The invisibility illusion
Harry threw the cloak around his shoulders and Ron gave a yell…
Harry looked down at his feet, but they had gone. He dashed to the mirror. Sure enough, his reflection looked back at him, just his head suspended in mid-air, his body completely invisible. He pulled the Cloak over his head and his reflection vanished completely.
—Harry Potter and the Philosopher’s Stone
Before Harry Potter wrapped his silvery cloak around him to save the Wizarding World from evil, invisibility played a key role in countless other stories. From Hades’ invisibility helmet to the transparency chemicals of The Invisible Man and the cloaking devices of Star Trek, various fictional devices have helped characters pursue their goals without being seen.
So, in the real world, where do we stand with cloaking technology? At a precipice, it would seem.
Scientists have struggled to replicate disappearing acts like those featured in Harry Potter or Star Trek since the whole “becoming invisible” idea first struck—but that doesn’t mean they’re done trying. Using modern metamaterials as well as traditional optics, science is still exploring every possible way to become invisible. But before we get too far into the details of present-day cloaking, let’s start with what, exactly, “invisibility” means.
The word invisible entered the English language in 1340 via poetry often ascribed to a mystic named Richard Rolle of Hampole, England. The poem, “The Pricke of Conscience,” is an exploration of Platonic metaphysics, describing God as unchangeable, endless, and “invysible.” But historically, the term has been used to describe things that can’t be seen for various reasons, whether they’re too small to detect, like germs, or are just plain hidden from view. In fact, one of the oldest mentions of an invisibility cloak in a science journal appeared in a 1944 article published in The Science News Letter in reference to military camouflage.
An important facet of this relatively loose interpretation of what it means to be invisible is the subjective role of the observer. For instance, a fawn’s spotted hide is perfectly camouflaged in dappled forest light of the visible spectrum, but if its predator sees in the infrared, that deer is no longer invisible. Similarly, when the sun is visible from a beach until it sets below the ocean’s horizon, it would still be visible for some time from the top of a nearby cliff. The relative location to the observed and the way in which the observer sees are crucial aspects of
invisibility.
At the turn of the 21st century, Sir John Pendry of Imperial College London published a study articulating the use of metamaterials—artificial materials engineered to respond to electromagnetism in exotic ways based on structure—to potentially create what he called the perfect lens. This lens is not a usual lens, however. It’s a thin slab of metamaterial (like a matrix of tiny gold or silver wires) with flat, parallel sides that can refract light negatively.
In Pendry’s perfect lens, no matter how light hits it, the light will bend backward at the point of incidence instead of being slightly rerouted. This uncanny result occurs because the individual components of the slab are smaller than the wavelength of light shining on them. When put together in certain ways, they react to both the electric and magnetic fields, forcing the light waves to do their bidding.
Unfortunately, a perfect lens doesn’t have much practical use—an image on one side of the lens will be exactly the same on the other side. That’s not very helpful when trying to visualize a cell in a microscope, for instance. So, Pendry developed a mathematical theory of optics, called transformation optics, to aid in identifying materials that warp light according to Maxwell’s equations, allowing for magnification at the expense of perfection. It wasn’t long before people realized that light could potentially be warped around a point in space, essentially cloaking that space and turning it invisible.
Illustration of a metasurface cloak composed of an ultrathin layer of nanoantennas (gold blocks) coating an object of arbitrary shape. Light reflects off the cloak similar to a mirror’s reflection. Photo credit: Xingjie Ni (Pennsylvania State University) and Xiang Zhang (University of Hong Kong)
Then, in 2006, a team of scientists led by David R. Smith from Duke University, worked with Pendry to create a working cloak for use with microwaves. In the experiment, scientists guided microwaves around a cylindrical object made of a copper metamaterial, turning it almost entirely invisible when viewed in that band of light. Sensational reports flooded the news and scientists were eager to develop the technology further. However, almost two decades have passed since Duke’s microwave cloak without much progress toward one that works for the entire visible spectrum.
“In terms of optical cloaking, it’s a very nice idea to allow light to go around an object,” says Debasish Banerjee, director of materials research at the Toyota Research Institute of North America. “But the problem is, light is not one wavelength.”
Banerjee explains that how light travels in a medium depends on the properties of both the light and the medium. “If we want to see visible cloaking for each wavelength, we will have to change the medium separately.” This means the metamaterials used would have to be designed to accommodate every wavelength of visible light individually to guide them around the object intended to be cloaked.
“It’s just too difficult of an engineering task,” he states. Because of this major limitation in metamaterials, some scientists have chosen to explore older, simpler ideas in the quest for broadband cloaking: ray optics.
Ray optics are intuitive compared to transformation optics—there’s no imagined warping of space needed. When looking in a mirror, it’s simple to trace the straight rays of light reflecting off the glass surface from one’s shoe to one’s eye, for instance. Similarly, with lenses, it can be easy to trace a ray of light as it travels from a light source to the curved lens surface, through its middle, and out the other side to where the light focuses. “That becomes more of a traditional optical problem,” says Joseph S. Choi, a systems engineer at Northrop Grumman. “Then we just worry about the power of light and the frequency of light.”
Before Choi joined the aerospace industry, he was a doctoral student at the University of Rochester and helped develop what’s now known as the Rochester cloak—an impressive optical cloaking system that uses nothing but lenses and light, and also works from more than one viewing angle. “I think a lot of people thought about that and said, ‘Oh, that’s too trivial,’ but some people thought, ‘Oh, that’s simple—it’s good you were able to simplify it.’”
The Rochester cloak uses a series of four lenses lined up at specific distances from each other, so the same light passes through each lens. As light rays enter the first lens, they’re positively refracted and focused on the other side, between the first and second lens. Once the rays pass through the focal point, they continue their journey toward the second lens, inverting whatever image the light is carrying. As the rays pass through the second lens, they’re refracted again to collimate—make parallel—the lines to their original angle of incidence. The third and fourth lenses are the same as the second and first (respectively), so when the rays pass through them, they’re refracted in reverse, inverting the image once more. The result is an uninverted image with the rays exiting at the same angle they originally entered in the very first lens.
The Rochester cloak. Photo credit: University of Rochester
This lens setup works as a cloaking device because objects can exist in the donut-shaped areas surrounding the focal points without interrupting the original light’s journey from one lens to the next. “This is the first device that we know of that can do three-dimensional, continuously multidirectional cloaking, which works for transmitting rays in the visible spectrum,” Choi said in a 2014 news release.
One of the biggest downsides to this lens setup, however, is the limited cloaking space and viewing angle. Even if larger lenses and mirrors are used to increase that space and angle, the cloak is still easily thwarted by taking a few steps to the left or right. Other “low-tech” optical cloaks have similar downsides, where backgrounds become misaligned, or images get distorted when the viewer or device moves. Plus, since light waves are refracted as they pass through the physical material of the cloak, they change phase, allowing the cloak to be “seen” by any instrument that measures phase.
A 2015 paper describes the use of a thin mesh of tiny golden cubes acting like antennas to hide small objects beneath the cloak. The material is called a metasurface skin cloak since the cube mesh is a metamaterial that rests on the surface of an object, like skin.
According to lead author Xingjie Ni, associate professor of electrical engineering at Penn State, the tiny cubes—dubbed nanoantennas—can receive and emit electromagnetic waves similar to the way bunny ears on old TVs receive and emit radio waves. “By designing those nanoantennas to differentiate geometry, you can control the emitted light,” he says. He calls this process “tuning.”
For instance, some nanoantennas can be programmed to hold light for a time to accumulate its phase and then re-emit it, or they could absorb all the light and then only re-emit half of it back. In this way, the scientists were able to individually tune the nanoantennas to act as if they were reflecting a flat surface instead of the bumpy objects they were cloaking. “We used the various antenna distributions to recover the phase from distortion of the object so we could erase all that information,” Ni says.
Unfortunately, limitations still dog this kind of cloaking. Ni says the metasurface cloak was made using electron beam lithography, a specialized printing technique that cuts a pattern from material using a single laser beam of electrons—a slow and costly process. “It’s very good for research, but not very good for mass production,” he says. In addition, the cloaks only work for a narrow bandwidth of light (around 700 nm), so Ni’s team is now attempting to broaden the wavelengths that can work with a cloak. They’re experimenting with applying metasurfaces to traditional lenses as well as shaping metasurfaces into sophisticated designs that control a wider band of the visible spectrum.
In the future, Ni says, he can see metasurface cloaks being used by the military to hide tanks, weapons, or even people. (Governments are already developing heat cloaks that hide from thermal cameras.) But Ni thinks there’s more to these metasurfaces than just espionage. “Since we can control light,” he says, “why don’t we just create something that’s actually seen in sci-fi movies?” He envisions new ways to display light, like the holodeck in Star Trek or holograms in Star Wars. They use the same principles as cloaking, he says, but the opposite. “Cloaking is making things disappear, but a hologram is making things [appear] from nowhere.”
As scientists have learned more about invisibility, new uses have been imagined and explored in thermal, acoustic, and even spacetime cloaking. But for now, at least, a true cloak of invisibility remains the stuff of fiction.
Rachel Lense is a freelance science writer, poet, and artist living in the greater Washington, DC area. She loves to tell stories at the intersection of science, technology, and culture, and emphasizes the dual roles history and language tend to play in therein.
