Plexcitons travel for 20,000 nanometers, a length which is on the order of the width of human hair. Graphic by Joel Yuen-Zhou
Scientists at UC SD, MIT and HarvardU have engineered “topological plexcitons,” energy-carrying particles that could help make possible the design of new kinds of solar cells and miniaturized optical circuitry. Within the Lilliputian world of solid state physics, light and matter interact in strange ways, exchanging energy back and forth between them.
“When light and matter interact, they exchange energy,” explained Assistant/Prof. Joel Yuen-Zhou. “Energy can flow back and forth between light in a metal (so called plasmon) and light in a molecule (so called exciton). When this exchange is much faster than their respective decay rates, their individual identities are lost, and it is more accurate to think about them as hybrid particles; excitons and plasmons marry to form plexcitons.”
Materials scientists have been looking for ways to enhance exciton energy transfer, or EET, to create better solar cells as well as miniaturized photonic circuits, much smaller than their silicon counterparts. The drawback with EET is the energy transfer is extremely short-ranged, 10 nm and quickly dissipates as the excitons interact with different molecules. A solution is to hybridize excitons in a molecular crystal with collective excitations within metals to produce plexcitons, which travel for 20,000 nm, a length which is on the order of the width of human hair.
Plexcitons are expected to become an integral part of the next generation of nanophotonic circuitry, light-harvesting solar energy architectures and chemical catalysis devices. But the main problem with plexcitons is that their movement along all directions, which makes it hard to properly harness in a material or device. The solution is engineering “topological plexcitons,” based on the concepts in which solid state physicists have been able to develop materials called “topological insulators.”
“Topological insulators are materials that are perfect electrical insulators in the bulk but at their edges behave as perfect one-dimensional metallic cables,” Yuen-Zhou said. “The exciting feature of topological insulators is that even when the material is imperfect and has impurities, there is a large threshold of operation where electrons that start travelling along one direction cannot bounce back, making electron transport robust. In other words, one may think about the electrons being blind to impurities.”
Plexcitons, as opposed to electrons, do not have an electrical charge. Yet they still inherit robust directional properties. Adding this “topological” feature to plexcitons gives directionality of EET. This should eventually allow for creation of plexcitonic switches to distribute energy selectively across different components of a new kind of solar cell or light-harvesting device. http://ucsdnews.ucsd.edu/pressrelease/scientists_design_energy_carrying_particles_called_topological_plexcitons
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