Interferometric tracking of nanoparticle orientation with quantitative optical anisotropy imaging

We introduce Quantitative Optical Anisotropy Imaging (QOAI), an interferometric spectral multiplexing technique that allows imaging and tracking of the orientation of individual nanoparticles at the microsecond timescale. In QOAI, incident light whose polarization is modulated in the spectral domain is scattered off particles and structures, interfered with a reference beam, and detected spectroscopically. The signal is directly proportional to anisotropies in the particle polarizability, and can therefore be used to extract both orientation and shape information about each individual particle, even at sizes well below the diffraction limit of conventional microscopy. We use this technique to categorize the aspect ratio of gold nanorods and to characterize their rotational diffusion near a solid interface. QOAI can be straightforwardly combined with existing particle tracking techniques so that position and orientation can be tracked simultaneously and can also be modified to provide quantitative measurements of the chirality of individual particles.