If you want to create a moving light source, you have a few possibilities. One is to mount a light emitter in some kind of mechanical housing — the approach used in, say, theatrical spotlights, which stagehands swivel and tilt to track performers.
Another possibility, however, is to create an array of light emitters and vary their “phase” — the alignment of the light waves they produce. The out-of-phase light waves interfere with one another, reinforcing each other in some directions but annihilating each other in others. The result is a light source that doesn’t move, but can project a beam in any direction.
Such “phased arrays” have been around for more than a century, used most commonly in radar transmitters, which can be as much as 100 feet tall. But in this week’s issue of Nature, researchers from MIT’s Research Laboratory of Electronics (RLE) describe a 4,096-emitter array that fits on a single silicon chip. Chips that can steer beams of light could enable a wide range of applications, including cheaper, more efficient, and smaller laser rangefinders; medical-imaging devices that can be threaded through tiny blood vessels; and even holographic televisions that emit different information when seen from different viewing angles.
In the 4,096-antenna chip — a 64-by-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 millimeters away, however, the interference of the antennas’ phase-shifted beams produces a more intricate image.
In the other chip, which has an eight-by-eight 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 says. “Four thousand wires is more than Jie wanted to solder up.”
Indeed, Watts explains, 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 says. “It’s kind of amazing that this actually worked,” he says. “It’s really nanometer precision of the phase, and you’re talking about a fairly large chip.”
Both chips represent the state of the art in their respective classes. No two-dimensional 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 says. “It’s now very believable that we could make a 3-D holographic display.”
ABSTRACT – Electromagnetic phased arrays at radio frequencies are well known and have enabled applications ranging from communications to radar, broadcasting and astronomy. The ability to generate arbitrary radiation patterns with large-scale phased arrays has long been pursued. Although it is extremely expensive and cumbersome to deploy large-scale radiofrequency phased arrays, optical phased arrays have a unique advantage in that the much shorter optical wavelength holds promise for large-scale integration. However, the short optical wavelength also imposes stringent requirements on fabrication. As a consequence, although optical phased arrays have been studied with various platforms and recently with chip-scale nanophotonics all of the demonstrations so far are restricted to one-dimensional or small-scale two-dimensional arrays. Here we report the demonstration of a large-scale two-dimensional nanophotonic phased array (NPA), in which 64 × 64 (4,096) optical nanoantennas are densely integrated on a silicon chip within a footprint of 576 μm × 576 μm with all of the nanoantennas precisely balanced in power and aligned in phase to generate a designed, sophisticated radiation pattern in the far field. We also show that active phase tunability can be realized in the proposed NPA by demonstrating dynamic beam steering and shaping with an 8 × 8 array. This work demonstrates that a robust design, together with state-of-the-art complementary metal-oxide–semiconductor technology, allows large-scale NPAs to be implemented on compact and inexpensive nanophotonic chips. In turn, this enables arbitrary radiation pattern generation using NPAs and therefore extends the functionalities of phased arrays beyond conventional beam focusing and steering, opening up possibilities for large-scale deployment in applications such as communication, laser detection and ranging, three-dimensional holography and biomedical sciences, to name just a few.