Determination of the Dipole Geometry of Fluorescent Nanoparticles Using Polarized Excitation and Emission Analysis

We demonstrate that the geometries of the absorption dipole 1/4_ab and emission dipole 1/4_em of nano-emitters such as quantum dots can be determined simultaneously by far-field polarimetry. The method involves plotting the emission polarization ratio against the absorption polarization ratio of single nano-emitters. Using Monte Carlo simulation, we show that these plots depend sensitively on the aspect ratio of the dipole shape. For example, the so-called 3D-2D dipole combination, that is, 1/4_ab of radius ratio 1:1:1 and 1/4_em of ratio 1:1:0, would give rise to a vertical line plot. Polarization ratios of commercial cadmium selenide/zinc sulfide (CdSe/ZnS) quantum dots are measured and plotted. The empirical data points are best-fitted to yield 1/4_ab of radius ratio 1:1:0.28 and 1/4_em of ratio 1:1:0.