Science 346, 828–831 (2014)

Metallic nanostructures can sustain collective oscillations of conduction electrons that have been stimulated by light. These oscillations have a specific frequency (the plasmon frequency) that can shift slightly to higher or lower values when the nanoparticle acquires or loses electrons, respectively. The number of charges can be modified by applying an external static electric field, for example. Now, Harry Atwater and colleagues have shown that the reverse process, in which light induces the creation of an electrostatic potential, can also occur.

The researchers — who are based at the California Institute of Technology and the FOM Institute AMOLF in the Netherlands — deposited a gold nanoparticle with a diameter of 60 nm on top of a conducting surface made of indium tin oxide (ITO). Then, using a Kelvin probe microscope, they measured the electric potential between the microscope tip and the nanoparticle, while scanning the frequency of the incoming light around the plasmon resonance. When the light frequency is slightly off-resonance, they can detect an electrostatic 'plasmoelectric' potential on the nanoparticle due to a change in the charge carrier density.

Atwater and colleagues suggest that if the two frequencies do not match, a transfer of electrons occurs from the nanoparticle to the ITO, or vice versa, depending on which side of the plasmon resonance they are working. This charge transfer is a thermodynamically favourable process, as the system tends to match the irradiation frequency to minimize free energy.