How to improve the transport efficiency of excitons by many orders of magnitude

Usually, the concept of exciton is linked to that of nonmetallic crystal: an electron-hole pair in a crystal that is bound in a manner analogous to the electron and proton of a hydrogen atom. It behaves like an atomic excitation (hence the name) that passes from atom to another and may be long-lived. Exciton behaviour in semiconductors is very important.

But excitons are not only present in standard crystals. Most systems composed of organic molecules are disordered and possess relatively large dissipation and dephasing rates, so that one would expect that the exciton transport would become diffusive over long distances; still, the transport of excitons is a fundamental process that plays a crucial role in photosynthesis, for example, where energy has to be transported to a reaction centre.

The transport of excitons is also important in artificial devices such as excitonic transistors or organic solar cells, where the power conversion efficiency can be improved significantly when the exciton diffusion length is increased.

In most cases, excitons are induced by an electromagnetic wave. If the exciton transition has a large oscillator strength, the quanta of the electromagnetic field in the medium (photons) are mixed with the quanta of “mechanical” (electronic) excitations, thereby forming exciton-induced polaritons (photoexcitons). In this case, there is no longer a sharp difference between the excitons and the photons in the system.

Actually, polaritons are an intriguing possibility to modify exciton properties. This is achieved when the Rabi frequency, i.e., the energy exchange rate between exciton and electromagnetic modes, becomes faster than the decay and/or decoherence rates of either constituent. Polaritons combine the properties of their constituents, in particular, mutual interactions and low effective masses, enabling new applications.

Now, Johannes Feist from IFIMAC and Francisco J. García-Vidal from DIPC, demonstrate ¹ that through the creation of polaritonic states, the exciton transport efficiency can be improved by many orders of magnitude. The strong coupling allows the excitons to bypass the disordered organic system, preventing localization and leading to dramatically improved energy transport properties. Even though they have focused on organic molecules, the results can readily be generalized to other systems such as quantum dots and Rydberg atoms, or even chains of trapped ions. The results are published in Physical Review Letters.

The researchers use a very simple one-dimensional model, but it captures the essential physics: a chain of two level emitters inside a cavity. In such a system, the emitter dipole transition is coupled to the single cavity mode, and, additionally, induces Coulombic dipole-dipole interaction between the emitters.

They find that when the coupling is strong enough and the polaritons are fully formed, the excitons can almost completely bypass the chain of quantum emitters and “jump” directly from one end to the other, leading to large exciton conductance. This robust effect persists almost independently of the exact parameters of the system, and most notably occurs efficiently even when the underlying excitonic system is strongly disordered and its transport is completely suppressed due to localization.

Still, the connection between exciton transport through polaritons and electrical conduction is currently unclear, as polaritons are, in principle, neutral quasiparticles.

Through this simple model, the researchers also show that transport through direct hopping and through the polariton modes constitute two effectively independent channels, which helps to explain why the polariton conductance is almost independent of the disorder in the system.

These results demonstrate a possible pathway for improving the efficiency of excitonic devices, where the electromagnetic mode could be provided by plasmonic structures to enable fully integrated nanometer-scale devices. In any case, they could have important implications in the fields of heat transport, photosynthesis, and biological systems in which exciton transport plays a key role.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance.