Laser Optical Antennas

Laser optical antennas represent a relatively new approach to getting around the old diffraction limit characterizing conventional optics, namely the inability of a lens to focus light for imaging purposes to any better than about half the wavelength of the light being used.

Like a rooftop antenna which grabs meter-sized radio waves and turns them (courtesy of a tuned circuit) into signals far smaller in physical extent, so the optical antenna converts visible light into an illuminating beam of much higher resolving power. For example, 800-nanometer light can produce images with a spatial resolution of no better than about 400 nanometer.

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 (see PNU 770 for background) 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. (For comparison images recorded with a force microscope, an electron microscope, and the new laser antenna, see Physics News Graphics).

Crozier (kcrozier@deas.harvard.edu, 617-496-1441) 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
Contact Ken Crozier, Harvard University
kcrozier@deas.harvard.edu, tel: 617-496-1441
See the Crozier Group Web site
Image at Physics News Graphics
See also PNU 701