Light and nano: quantum mechanics vs. classical optics

Rice physicists show that quantum methods can predict nanophotonic behavior

According to new research from Rice University, scientists studying the way light interacts with metallic nanostructures should throw out their old optics textbooks and bone up on their quantum mechanics instead.

The new findings, which are described in the Oct. 17 issue of the journal Science, offer a new understanding of plasmonics, an emerging field of optics aimed at the study of light at the nanometer scale — at dimensions far smaller than a wavelength of light, smaller than today’s smallest electronic devices. Rice’s findings will make it easier for scientists and engineers to design new optical materials and devices “from the bottom up,” using metal particles of specifically tailored shapes.

The field of plasmonics, which has existed for only a few years, has already attracted millions of research dollars from industry and government. One primary goal of this field is to develop new optical components and systems that are the same size as today’s smallest integrated circuits and that could ultimately be integrated with electronics on the same chip. In the field of chemical sensing, plasmonics offers the possibility of new technologies that will allow doctors, anti-terror squads and environmental experts to detect chemicals in quantities as small as a single molecule — a prospect so intriguing the National Nanotechnology Initiative chose it as one of this past year’s primary funding objectives.

“What this work gives us is a simple, intuitive model that describes how ultrasmall metal structures of various shapes capture and manipulate light,” said Naomi Halas, the Stanley C. Moore Professor of Electrical and Computer Engineering and professor of chemistry. It provides a practical design tool for nanoscale optical components.”

The fact that light interacts with nanostructures at all flies in the face of traditional optics, which held for more than a century that light waves couldn’t interact with anything smaller than their own wavelengths.

Research over the past five years has turned that assumption on its head, showing that nanoscale objects can amplify and focus light in ways scientists never imagined. The “how” of this involves plasmons, ripples of waves in the ocean of electrons flowing across the surface of metallic nanostructures. The type of plasmon that exists on a surface is directly related to its geometric structure – the precise curvature of a nanoscale gold sphere or a nano-sized pore in metallic foil, for example. When light of a specific frequency strikes a plasmon that oscillates at a compatible frequency, the energy from the light is harvested by the plasmon, converted into electrical energy that propagates through the nanostructure and eventually converted back to light. Researchers at Rice, Caltech, Stanford and UCLA, as well as European teams at Imperial College, UK, and Strasbourg, France, have all reported advances in plasmonics in recent years.

Some nanostructures act as superlenses, capturing specific wavelengths of light, focusing the light to ultrasmall spots at high intensities and converting some electrical energy back into light that is reflected away. One such nanoparticle is the nanoshell, which was developed at Rice five years ago in Halas’ laboratory.

In the research described in the Science report, the Rice team show that the equations that determine the frequencies of the plasmons in complex nanoparticles are almost identical to the quantum mechanical equations that determine the energies of electrons in atoms and molecules. Their method is called “plasmon hybridization.” Just as quantum mechanics allows scientists to predict the properties of complex molecules, the work performed by the Rice team shows how the properties of plasmons in complex metallic nanostructures can be predicted in a simple manner.

“What we’ve found is that plasmons in nanoparticles hybridize with each other in the same way that atomic energy levels hybridize with each other when atoms form molecules,” said Peter Nordlander, the theoretical physicist who led the study. “The findings are applicable not only to nanoshells, but to nanoscale wave guides and any other nanophotonic structures.”

Nordlander, a professor in both the physics and astronomy and the electrical and computer engineering departments at Rice, said the importance of the research is that it frees researchers from having to describe nanophotonic structures in terms of classical optics, something that plasmonic scientists have struggled with since the field was formed.

“Electromagnetism on the nanoscale is a messy subject. The equations are very complicated, which restricts our intuition from playing a role in the rational design of nanostructures with specific optical properties,” said Nordlander. “Extensive calculations on powerful computers give you the right answer, but they don’t provide the kind of information that guides your thinking. Our approach provides a conceptual foundation for designing nano-optical components of arbitrary shape and understanding in advance what they will do to light. It is based on our intuition developed from quantum mechanics, and for us that makes things much easier.”

The research was sponsored by the Army Research Office and the Robert A. Welch Foundation.

The paper, titled “A Hybridization Model for the Plasmon Response of Complex Nanostructures,” was co-authored by Nordlander, Halas and doctoral graduate students Emil Prodan and Corey Radloff.