Watching “Excitons” on the Move

Dr. John M. Lupton
University of Utah 
Department of Physics

 The absorption of light generates an electronic polarization, spatially displaced net positive and negative charges. Given sufficient electrostatic correlation between the charges, neutral quasi-particles are formed: excitons. Nature has evolved extraordinary machinery to move such charge pairs around, ultimately leading to the conversion and storage of energy in photosynthesis. Replicating nature’s powerhouse requires precise understanding and control of the elementary processes of light-matter interaction and excited-state coupling: how do neutral species move in space, transferring energy from one entity to another?

 This problem is akin to the electrodynamical coupling of two antennae – transmitting and receiving a signal – with the difference that the dipoles involved are only a few nanometers in size, much smaller than the wavelength of radiation. As disorder affects the quality of the resonance, microscopic experimental techniques are required to unravel intrinsic dipole-dipole coupling in prototypical donor-acceptor systems such as large molecules and nanocrystalline quantum heterostructures. Recent experimental advances now enable a complete microscopic tracking of excitation energy migration on the nanometer scale.