by Eric Smalley, Technology Research News
Researchers are attempting to bring light and its useful properties to the realm of nanotechnology, but the effort is challenging because visible light can't be focused more tightly than about 200 nanometers, or about one-fifth the width of an E. coli bacterium.
Computer circuits and nanodevices are already smaller than that, and getting still smaller.
The key is figuring out how to guide near-field light, which forms a standing field rather than light's usual moving wave. Near-field light is produced by shining light through a tiny hole or lens that is positioned closer to a surface than the wavelength of the light; its size is limited by the hole or lens size rather than the wavelength of the light. When light is farther away from a surface, it can only be focused to about half its wavelength.
Researchers from the California Institute of Technology and the University of Southern California have found a way to guide near-field light over short distances through channels that are several times narrower than the wavelengths of light. The method sets the stage for nanoscale sensors and communications devices that use light to measure individual molecules and carry signals between tiny, closely spaced electronic components.
The researchers have developed a waveguide that ushers plasmon waves along a chain of metal nanoparticles. Plasmons are oscillations of the electrons on the surface of metal stimulated by near-field light.
The waveguides could be used to optically connect components within computer chips or to channel light to nanoscale biological and chemical sensors, said Stefan Maier, an applied physics researcher at the California Institute of Technology. Spectrographic sensors can identify molecules by the way they interact with light. The waveguides could also channel bits in ultrahigh-capacity optical data storage devices.
The researchers' prototype plasmon waveguide is made from silver rod-shaped nanoparticles 90 nanometers long by 30 nanometers wide and spaced 50 nanometers apart. The nanoparticles are parallel, like railroad ties, and form a 90-nanometer-wide path. The researchers directed a spot of light onto one of the particles using a near-field scanning optical microscope that contained a 100-nanometer opening.
The near-field light generated a plasmon wave. "The electrons in the particle slosh back and forth, driven by the light field," said Maier. The oscillations excite adjacent rods, allowing a plasmon wave to propagate along a particle chain, he said. The light energy is confined to the particle array even though the particles are smaller than the diffraction limit of light, he said.
It is the narrow field of near-field light that makes this type of propagation possible, said Maier. Regular lightwaves are large enough that they hit the entire nanoparticle chain at once, and so cannot move from one end to the other. "Think of a guitar string," he said. In order to have a wave pass through it, causing the string to undulate, the energy must originate at one point rather than encompassing the whole string at once, he said.
The researchers demonstrated the effect by adding to the waveguide fluorescent nanospheres that glowed more brightly when they absorbed the plasmon energy. An increase in fluorescence from a nanosphere that was too far from the microscope to have been directly excited proved that plasmon energy was traveling along the guide. "The energy propagates along the waveguide to the [nanosphere]," said Maier.
The plasmon waves fade quickly as they travel along the nanoparticle waveguides, which means the waveguides can only be used to transfer energy over distances of a micron or less, said Maier.
The researchers are now working to reduce the energy loss of current prototypes by finding alternative substrates on which to build the waveguides, said Maier. They currently make the nanorods using electron beam lithography, which requires that they use substrates that conduct electricity, he said.
The researchers are also working on constructing more complicated optical devices like modulators and switches that will give them more control over plasmon waves, said Maier. Their ultimate goal is to integrate plasmon devices into optical chips, he said.
They are also looking to find methods of focusing near-field light onto the waveguides that can be integrated into practical devices, said Maier.
Other researchers, including a team at Karl Franzens University in Austria, are pursuing similar research using nanowires instead of chains of nanoparticles.
Prototype plasmon waveguide interconnects could be developed within five years, said Maier.