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Raman probe beats resolution record

14 Mar 2003

Researchers unveil an optical image with a spatial resolution of 17 nm, which they say is the highest reported to date.

Using a technique known as near-field Raman spectroscopy, researchers in the US claim to have taken the highest resolution optical image to date. The image has a spatial resolution of 17 nm and shows detailed structures of single-walled carbon nanotubes. (Physical Review Letters 90 095503)

The researchers from the universities of Rochester, Portland State and Harvard now hope to improve the technique to study proteins in cell membranes. This may help scientists produce designer medicines and identify new disease strains.

Raman spectroscopy involves measuring how laser light is scattered in a sample and gives detailed structural information. The only other technique that can match and better this resolution is atomic force microscopy (AFM), but Lukas Novotny, who heads the Rochester group, argues that AFM does not produce an optical image.

The Raman technique involves placing a sharp 15-nm-diameter silver tip close to the focus of a 633 nm laser beam. The tip itself is held just 1 nm above the sample and scanned across it. "The exciting laser produces a localized field at the tip apex," explained Novotny. "This field interacts with the vibrating atoms in the sample." The scattered light is gathered to form an image.

"Every sample has its natural vibration," Novotny told Optics.org. "These vibrations form a highly specific fingerprint for the chemical composition of the sample."

According to Novotny, technological advances lie behind this breakthrough. He says the secret lies in sharp tips, illumination polarized along the tip axis, sensitive detectors, a favorable sample system and being able to hold the tip within 1 nm of the sample surface without damage.

The team chose to study carbon nanotubes as they produce a strong Raman scattering signal. "To image an arbitrary sample using the same contrast mechanism we would need to increase our sensitivity by several orders of magnitude," Novotny said. "That's what we are currently working on." The group is now looking at different tip shapes and materials.

Author
Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.

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