Holograms in the Palm of Your Hand
A grid of 4,096 miniature antennas steer beams of infrared light to create patterns and images. Jie Sun, MIT
Holograms are a science-fiction staple from Star Trek’s holodeck to the famous scene in Star Wars where a holographic Princess Leia implores, “Help me, Obi-Wan Kenobi.” But the reality has never lived up to the dream.
That might change. Scientists at the Massachusetts Institute of Technology built a tiny device that contains a grid of 4,096 miniature antennas (64 by 64) that steer beams of infrared light to create patterns. Their so-called phased array was able to generate an image -- in this case a tiny MIT logo -- and "float" it a few millimeters out in front of the grid.
It's the first time anyone has built an array with so many components, as previous attempts only managed 16. It's also the first device of its kind that can steer each beam from an individual antennae in both the vertical and horizontal direction, making it possible to create three-dimensional pictures.
“At a basic level we’re showing that not only can you steer beams actively but also generate new and arbitrary patterns,” said Michael Watts, a professor in the Research Laboratory of Electronics at MIT. That opens up a number of possibilities in holography as well as imaging devices such as biomedical sensors, akin to radar. Communications is also a possibility, since fine control of light waves can reduce interference and noise.
Watts and graduate student Jie Sun, the lead author, presented their work in the Jan. 9. Issue of Nature.
Watts and his colleagues made antennas that control both the phase and intensity of the light it transmits. Two light beams that are 180 degrees out of phase will, if transmitted together, cancel each other out. Meanwhile light waves that are slightly out of phase will interfere with and reinforce each other in certain patterns, making the light look brighter or dimmer depending on how far in or out of phase they are.
That makes an image in the “far field” -- a technical way of saying that it’s some distance away. If one were to build a display like this in a living room, it would mean that the image would be out in front of it.
Phased arrays aren’t new: modern radar uses them all the time. But Watts and Sun transmitted signals at short wavelengths, in the near infrared as opposed to the radio waves of radar. They also made images, which hadn't been done before with a phased array at those wavelengths.
And because it’s possible to control the phase and intensity of the light, you get more than the illusion of depth from the front: a person standing on any side of the image could be shown a different perspective. A hologram would be truly 3-D, and if built with billions of antennas, would produce an image as detailed as any ordinary display. That's because each antennae essentially represents one pixel.
“The exciting part is that you can project an image,” said Thomas Krauss, a physicist at the University of York in the U.K., who was not involved in the research. “It’s the first time anyone has done it with so many pixels.” Previous attempts had never managed more than a dozen or so.
Sun and Watts didn't just set records for the size and number of antennas: they did it using ordinary microchip manufacturing methods. That means building a larger-scale device won't require retooling or building whole factories.
Jonathan Doylend, a postdoctoral researcher at the University of California santa Barbara’s Optoelectronics Research Group, noted that being able to build such an array is an important step. “Were all working in this field with that sort of end goal in mind –- there’s always a push towards higher array counts and higher density (of antennas),” he said.
The MIT device used near infrared light. To make it work for visible light the only change would be the material the antennas and waveguides are made of -– it has to be something other than silicon. “We’re working on making it in the visible,” Watts said.