When light interacts with matter, it can be absorbed to create higher energy (excited) quantum states. These excited states are well-studied for isolated atoms and molecules. But when the light interacts with a collection of absorbing units (for example molecular assemblies, inorganic semiconductor nanocrystals, or a group of biological pigments embedded in a protein) the excited state is an exciton, a collective excitation with new properties that are qualitatively different from those of isolated absorbers. These novel properties include the ability to transport energy over large distances, as well as undergo fission/fusion processes that repackage energy.

There is a new urgency to achieve a quantitative understanding of the structure and dynamics of excitons because they play a key role in vital photonic processes, like solar energy conversion, lasing, and biological light harvesting. However, the fact that the exciton was introduced as a loosely defined virtual particle, often lacking a clear microscopic basis, has hindered the derivation of an unambiguous theoretical description. Furthermore, the large number of atoms involved in systems that exhibit excitonic behavior, like semiconductor quantum dots and light-harvesting membranes, makes first principles ab initio quantum calculations prohibitively expensive. Workers in the field often refer to any excited state in a solid as an “exciton”, making it challenging to compare different experimental results and material properties. For example, physicists usually describe excitons in terms of the Wannier framework of a bound electron-hole pair, while chemists and biologists typically rely on the Frenkel model of interacting multilevel systems.

The goal of this workshop is to focus attention on three key issues that we think can define the field and determine future directions.
1) How big is an exciton? Its spatial extent should determine its most convenient representation in momentum versus real space.
2) How does an exciton move? Excitons can move through both coherent and diffusive motion.
3) What are potential applications of excitons? There may be advantages to energy versus charge manipulation in optoelectronic devices and information processing.

We believe the development of new experimental tools that can capture both spatial and time domain information simultaneously, combined with advances in theory and computation, now make it possible to address these issues in a comprehensive way.