How the fantasy of invisibility becomes reality in the sky
When asked, “What kind of superpower would you like to have,” most of us say “invisibility.” Even Derek Jeter. Invisibility, like the ability to fly, is the stuff of childhood dreams. And for decades, cloaking devices have been a favorite plot device of science-fiction and fantasy classics like “Star Trek,” “Harry Potter” and “Doctor Who.”
Today, the F-35 strike fighter jet makes this fantasy a reality, as it navigates airspace with the most advanced powers of hide and seek. Its multiple stealth devices – radar-absorbing materials and internal infrared sensors – comprise the ultimate invisibility cloak. In the F-35 and elsewhere, stealth and cloaking technologies have become more comprehensive and durable, with applications for military and other industries. This is what happens when science meets imagination.
“With improvements, tanks or planes can be cloaked from human observation, car trunks can be made see-through, blind spots can be cloaked to be seen easily or cloaking can even be used as art or included for architectural effects,” said Joseph Choi, a researcher with the University of Rochester’s Institute of Optics.
Cloaking, which makes objects partly or wholly invisible, manipulates the direction of visible and near-infrared light or electromagnetic waves around an object as if it weren’t there. For an invisibility cloak or shield to work, the material needs to curve waves completely around all dimensions of an object, and work with all backgrounds and angles of view.
The Lawrence Berkeley National Laboratory at the University of California recently created an ultrathin invisibility cloak – a thin film of magnesium fluoride topped by small gold antennae – that can flexibly wrap light waves around any shape and create illusions to match different backgrounds. It does that by controlling how reflected light is scattered, and therefore what the viewer sees. The cloak’s creators say it can be draped over any object to obscure it or make it look like something else.
Cloaking with specifically engineered, artificial materials to bend light waves – or metamaterials – also holds great promise for electromagnetic field cloaking. “For more practical cloaking that can make large objects disappear for the human eye and work for all visible colors (frequencies), we think using standard optics (lenses and mirrors) has a lot of potential,” Choi said.
Late last year, Choi and John Howell at the University of Rochester investigated invisibility principles in optical spatial cloaking. They produced a scalable cloaking device that uses four standard optical lenses to manipulate light waves to hide objects, no matter the position of the viewer.
“Currently, the Rochester Cloak works for small angles, but as designs that work for much larger angles are developed, and the design becomes more portable, this option can be quite useful,” Choi said.
The scientific understanding of cloaking devices has made exponential leaps in the last few years. Previous two-dimensional cloaking technologies lost their invisible power as a viewer changed angles, and were difficult to scale for larger objects. The Rochester team’s solution is three-dimensional and multidirectional. By adjusting the lens type, power and precise distance, there is no discontinuity as the viewer changes positions.
In the F-35 Lightning II, invisibility is not just about hiding from the enemy, but also about locating and attacking. With its infrared sensors, the F-35’s integrated airframe design allows it to skirt detectability by enemy radar while it stealthily identifies and tracks targets from long ranges. It does this with its advanced electro-optical targeting system (EOTS), the world’s first sensor to combine forward-looking infrared and infrared search and track functionality.
This infrared imaging system is a passive sensor that is housed in a small, low-profile window. Among its other functions, EOTS tracks targets without emitting radar – allowing the plane to operate with stealth.
“The military services see infrared as an absolute necessity. It became a mantra that the U.S. military owned the night because of this technology,” said Don Bolling, director of business development at Lockheed Martin’s missiles and fire control division.
The F-35’s fused sensor solution and cloaking techniques have broader military and commercial applications – they can be deployed in automobiles and energy efficiency systems, and in infrared cameras and night vision devices.
Invisibility is power. By being invisible to enemy radar, F-35 pilots can “identify friend from foe and make difficult decisions in challenging situations,” Bolling said.