Nano triangles concentrate light

Seeing an object means sensing the lightwaves bouncing off it. The challenge in building a microscope that shows objects that are the size of molecules is that they are considerably smaller than the size of lightwaves, and light can only be focused down to about half its wavelength.

The diffraction limit of light also limits the size of electrical components like computer chips that are etched using light beams. Visible light waves range from 400 to 700 nanometers. A nanometer is one millionth of a millimeter, or the span of 10 hydrogen atoms. The smallest feature that can be imaged in a practical way using visible light is around 300 nanometers.

Researchers from Stanford University have sidestepped the diffraction limit of visible light with an extremely small gold bowtie nanoantenna that focuses visible and near-infrared light to extremely small, intense spots about 20 nanometers in diameter.

The intense spots of light could eventually be used to allow microscopes to focus at the nanoscale, and to extend lithographic manufacturing processes like those used to make computer chips to support smaller components. They also promise to be useful in optical studies.

The bowtie nanoantenna is made from a pair of 75-nanometer long gold triangles separated by a gap of about 20 nanometers. The device combines a lightning-rod-like effect present at sharp metallic points with the tendency of metal nanoparticles to attract electrons between them to capture nearby light and concentrate it in the gap. Because these effects confine the light to a very small area, the light is more than one thousand times more intense than a beam focused using conventional optics.

The researchers are aiming to have a practical implementation of the bowtie nanoantenna built within a year. The work appeared in the January 14, 2005 issue of Physical Review Letters.