System measures limits of plasmonic enhancement

Photonic interactions have been measured for the first time on the scale of a single atom. The ability to quantify the unique properties of light gives physicists a road map to precise control of scattering in metal-based devices such as biosensors and photonic integrated circuits.

The work, performed at Duke University and Imperial College London, measures plasmons on an unprecedented scale, and the researchers believe that they have characterized the limits of such surface plasmons on metal. The electromagnetic field enhancement produced at the nanoscale by surface plasmons on metal is significantly higher than that achievable with any other material. Knowing the maximum limit of the field enhancement will give researchers an advantage when working with metal-based devices that enhance light.

An artistic representation of a new film-nanoparticle plasmonic system that enables the science of light on a scale of a few tenths of a nanometer. Spherical gold nanoparticles are coupled to a gold film substrate by means of an ultrathin layer that forbids the particles from directly touching the film. Electromagnetic ultrahot spots are excited in the gaps, the diameter of a typical atom.

“Once you know maximum field enhancement, you can then figure out the efficiencies of any plasmonic system,” said David R. Smith, William Bevan Professor of Electrical and Computer Engineering at Duke. “It also allows us to ‘tune’ the plasmonic system to get exact predictable enhancements, now that we know what is happening at the atomic level. Control over this phenomenon has deep ramifications for nonlinear and quantum optics.”

Plasmonic devices typically involve the interactions of electrons between two metal particles separated by a very short distance. For 40 years, scientists have been trying to determine what happens when these particles are brought closer and closer, at subnanometer distances.

The Duke researchers began with a thin gold film coated with an ultrathin monolayer of organic molecules studded with precisely controllable carbon chains and dispersed nanometric gold spheres on top of the monolayer. Essential to the experiment was that the distance between the spheres and the film could be adjusted with the precision of a single atom. This way, they overcame the limitations of traditional approaches and obtained a photonic signature with atom-level resolution.

“We were able to demonstrate the accuracy of our model by studying the optical scattering from gold nanoparticles interacting with a gold film,” said Cristian Ciracì, a postdoctoral researcher at Duke’s Pratt School of Engineering. “Our results provide a strong experimental support in setting an upper limit to the maximum field enhancement achievable with plasmonic systems.”

The experiments were conducted in Smith’s lab; the team worked with colleagues at Imperial College, specifically Sir John Pendry, who has long collaborated with Smith.

“This paper takes experiment beyond nano and explores the science of light on a scale of a few tenths of a nanometer, the diameter of a typical atom,” said Pendry, a physicist and co-director of the Centre for Plasmonics and Metamaterials at Imperial College. “We hope to exploit this advance to enable photons, normally a few hundred nanometers in size, to interact intensely with atoms, which are a thousand times smaller.”

The work, which appears in Science (doi: 10.1126/science.1224823) was supported by the Air Force Office of Scientific Research and by the Army Research Office’s Multidisciplinary University Research Initiative.