The interaction between matter and light has been intensively studied over the last 10 years and scientists have succeeded in making devices capable of controlling this interaction. Devices made of metallic nanostructures are particularly good in this respect because they can focus and trap light into tiny regions, much smaller than the wavelength of light. These systems can also strongly interact with other photonic elements, such as quantum emitters.

Thanks to recent advances in nanofabrication, we are now getting close to length scales in which the quantum behaviour of these structures becomes important. This is a new area of study, known as quantum plasmonics.

Switching the coupling on and off
Alejandro Manjavacas and colleagues studied a system consisting of a quantum emitter placed in the gap between two metallic nanoparticles. The interesting non-linear behaviour observed in this set-up comes thanks to the fermionic character of the emitter, say the researchers. The model that they used, based on "Zubarev's Green Functions" shows that – depending on the initial state of the quantum emitter – it is possible to switch the coupling between the emitter and the plasmons (on the metallic nanoparticles) on and off. Plasmons are collective oscillations of electrons and propagate on the surface of a metal. They can interact strongly with light.

In Manjavacas and colleagues' experiment, the interactions between plasmons and excitons (electron-hole pairs) in the emitter are possible thanks to the strong electromagnetic fields that exist in the gap of the dimer. The researchers describe this interaction using two terms in their model: one associated with the creation of a plasmon and the annihilation of an exciton; and the other involving the opposite process (the annihilation of a plasmon and the creation of an exciton).

Simple approach
Zubarev's Green Functions are a mathematical model developed by Russian physicist Dmitry Nikolaevich Zubarev in 1960 that can be successfully applied to solve different problems in statistical physics. "In our work, we adapted this methodology to model the optical response of plasmonic systems strongly interacting with excitonic systems," explained Manjavacas. "This approach is simple to use compared with standard quantum modelling methods."

According to the team, the model could describe many other plasmonic-exciton interactions in systems such as plasmonic transistors, modulators and quantum information devices. It may even be used to help design novel optical devices, such as optical switchers.

The team would now like to use its method to study more complex systems. For example, one possible scenario involves placing two quantum emitters in the gap between two quantum particle emitters.

The work was reported in Nano Letters.