As gigabytes of movies, pictures, audio, and text fill up more and more CDs and DVDs, there’s clearly a need for better ways to save more data. A research team at Harvard University has developed a technique that could help to significantly boost the capacity of conventional optical discs. They’ve fabricated a nano antenna–built directly onto an inexpensive, off-the-shelf laser–that focuses light to a much smaller spot size than is possible with even the best traditional lenses, potentially enabling more bits to be written onto an optical disc.
The storage capacity of a disc increases as the wavelength of light used to write data to it decreases; CDs are written and read using light with a wavelength of 780 nanometers, DVDs use 650 nanometers, and HD-DVDs and Blu-ray discs use 405 nanometers. Wavelengths shorter than 405 nanometers would require light sources far too expensive for consumer electronics.
The problem is that conventional lenses can only focus light to half their wavelength, a physical barrier called the diffraction limit. The Harvard researchers sidestepped this limit, however, by abandoning traditional optics in favor of nano-optical techniques. “We can get around the wavelength limitation by using an antenna,” says Ken Crozier, assistant professor of electrical engineering at Harvard.
The team of Crozier, Federico Capasso, professor of applied physics at the university, and graduate students Eric Kort and Ertugrul Cubukcu designed the optical antenna to focus light from a commercial laser (with a wavelength of 830 nanometers) to a spot size of 40 nanometers. With this resolution, “you’d be able to pack more than three terabytes [about 3,000 gigabytes] worth of data onto something the size of a CD,” Crozier estimates. That’s enough to hold more than 300 feature-length movies. By comparison, a dual-layer HD-DVD or Blu-ray disc can hold 30 gigabytes or 50 gigabytes, respectively.
The antenna consists of two gold-coated nano rods, separated by a 30-nanometer-wide gap, according to Crozier. When light from the laser hits the nano rods, it applies a force to the electrons in the gold, nudging them out of place. The electrons don’t stay displaced for long, however, and are pulled back toward their original position. But they overshoot it, Crozier says, and bounce back out of place, oscillating “like a mass on a spring.”
These oscillating electrons affect the tiny gap between the nano rods. If you took a snapshot of the antenna, Crozier says, you’d see that positive charges collect on one side of the gap, and negative charges on the other. The nano rods and gap act as a tiny capacitor–with opposite charges on opposite sides of the gap–that effectively concentrates the energy from the laser light into a spot about the size of the gap. This spot maintains its size to about 10 nanometers away from the antenna before it starts to spread out.
Although the 10-nanometer gap is minuscule, researchers could build a new type of optical reading and writing head using the technology, suggests Crozier. The magnetic storage industry, he points out, works with a similarly small gap between the head and medium.
Using nano antennas to focus optical light is not an entirely new idea, Crozier says, but their work, published in Applied Physics Letters, is the first time an antenna has been integrated directly onto a laser. This offers an advantage in production because the light source and antenna are in one package. “It’s extremely compact and easier to use because alignment with the laser and the antenna is all done in fabrication,” he says.
There’s a lot of research activity to reduce the spot size of light, but it’s especially attractive to the data storage industry, says Bae-Ian Wu, a research scientist in the Research Laboratory of Electronics at MIT. Using a nano antenna is just one way to gain “super resolution smaller than the wavelength of light.” But, he says, the Harvard researchers work “is very good in the sense that they are doing optical experiments to back up their theory, while some papers are only in the realm of theory.” The Harvard scientists, he adds, “just did it.”
Crozier says his team is exploring fabrication techniques that can further decrease the spot size to 20 nanometers. They’re also exploring alternatives to the gold metal that currently coats their nano rods. Silver, for instance, could focus light more efficiently than gold at the wavelengths used by the consumer electronics industry.
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