A New Focus for Light

 

Researchers trying to make high-capacity DVDs, as well as more-powerful computer chips and higher-resolution optical microscopes, have for years run up against the "diffraction limit." The laws of physics dictate that the lenses used to direct light beams cannot focus them onto a spot whose diameter is less than half the light's wavelength. Physicists have been able to get around the diffraction limit in the lab--but the systems they've devised have been too fragile and complicated for practical use. Now Harvard University electrical engineers led by Kenneth Crozier and Federico Capasso have discovered a simple process that could bring the benefits of tightly focused light beams to commercial applications. By adding nanoscale "optical antennas" to a commercially available laser, Crozier­ and Capasso have focused infrared light onto a spot just 40 nanometers wide--one-­twentieth the light's wavelength. Such optical antennas could one day make possible DVD-like discs that store 3.6 terabytes of data--the equivalent of more than 750 of today's 4.7-gigabyte recordable DVDs.

Crozier and Capasso build their device by first depositing an insulating layer onto the light-emitting edge of the laser. Then they add a layer of gold. They carve away most of the gold, leaving two rectangles of only 130 by 50 nano­meters, with a 30-­nanometer gap between them. These form an antenna. When light from the laser strikes the rectangles, the antenna has what Capasso calls a "lightning­-rod effect": an intense electrical field forms in the gap, concentrating the laser's light onto a spot the same width as the gap.

"The antenna doesn't impose design constraints on the laser," Capasso says, because it can be added to off-the-shelf semiconductor lasers, commonly used in CD drives. The team has already demonstrated the antennas with several types of lasers, each producing a different wavelength of light. The researchers­ have discussed the technology with storage-device companies Seagate and Hitachi Global Storage Technologies.

Another application could be in photo­lithography, says ­Gordon Kino, professor emeritus of electrical engineering at Stanford University. This is the method typically used to make silicon chips, but the lasers that carve out ever-smaller features on silicon are also constrained by the diffraction limit. Electron-beam lithography, the technique that currently allows for the smallest chip features, requires a large machine that costs millions of dollars and is too slow to be used in mass production. "This is a hell of a lot simpler," says Kino of Crozier and Capasso's technique, which relies on a laser that costs about $50.

But before the antennas can be used for lithography, the engineers will need to make them even smaller: the size of the antennas must be tailored to the wavelength of the light they focus. Crozier­ and Capasso's experiments have used infrared lasers, and photo­lithography relies on shorter-wavelength ultraviolet light. In order to inscribe circuitry on microchips, the researchers must create antennas just 50 nanometers long.

Capasso and Crozier's optical antennas could have far-reaching and un­predictable implications, from superdense optical storage to ­superhigh-resolution optical microscopes. Enabling engineers to simply and cheaply break the diffraction limit has made the many applications that rely on light shine that much brighter.