The optical anisotropy of SC NRs is typically determined by measuring the polarization-dependent PL emission of individual NRs under a microscope or by performing ensemble anisotropy measurements of samples in solution. The latter strategy is generally favored as it samples the entire NR ensemble. Here, we measure the optical anisotropy of CdSe/CdS NRs using ensemble fluorescence anisotropy measurements. In a typical experiment, a random ensemble of NRs is excited by a vertically polarized source that selectively excites transition dipoles based on the projection of these dipoles parallel to the polarization axis of excitation. The PL emission from excited NRs is measured at 90° relative to the excitation beam, through a vertical or horizontal polarizing filter. The ensemble anisotropy (𝑟) is defined according to eq 2: