MICROSCOPY: Plasmonic laser antenna sharpens nanovision

Oct. 1, 2006

In microscopy, diffraction limits spatial resolution to about half a wavelength, or several hundred nanometers with visible light.

Hassaun A. Jones-Bey

FIGURE 1. A plasmonic laser antenna integrates an optical antenna on a facet of a commercial laser diode.

In microscopy, diffraction limits spatial resolution to about half a wavelength, or several hundred nanometers with visible light. One way of overcoming this is to use an antenna, rather than a lens, to concentrate light to the subwavelength scale. A challenge that has arisen with such optical antennae is that surface plasmons (collective electron resonances caused by the penetration of radiation into metals) prevent a direct downscaling of traditional antenna designs and hence demand a careful study of surface modes in metal nanostructures or plasmonics (see www.laserfocusworld.com/articles/259934).

FIGURE 2. Optical-antenna facets are shown in atomic-force-microscope (AFM) and scanning-electron-microscope images at top (left and right, respectively). The small bump below the right antenna facet in the AFM image is probably due to nanomasking and shows up as a perturbation to the right of the main lobe of illumination, both in the apertureless near-field scanning optical-microscope image and line scan of near-field distribution at bottom (left and right, respectively).

In August of this year, researchers at Harvard University (Cambridge, MA) reported research results that appear to address this conundrum directly. They successfully made use of plasmonic effects to integrate an optical antenna onto a laser diode. In doing so, they also concentrated the laser radiation down to dimensions that are an order of magnitude less than its wavelength.¹

The Harvard surface-plasmon device consists of a dipole antenna in which two 130-nm-long and 50-nm-wide gold nanorods are separated by a 30 nm gap and integrated onto the facet of a commercial 830 nm laser diode (see Fig. 1). Laser excitation of surface plasmons in the gold nanorods generated enhanced and spatially confined optical near fields, and a spot size measuring 40 × 100 nm was obtained (see Fig. 2). Resonant dimensions for the optical-antenna gap and segments were obtained prior to fabrication by modeling optical-antenna structures using finite-difference time-domain software.

The Harvard team was from the Division of Engineering and Applied Sciences. It included Ertugrul Cubukcu and Eric Kort, graduate students in the Ken Crozier and Frederico Capasso research groups at Harvard. Their device appears to be the first integration of an optical antenna onto a laser, providing a platform that effectively combines plasmonics with near-field microscopy.

“The laser diode illuminates the antenna, exciting surface-charge oscillations or surface plasmons,” Crozier said. “This leads to an intense nanospot of light in the antenna gap.” The intensity of the spot is due to a capacitive effect between the gold nanorod facets that creates a huge electric field. In pulsed operation, the antenna can generate a peak intensity of more than a gigawatt per square centimeter.

The researchers believe that spot sizes of 20 nm should be possible and point out that the technology can be implemented in spectral regions ranging from the visible to the far-infrared, and can also be implemented using quantum-cascade lasers. Potential application areas include near-field optical microscopes, optical data storage, and heat-assisted magnetic recording.