Coupling plasmons with semiconducting nanocrystals (quantum dots)

Coupled metal nanoparticles supporting localized surface plasmon resonances act like electronic components in an electrical circuit - one that is powered by light and operates at the frequency of light. However, these nanoparticles act as simple linear components - there are no non-linearities such as occur with diodes and transistors that lead to rectification or amplification. The idea is to use non-linear effects to try to make active components in plasmonic circuits. One way of doing this is to exploit the quantum properties of semiconducting nanocrystals.

Semiconducting nanocrystals, made from materials such as cadmium selenide, behave like the archetypal particle-in-a-box from quantum mechanics. Light incident on the nanocrystal can excite a valence electron into the conduction band. It becomes bound to the positively charged hole left behind, forming a quasiparticle called an exciton. Because the crystal is so small, typically a few nanometres in diameter, the electron can only have a discrete set of energies related to its wavelength and the size of the crystal , or "box". When the electron falls back into the valence band, it emits light (fluorescence) with a wavelength characteristic of the energy difference between the excited state and the ground state. This energy depends on the size of the crystal. Growing bigger crystals leads to fluorescence with red-shifted wavelengths; growing smaller crystals leads to blue-shifted fluorescence.

In experiments with quantum dots coated on a silver film supporting surface plasmons, we found that the quantum dots did not interact with one another (or if they did, only weakly). Instead they behaved like a large number of simple "particle-in-a-box" systems. This is very interesting because their properties are relatively simple to describe. Moreover, the surface plasmons pumped energy into the quantum dots which, in turn, pumped energy back into the surface plasmons. This led to forbidden energy states and Rabi oscillations associated with the resonant pumping of energy back and forth between the two systems. By using direct excitation of the quantum dots by an external light beam we hope to disrupt this interaction leading to some control of the surface plasmons. This would be a first step towards making active plasmonic/optical components.

A concept of the "plasmonic" circuit consisting of an optical antenna, a plasmonic resonator and a non-linear amplifying component!

A silver nanowire coated with quantum dots. Laser light focused at the centre of the wire excites the QDs and generates a surface plasmon that propagates to the ends where light is re-emitted - see the two faint emissions above and below the centre.

[1] D. E. Gomez, K. C. Vernon, P. Mulvaney, T. J. Davis: "Surface Plasmon Mediated Strong Exciton-Photon Coupling in Semiconductor Nanocrystals" Nano Letters 10, 274-278 (2010); [2] D. E. Gomez, K. C. Vernon, P. Mulvaney, T. J. Davis: "Coherent superposition of exciton states in quantum dots induced by surface plasmons" Applied Physics Letters 96, 073108-3 (2010)

Copyright Tim Davis 2012