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A tiny "bowtie" could be key to improving Raman spectroscopy by improving the optical mismatch between nanoscale objects and light according to researchers at Stanford University. The nanoscale device can effectively compress ordinary light waves into an intense optical spot only 20 nm wide and so could also allow high-resolution microscopic images to be obtained for a wide range of applications.
Optical imaging in nanotechnology has until now remained something of a non-sequiteur given that by definition nanoscopic objects are smaller than the wavelength of visible light. Even the larges molecules, such as DNA, which might form the components of a nanodevice are only a few nanometres across, whereas the wavelength of light is of the order of hundreds of nanometres long.
However, W.E. Moerner, Gordon Kino and colleagues Stanford University want to shed light on single molecules and so have developed a nanoscale antenna, which resembles a bowtie in shape, that they say can compress ordinary light waves and so circumvent the mismatch between light's wavelength the length scales of nanotechnology. The team hope that one day such nano-antennae will provide snapshots of proteins, DNA molecules and synthetic objects, such as bundles of carbon nanotubes. "One of our goals is to build a microscope with bowtie antennae that we can scan over a single molecule," explains Moerner.
Moerner and colleagues James Schuck, David Fromm, Gordon Kino, and Arvind Sundaramurthy first introduced their bowtie antennae earlier this year, but discussed new details at the recent meeting of the American Chemical Society. The bowtie comprises two triangular pieces of gold, each just 75 nm long. The antenna operates like a radio receiver, but rather than gathering radio waves it grabs energy from an incident beam of near infrared at 830 nm and compresses it into the 20 nm gap that separates the tips of the two gold triangles. The resulting speck of light is rendered a thousand times more intense than the incident NIR beam.
Kino explains how this intense spot of light can be used to scan a nanoscopic object, "What you end up with is a very small optical spot that you could scan to make detailed images of molecules and other nano-particles," he says, "Normally, we use lenses to focus, but it's not possible to resolve detail in objects smaller than one-half the wavelength of light." The shortest visible light is 400 nm, so objects smaller than 200 nm could not be resolved until now. "The bowtie antenna produces an optical spot that's 20 nm wide, so we're improving the resolution by a factor of 10," Kino adds.
From the spectroscopist's point of view the nano bowties could be rather useful in Raman applications allowing much smaller samples to be analyzed at the molecular level. "The bowties provide a controllable way to generate a locally enhanced electric field," Moerner told spectroscopyNOW, "This is critical to increasing Raman scattering signals, because the detected Raman signal depends upon the fourth power of the local electric field, or the second power of the intensity." Their measurements of the intensity enhancement give about a factor of 1000 increase in intensity, so the Raman signal should be 1 million times stronger than it would have been without the antenna. This occurs only from an approximately 20 nm region around the bowtie gap, so the potential for local Raman spectroscopy is high.
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The bowtie nanoantenna consists of two triangular pieces of gold separated by a 20-nanometer gap (Credit: Arvind Sundaramurthy)
W. E. Moerner |
