Anisotropic absorption in PbSe nanorods.

We present absorption anisotropy measurements in PbSe nanostructures. This is accomplished via a new means of measuring absorption anisotropy in randomly oriented solution ensembles of nanostructures via pump-probe spectroscopy, which exploits the polarization memory effect. We observe isotropic absorption in nanocrystals and anisotropic absorption in nanorods, which increases upon elongation from aspect ratio 1 to 4 and is constant for longer nanorods. The measured volume-normalized absorption cross section is 1.8 ± 0.3 times larger for parallel pump and probe polarizations in randomly oriented nanorods as compared to nanocrystals. We show that this enhancement would be larger than an order of magnitude for aligned nanorods. Despite being in the strong quantum confinement regime, the aspect ratio dependence of the absorption anisotropy in PbSe nanorods is described classically by the effects of dielectric contrast on an anisotropic nanostructure. These results imply that the dielectric constant of the surrounding medium can be used to influence the optoelectronic properties of nanorods, including polarized absorption and emission, phonon modes, multiple exciton generation efficiency, and Auger recombination rate.