ALBUQUERQUE, N.M. - The U.S. Department of Energy's Sandia National Laboratories recently reported an optical light-bending technology that makes current wave guides obsolete.
Called a photonic lattice, the technology has been the holy grail of optical communications researchers because it enables coherent light to bend as much as 90 with 95 percent efficiency, compared with 30 percent efficiency with conventional wave guides. In addition, it takes only one wavelength, not 10, to make a bend.
"Others have tried to build photonic lattices with gallium arsenide, because it is a conventional material for optics, but they were not successful," said Sandia researcher Jim Fleming. "We succeeded by using silicon fabrication equipment, because commercial equipment for fabbing silicon is more advanced and readily available."Fleming is a silicon fab expert. The other member of the team that created the photonic lattice, researcher Shawn Lin, is the expert in optical technologies.
The lattice functions as the photonic equivalent of an artificial single-crystal material, because the elements of the lattice have a perfect symmetry that is constant throughout. While its microscopic structure is that of a lattice, its crystal-like function leads many to call it an artificial crystal.
Conventional wave guides use crystals of different refractive indices to bend light, but the photonic lattice achieves a more efficient result by creating a 3-D version of a defraction grating.
Physically, the photonic lattice resembles a stack of 2-D defraction gratings one atop the other, with each layer turned 90 to the one above. Thus, every other layer has its gratings parallel to those two layers above and below. The gaps in the grating are staggered so that every other layer's gratings align with the gaps two layers above and below, and thus the pattern repeats with every fourth layer. It takes about 10 such layers to create a working photonic lattice.
Rather than scoring a thin 2-D material to create each layer, the Sandia team fabricated individually sized grating elements separately, leaving air gaps between them when they are stacked. By carefully choosing the gap size, one wavelength is reflected while all others pass through. Each grating element was etched from silicon using 0.18-micron design rules, resulting in a photonic lattice that operates in the 1.5-micron wavelength range-the most popular wavelength for optical communications equipment. The team last year reported success in the 12-micron region, but scaled the device down to prove the concept for commercially useful wavelengths.
Fleming, an expert in building surface-etched micromachines, built the photonic lattice by first coating a silicon wafer with silicon dioxide, then cutting trenches into it and bathing the chip in polysilicon until the trenches were filled. The surface was then polished until smooth and the chip was bathed in another layer of silicon dioxide into which new trenches were laid down at right angles to the first. Those gaps were then also filled with polysilicon. This process was repeated for the number of de-sired layers, after which the silicon dioxide was removed using hydrofluoric acid. The result was polysilicon elements 1.2 microns wide and 1.5 microns high, with a pitch of 4.8 microns. "The advantage of building the photonic lattice in 3-D is that it works the same no matter what size you make it-it's just a matter of what wavelength of light is applicable," said Fleming.
The next step will be fabricating working wave guides and filter cavities. A filter cavity traps light inside the lattice and allows only a particular wavelength to pass, creating a high-Q bandpass filter. The first demonstration of a working filter cavity will probably be in 6-micron wavelength, but the researchers say they will subsequently scale that down.
"Ours will be the first to use silicon fabrication equipment to shrink the technology down to a commercially viable size," said Fleming.
Ultimately, the photonic lattice has the potential to displace GaAs as the industry's favorite optical material, by enabling vertical-cavity surface-emitting lasers and other GaAs devices to be built in silicon. Tens of thousands of waveguides can be fabricated on a 6-inch silicon wafer-about 100 times more than can be fabricated using GaAs.
"Silicon has an indirect bandgap, so it doesn't emit or absorb light like gallium arsenide does," said Fleming. "But with the photonic lattice technology, we will eventually be able to make LEDs, microlasers requiring minimal startup energy and other optical-interconnection components directly on silicon chips."