Thin Is in for Invisibility Cloaks

New metasurface could point the way toward practical invisibility cloaks

2 min read
Thin Is in for Invisibility Cloaks
Illustration: Xiang Zhang group, University of California, Berkeley

When Xiang Zhang first invented an invisibility cloak that worked at optical wavelengths, his son complained that it wouldn’t allow him to easily stroll around unseen, like Harry Potter sneaking through the library.

“My son always joked that, ‘Daddy, if I want to make myself disappear I have to carry a huge cylinder around myself,’” says Zhang, a professor of mechanical engineering at the University of California, Berkeley. The boy wondered why he couldn’t have something more like a sunblock, a spray-on material that rendered him invisible. That may not be such a far-fetched vision, Zhang says.

He and his colleagues have created an extremely thin cloak that works in visible light, made of a metasurface, a special type of metamaterial that is thin enough to be considered two-dimensional. They described the device in last week’s Science.

The metasurface is a thin layer of silicon dioxide, on top of which the team inscribed gold blocks, surrounded by the dielectric material magnesium fluoride, that act as nanoantennas. The blocks are approximately 80 nm thick, but they vary in size by as much as 30 percent, so that the metasurface contains six different sizes of nanoantenna.

The antennas resonate at particular wavelengths, absorbing the incoming light and re-emitting it to make it seem as if it’s coming off a flat surface. Because they vary in size, they can cancel out local variations in the phase of the light, a type of interference that would render the hidden object visible.

The cloak worked at wavelengths from 710 to 730 nm, but Zhang says it’s theoretically possible to make materials that can span 300 nm of the spectrum, enough to cover the visible wavelength range.

And while this cloak can only hide stationary objects, it might be possible to use a current to tune the antennas and change their resonance to cope with changes in the object underneath. “That’s our next task, how you make things more adaptive, how you change the shape of something,” Zhang says.

So how long until there’s a practical cloaking device that can effectively cover a boy wizard as he moves about? Zhang says he wouldn’t be surprised to see—or not see—something like that within the next decade. “I think we are getting close to that.”

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3D-Stacked CMOS Takes Moore’s Law to New Heights

When transistors can’t get any smaller, the only direction is up

10 min read
An image of stacked squares with yellow flat bars through them.
Emily Cooper

Perhaps the most far-reaching technological achievement over the last 50 years has been the steady march toward ever smaller transistors, fitting them more tightly together, and reducing their power consumption. And yet, ever since the two of us started our careers at Intel more than 20 years ago, we’ve been hearing the alarms that the descent into the infinitesimal was about to end. Yet year after year, brilliant new innovations continue to propel the semiconductor industry further.

Along this journey, we engineers had to change the transistor’s architecture as we continued to scale down area and power consumption while boosting performance. The “planar” transistor designs that took us through the last half of the 20th century gave way to 3D fin-shaped devices by the first half of the 2010s. Now, these too have an end date in sight, with a new gate-all-around (GAA) structure rolling into production soon. But we have to look even further ahead because our ability to scale down even this new transistor architecture, which we call RibbonFET, has its limits.

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