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 Atomic force microscopy image of optical antenna. Picture: www.aip.org
 Near-field optical scanning microscopy image of optical antenna. Picture: www.aip.org
A new device, built by the groups
of Ken Crozier and Federico Capasso at Harvard, producing spot sizes
as small as 40 nanometer using 800-nanometer light, is the first optical antenna
to be fully integrated (laser and focusing apparatus on one
platform) and the first to prove (by directly measuring light
intensities) the narrowness of the focused spot of light.
Their
method combines two proven techniques -- plasmonics, in which light
waves, striking a metal surface, can create plasmons, which are a
sort of electromagnetic disturbance with a
wavelength less than that of the incoming light; and near-field
microscopy, in which the diffraction limit is avoided by placing the
specimen very close to the imaging device.
In the Harvard setup the
antenna consists of two gold patches (130 nanometer long by 50 nanometer wide)
separated by a 30 nanometer gap. Light falling on the gold strips (which
sit right on the facet of an ordinary commercial laser diode)
excites a huge electric field in the gap. A specimen located
beneath this gap sees it as a 30-nanometer wide burst of light (although at
this stage in the work the spot size is more like a 40 x 100 nanometer
rectangle).
In many forms of subtle microscopy, power is sometimes
feeble, but here, in pulsed operation, the antenna can generate a
robust peak intensity of more than a gigawatt per square centimeter.
Crozier says that spot
sizes of 20 nanometer should be possible and that likely applications for
their laser antenna will be found in the areas of optical data
storage (where 3 terabytes of data could be stored on a CD),
spatially-resolved chemical imaging, and near-field scanning optical
microscopy (NSOM).
Cubukcu et al.,
Applied Physics Letters, 28 August 2006
Credits: AIP
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