Going negative for a fast switch

Hank Hogan, hank@hankhogan.com

The days of high-speed, all-optical communication are literally a bit closer. Researchers at Los Alamos National Laboratory and at the University of New Mexico in Albuquerque have demonstrated a nanoscale negative-index metamaterial device that can switch a bit on or off in 600 fs – 100 times faster than any previously reported.

“In principle, this gives us more than a terabit per second communication speeds,” said Keshav M. Dani, a postdoctoral fellow at Los Alamos and lead author of a September 2009 Nano Letters paper describing the research.

He cautioned that the demonstration involved only a single bit and thus is still far from a commercial switch capable of modulating a trillion bits per second. However, with structural tuning, the demonstration device will work in the near-IR from 1 to 2 μm – a wavelength range widely used in telecommunications. Thus, all-optical switching based on this operating principle could be of interest to industry. Dani doesn’t foresee fundamental barriers to producing such products, although many engineering hurdles may have to be overcome.

The negative-index metamaterial devices were composed of two 28-nm-thick layers of silver sandwiching a 60-nm layer of amorphous silicon. Using a liftoff process, the researchers created elliptical holes in the film stack, with a spacing of about 100 nm between the roughly 200-nm holes. The combination of film and hole geometries resulted in a metamaterial with a negative index of refraction at wavelengths of 1.13 and 1.68 μm. Other geometries would shift those regions around, allowing anything between 1 and 2 μm to be covered.

Shown is a nanoscale fast-switching metamaterial composed of three layers: silver, amorphous silicon and silver (inset). A visible pulse changed this fishnet metamaterial’s near-IR transmission, opening up the possibility of all-optical ultrafast switching. Courtesy of Zahyun Ku, University of New Mexico, and Keshav M. Dani, Los Alamos National Laboratory.

The key to the rapid switching was the use of amorphous silicon, a semiconductor. When the researchers photoexcited the silicon with a laser pulse at a wavelength short enough for the photon energy to be above the bandgap, they flooded the silicon temporarily with holes and electrons. That altered the optical behavior of the metamaterial. The change lasted only as long as the carriers did, or 600 fs in the demonstration device.

The researchers used a laser and an optical parametric amplifier to generate a visible pulse for photoexcitation. Using a different optical parametric amplifier, they generated a near-IR pulse and measured its transmittance through the material. They were able to alter transmission at the shorter negative refraction wavelength by 20 percent, a figure determined by the photoexcitation pulse power.

Besides telecom applications, other possible uses involve data communications. The demonstration device involved transmission, but other optical properties could be used, such as phase or polarization.

As for the future, University of New Mexico graduate student Zahyun Ku noted that the fabrication of the devices could be improved, resulting in more nearly vertical hole sidewalls, better film quality, additional functional layers and greater device uniformity. These enhancements would increase resonance intensity and improve device performance, he said.

“If we would have stronger resonance peaks at the negative refractive index region, we could expect a larger switching ratio for applications,” he added.