Big Holograms from Tiny
Antennas


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Antennas receive, antennas transmit. It’s thanks to this long established technology that our TVs, radios, and cell phones can harvest signals from waves on the electromagnetic spectrum. Usually these waves are on the long side, of course. For smaller waves, say, on the visible part of the spectrum, antennas would have to be much smaller. Impossibly small, in fact, were it not for the age of nanotechnology.

“Antennas scale pretty well,” says Yuval Yifat, a doctoral student studying nanotechnology at Tel Aviv University. “Theoretically you should be able to push them down to optical frequencies, should be able to do whatever a classical antenna does.” Several years ago, researchers took that idea out of the theoretical realm and into the actual, showing that nano antennas could control the amplitude, direction, and phase of light. They put such tiny antennas on lasers splitting beams in two, with one half breaking off at an angle.

Yifat saw the potential to make a holgoram.

Holographic projection. Image: Wikimedia Commons

To do that, though, the nano antennas of 2012 had to be made more efficient. Those years-old nano antennas performed their tricks on about 5% of the light that hit them, with 95% of it passing through unaffected. They used simple dipole configurations, not unlike the rabbit ears on the TVs of the pre-digital age. “We went back to the classic antenna theory,” says Yifat. He and his colleagues settled on a dipole and patch configuration. They fabricated the antennas over a reflective slab of gold, so light passes through the antennas again as it bounces away.

With the new configuration, they managed to get efficiencies in the 40% to 50% range.

With the ability to efficiently control phase, they simply needed to put the antennas in the right place to make a hologram that would project the right depth information to the human eye. “We needed to determine a phase map,” says Yifat. “We wrote some home-grown code, fabricated a chip in our nano-center, and—voila—it actually worked.”

The hologram they created, of the Tel Aviv University logo in infrared, was not only more efficient, it projected a much wider angle than any hologram before it.

To increase the efficiency, Yifat’s team is currently trying to come up with a better algorithm.  They may also use 12 phase elements instead of the current six. “The more you have, the more fidelity you have to the original phase map,” he says. “You go point over point and decide which antenna element to use for each point.” With such techniques he hopes to get the efficiency up a few more tens of percentage points.

The uses for the new holograms are many, including data storage, particle trapping, and security applications. Shoppers could use a scan of a couch, or create their own in a CAD file, and project it in their living room to find the perfect spot for the furniture before buying it. 3D movies may ditch the need for glasses, and television may move into the third dimension as well, able to project different images to different viewers.

The Tel Aviv University logo, was, of course, static. To create a hologram for moving images would require a dynamic setup at the nano level. “People have pretty interesting ideas about how to control the phase of light,” says Yifat. Some of those ideas involve changing the antennas, some changing the environment they sit in, by heating it up or injecting electrons. “But, long story short, no one has been able to do it so far at our scale.”

Michael Abrams is an independent writer.

We wrote some home-grown code, fabricated a chip in our nano-center, and—voila—it actually worked.

Yuval Yifat, doctoral student,
Tel Aviv University

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October 2014

by Michael Abrams, ASME.org