in fluids is\nan outstanding\nchallenge with applications in characterizing synthetic functional\nnanoparticles and in early warning detection of rod-shaped pathogens\nin water supplies. Here we introduce a novel technique to measure\nthe aspect ratio of rod-shaped particles by analyzing changes in the\npolarization state of a laser beam transmitted through an optical\nmicrocavity through which the particle diffuses. The resolution in\naspect ratio measurement is found to be around 1%. Our work opens\nthe new possibility of in situ and single-particle shape measurement,\nwhich has promising applications in nanoparticle characterization,\nwater monitoring, and beyond.