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  • Focusing on nanoplasmonics

Photonics Spectra
Apr 2011
BERKELEY, Calif. – By imaging fluorescence from gold within a bow tie-shaped plasmonic device, scientists can now study plasmonic fields in nanostructures such as a strand of DNA or a quantum dot without altering the structures’ behavior.

Typical plasmonic devices crowd electromagnetic waves into tiny metal structures that concentrate the energy into nanoscale dimensions. By coupling electronics with photonics in these nanostructures, these devices could be harnessed for high-speed data transmission or ultrafast detector arrays. However, studying nanostructures is problematic because examining them means changing their behavior.

Scientists at Lawrence Berkeley National Laboratory explain that wavelengths of light, whether the source is a laser or a lightbulb, are too large for studying plasmonic fields in nanostructures. Another limitation is that when examining a typical nanoscale object, distinguishing one closely spaced nanostructure from another is difficult.

To make that distinction, the researchers are using a previously developed bow tie-shaped plasmonic device to capture, filter and steer light on the nanoscale. Serving as an antenna, the nanocolor sorter device can divert light in tiny spaces to a desired set of colors, which is crucial for filters and other detectors. It was engineered by staff scientist Jim Schuck in collaboration with researchers at Molecular Foundry, a US Department of Energy Nanoscale Science Research Center.

The team is using this device to visualize plasmonic fields. By imaging fluorescence from gold within the bow-tie device and maximizing the number of photons collected, they can determine the position of plasmonic modes as close together as a few nanometers. They also found that by shifting the structure of a double bow-tie antenna by only a few nanometers, they could focus light at different positions inside the bow tie with accuracy and predictability.

Their findings appeared in Physical Review Letters, Jan. 18, 2011 (doi: 10.1103/PhysRevLett.106.037402). Support for their work was provided by the National Science Foundation through its Network for Computational Nanotechnology.

1. With respect to radiation, a device used to attenuate particular wavelengths or frequencies while passing others with relatively no change. 2. See digital filter.
quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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