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Research unveils chips that can steer light

11 Jan 2013

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Researchers from Massachusetts Institute of Technology (MIT) Research Laboratory of Electronics (RLE) have developed chips consisting of a 4,096-emitter array that can steer beams of light and enable a range of applications such as cheaper, more efficient and smaller laser rangefinders, medical-imaging devices that can be threaded through tiny blood vessels, and even holographic TVs that emit different information when seen from different viewing angles, detailed the researchers.

The MIT authors—Michael Watts, an associate professor of electrical engineering, Jie Sun, a graduate student in Watts' lab and first author on the paper, Sun's fellow graduate students Erman Timurdogan and Ami Yaacobi, and Ehsan Shah Hosseini, an RLE postdoc—report on two new chips. Both chips take in laser light and re-emit it via tiny antennas etched into the chip surface.

MIT chips steer light

Figure 1: The new chip consists of tiny antennas arranged in a 64-by-64 grid.

A wave of light can be thought of as a sequence of crests and troughs, just like those of an ocean wave. Laser light is coherent, meaning that the waves composing it are in phase: their troughs and crests are perfectly aligned. The antennas in the RLE researchers' chips knock that coherent light slightly out of phase, producing interference patterns.

In the 4,096-antenna chip—a 64 x 64 grid of antennas—the phase shifts are precalculated to produce rows of images of the MIT logo. The antennas are not simply turned off and on in a pattern that traces the logo, as the pixels in a black-and-white monitor would be. All of the antennas emit light, and if you were close enough to them (and had infrared vision), you would see a regular array of pinpricks of light. Seen from more than a few millimetres away, however, the interference of the antennas' phase-shifted beams produces a more intricate image.

In the other chip, which has an 8 x 8 grid of antennas, the phase shift produced by the antennas is tunable, so the chip can steer light in arbitrary directions. In both chips, the design of the antenna is the same. In principle, the researchers could have built tuning elements into the antennas of the larger chip. But "there would be too many wires coming off the chip," Watts noted. "Four thousand wires is more than Jie wanted to solder up."

Indeed, Watts said, wiring limitations meant that even the smaller chip is tunable only a row or column at a time. But that's enough to produce some interesting interference patterns that demonstrate that the tuning elements are working. The large chip, too, largely constitutes a proof of principle, Watts added. "It's kind of amazing that this actually worked. It's really nanometre precision of the phase, and you're talking about a fairly large chip."

 MIT chips steer light

Figure 2: Images of the MIT logo appear to hover above the surface of the chip of the because interference of the phase-shifted light beams emitted by the antennas.

In both chips, laser light is conducted across the chip by silicon ridges known as "waveguides." Drawing light from the optical signal attenuates it, so antennas close to the laser have to draw less light than those farther away. If the calculation of either the attenuation of the signal or the variation in the antennas' design is incorrect, the light emitted by the antennas could vary too much to be useful.

Both chips represent the state of the art in their respective classes. No 2D tunable phased array has previously been built on a chip, and the largest previous non-tunable (or "passive") array had only 16 antennas. Nonetheless, "I think we can go to much, much larger arrays," Watts stated. "It's now very believable that we could make a 3D holographic display."

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