03/19/2003 08:00:00

Caltech applied physicists invent waveguideto bypass diffraction limits for new optical devices

Four hundred years ago, a scientist could peer into one of the newfangled optical microscopes and see microorganisms, but nothing much smaller. Nowadays, a scientist can look in the latest generation of lens-based optical microscopes and also see, well, microorganisms, but nothing much smaller. The limiting factor has always been a fundamental property of the wave nature of light that fuzzes out images of objects much smaller than the wavelength of the light that illuminates those objects. This has hampered the ability to make and use optical devices smaller than the wavelength. But a new technological breakthrough at the California Institute of Technology could sidestep this longstanding barrier.

Caltech applied physicist Harry Atwater and his associates have announced their success in creating "the world's smallest waveguide, called a plasmon waveguide, for the transport of energy in nanoscale systems." In essence, they have created a sort of "light pipe" constructed of a chain-array of several dozen microscopic metal slivers that allows light to hop along the chain and circumvent the diffraction limit. With such technology, there is the clear possibility that optical components can be constructed for a huge number of technological applications in which the diffraction limit is troublesome.

"What this represents is a fundamentally new approach for optical devices in which diffraction is not a limit," says Atwater.

Because the era of nanoscale devices is rapidly approaching, Atwater says, the future bodes well for extremely tiny optical devices that, in theory, would be able to connect to molecules and someday even to individual atoms.

At present, the Atwater team's plasmon waveguide looks something like a standard glass microscope slide. Fabricated on the glass plate by means of electron beam lithography is a series of nanoparticles, each about 30 nanometers (30 billionths of a meter, in other words) in width, about 30 nanometers in height, and about 90 nanometers in length. These etched "rods" are arranged in a parallel series like railroad ties, with such a tiny space between them that light energy can move along with very little radiated loss.

Therefore, if light with a wavelength of 590 nanometers, for example, passes through the nanoparticles, the light is confined to the smaller dimensions of the nanoparticles themselves. The light energy then "hops" between the individual elements in a process known as dipole-dipole coupling, at a rate of propagation considerably slower than the speed of light in a vacuum.

In addition to their functionality as miniature optical waveguides, these structures are also sensitive to the presence of biomolecules. Thus, a virus or even a single molecule of nerve gas could conceivably be detected with an optical device designed for biowarfare sensing. The potential applications include electronic devices that could detect single molecules of a pathogen, for example.

The ultrasmall waveguide could also be used to optically interconnect to electronic devices, because individual transistors on a microchip are already too small to be seen in a conventional optical microscope.

A description of the device will appear in the April 2003 issue of the journal Nature Materials. The other Caltech authors of the paper were Stefan A. Maier, a former graduate student and now postdoctoral researcher at Caltech, who was responsible for the working device, and Pieter G. Kik, also a postdoctoral researcher. Other authors were Sheffer Meltzer, Elad Harel, Bruce E. Koel, and Ari A.G. Requicha, all from the University of Southern California.

The nanoparticle structures were fabricated at the Jet Propulsion Laboratory's facility for electron beam lithography, with the help of JPL employees Richard Muller, Paul Maker, and Pierre Echternach.

The research was sponsored by the Air Force Office of Scientific Research and was also supported in part by grants from the National Science Foundation and Caltech's Center for Science and Engineering of Materials.

Contact: Robert Tindol (626) 395-3631