The sci-fi inspired “cloaking device” makes invisibility possible

For as long as human beings have been writing about fantasy, myth, and science fiction, the dream of invisibility has always been a top priority. There would be so much you could do or accomplish, under a stealthy guise, if only you didn’t have to worry about being seen. While Star Trek brought the idea of a cloaking device into the popular consciousness, and Harry Potter brought with it the widespread idea of an actual invisibility cloak, there haven’t been many useful applications of invisibility-based technology on a large scale. In fact, the closest we’ve come to achieving actual invisibility has been through the development of stealth technology, which only bestows a rudimentary invisibility at far longer wavelengths than human eyes can perceive by making the your target effectively smaller than the device itself.

The invisibility to radar, which is microwave-to-radio wavelength electromagnetic radiation, might have been the first step, but recent developments in metamaterials have extended this even further, bending light around an object and rendering it truly undetectable. Perhaps the critical advance that could finally bring an invisibility cloak to reality occurred in 2018, in a novel material called a broadband achromatic metalens. For the first time, it rendered an object undetectable across the entire visible light spectrum.

The fusion of this technology with metamaterial cloaking — another recent nanotechnology advance — could finally enable the first visible-light cloaking device. Here’s the scientific story behind what’s actually under development in today’s reality.

The B-2 Spirit stealth bomber, shown here, It is a low-observable aircraft, meaning that its cross-section for being spotted by radar devices is far smaller than its actual, physical size. A combination of reduced signatures to visual, infrared, radar, thermal, and acoustic measurements makes it a low-observable aircraft. Its wing design, composite materials, and (classified) special coatings all serve to reduce its observability, making it one of the most stealthy and difficult-to-detect aircraft on planet Earth today.

The concept of stealth technology is the closest thing we have, at present, to an implementation of the idea of invisibility. Although these craft are not invisible in any sense, they are optimized to greatly reduce their sensitivity to being detected by conventional means, including:

• by radar waves,
• through visible or infrared observations,
• through thermal signatures,
• or via sonic/acoustic vibrations.

A wide combination of interventions are used to make a craft or vehicle as invisible to these conventional means of detection as possible. The craft is shaped in such a way that it produces a minimal cross-section towards whatever territory it’s approaching, reducing its signature in the direction that it would be most threatened by. The materials that it’s made out of and coated in are designed to convert any absorbed, long-wavelength light into heat, which ensures that signals (such as radar signals) aren’t reflected back towards a detector that’s looking for them. Then the emitted heat and noise signatures are masked through a variety of special coatings.

Stealth technology goes way beyond old-fashioned ideas of camouflage, but is still, even in its optimized, modern incarnation, extraordinarily different from the notion of “cloaking,” or true invisibility.

This schematic view of the Star Trek spacecraft, the Romulan Bird of Prey, shows arguably the most famous and ubiquitous ship to routinely deploy a cloaking device. Seen with a side view, bottom-up view, top-down view, and forward-facing/rear-facing view, the craft alternately looks bird-like or ship-like when uncloaked, but is completely invisible when cloaked, as light is bent around it by the cloaking device itself.

The idea of an invisibility cloak, instead, goes back to the science-fiction franchise of Star Trek, which popularized the notion of a cloaking device. In reality, the H.G. Wells story The Invisible Man was the first to hypothesize a human being becoming literally invisible: where someone looking in their direction would only see the background of what’s behind them, as though they had become completely transparent. Ideas such as refraction screens or ray-bending devices became common in science-fiction stories in the 1930s and 1940s, and real-world radar stealth became commonplace in the 1950s. Nevertheless, Star Trek introduced a practical invisibility screen, the “cloak,” in a television format in 1966, and by 1968, were using the term “cloaking device” in the show’s dialogue.

The idea of a cloaking device was that, when the “cloak” was active, light from all background sources would be bent around the object itself and then would continue on in a straight line. To someone who was looking in the direction of the cloaked object, they would only see whatever light sources were behind the object itself, with no indications, either in emission or absorption in any form of light or particles, that the object was actually present. This provided a remarkable plot device to allow for surprise attacks and reveals, but also was rooted in physical reality: without an emitted or absorptive signature, there would be no way to know whether an object was actually there or not.

The ability to bend light around an object and show the background, incoming light from any angle-and-distance could become real due to combined advances in metamaterials, nanolenses, and transformation optics.

But this is still fairly conventional, in the sense that it deals with what we consider “normal circumstances” in our everyday experience. Under those normal circumstances, when you bombard any material with light of any wavelength, there are two typical behaviors you’re likely to encounter: either absorption or reflection. If you have an object and it’s a good absorber of light, then any background light and signals will be obscured as soon as the object enters your field of vision, alerting you to its presence. In other words, the object won’t be transparent, it will cast a shadow, and it will block the presence of any signal that’s normally present from behind it.

Similarly, if any of the light is reflected upon striking an object, then any signal you send out will be bounced back to you, illuminating the object and allowing you to observe it directly. Again, the object won’t be transparent or invisible, and will therefore be detectable.

The only way to achieve actual transparency would be if the light coming from behind the object could somehow still arrive, with the same trajectory, in front of the object, as though the light were transmitted directly through the object. Similarly, any signal sent towards the object wouldn’t be reflected back at all, but would pass through or around that object so that it couldn’t be detected from its reflection signatures either.

The way a true “cloaking device” would work, then, to hide a material that wasn’t intrinsically transparent would be to divert the light around an object from all directions. This way, any observer, looking from any location and orientation, would simply see the background signals, as though the cloaked object weren’t there at all.

Nearly two full decades ago, the first 2D cloaks were developed, hiding objects when viewed from a particular angle only. Today, we are working towards a true 3D cloak, where an object could be hidden from view in an omnidirectional fashion in three-dimensional space.

The way we’re working towards making this possible is through the development of a special, multi-layered coating that forms a substance known as a metamaterial. Over the course of the early 21st century, metamaterials have become an exciting new field of physics and materials science, enabling electromagnetic radiation of certain specific wavelengths to pass freely around an object. This is different from transparency, where light transmits through a material; the structure of a metamaterial guides light around an object, sending it off again, unperturbed, in a direction that’s identical to the one it came in at.

Starting in 2006, the science of transformation optics allowed us to map an electromagnetic field onto a twistable, spacelike grid. When the grid gets distorted, so does the electromagnetic field associated with it, and when the right configuration is achieved, an object that’s interior to that field (and the associated grid) can be completely hidden. By bending and then un-bending the light that’s incident on that object by the proper amount, objects can be cloaked to particular wavelengths of light.

Ten years after that transformation optics breakthrough, in 2016, a 7-layer metamaterial cloak was developed, extending the range of such a metamaterial from the infrared all the way through the radio portions of the spectrum: an enormous set of wavelength coverage. However, a key set of wavelengths was missing from that range: the component of light that our eyes are sensitive to, the optical.

Left: Cross section of an infinitely long PEC cylinder, subject to a plane wave. The scattered fields can be observed. Right: a 2 dimensional cloak, designed using transformation optics techniques is used to cloak the cylinder. There is no scattering in this case and the cylinder is electromagnetically invisible.

There’s also a related nanotechnology to metamaterials that plays a vital role for bending light around object: the scientific field of metalenses. Lenses are well known for being transparent, and for changing the angle at which background rays of light emerge. Converging and diverging lenses are the most common forms of lens and are frequently used to correct human vision in the form of eyeglasses and contact lenses. Most normal materials that you can create a lens out of will have the same dispersive property as a prism: when you pass light through it, light slows down relative to the speed of light in a vacuum, and different wavelengths are free to move at different speeds within that prism-like material.

In any lens, that’s an unfortunate property of light if you want to keep your original light wave consistent and coherent. If different wavelengths of light slow down by different amounts, you’ll inevitably get a “rainbow” effect when light passes through a medium, as red light travels at a different speed than blue light. Coatings can be applied to carefully-shaped lenses to minimize this chromatic aberration effect, but it’s always present in some amount. Modern cameras use multiple lenses to eliminate chromatic aberration as much as possible, but they are heavy, bulky, expensive, and still not even 100% successful.

The behavior of a beam of sunlight, perhaps the greatest example of white light, as it passes through a prism demonstrates how light of different energies moves at different speeds through a medium, but how they all move at the same speed through a vacuum, which is why the light that doesn’t pass through a refractive medium remains white in color. Note that the visible light shown here is only illustrative of the sunlight that we can see; other wavelengths of light still exist beyond the portion that’s visible.

That’s where the idea of a metalens comes in, offering a substantial improvement over traditional lenses in this regard, even at visible light wavelengths. A metalens, ideally, would shape the wavefronts of incoming light waves regardless of wavelength, allowing that light to be focused down to a single point on even the smallest of scales. A metalens has numerous attractive properties when it comes to the possibility of creating a cloak that effectively keeps different colors/wavelengths of light together, without having them spread out or disperse the way a standard prism would. Metalenses can, both in principle and in practice:

• be very thin (on the order of a single wavelength of light), or less than a micron thick,
• become very easy to fabricate, often easier than traditional lenses which must be ground very precisely, and
• focus light of a variety of wavelengths all onto the same point, without that chromatic dispersion or aberration.

A critical breakthrough occurred in 2018, which was published in Nature Nanotechnology, which brought in the application of titanium-based nanofins. Based on the wavelength of the incident light, these nanofins then guide the light through a different part of the material, allowing it to bend by exactly the proper, necessary amount to have it wind up where we need it to in a wavelength-dependent fashion, eliminating chromatic aberration entirely, and for the first time.

Through the novel technology associated with a 2018 breakthrough of a novel metalens, light from across the spectrum can be focused onto a single point, virtually eliminating chromatic aberration, which had never been eliminated prior to this breakthrough in any dispersive lens system.

Immediately, this allows for the development of a cheaper, lighter, more effective lens. As one of that key paper’s authors, Wei Ting Chen, explained:

“By combining two nanofins into one element, we can tune the speed of light in the nanostructured material, to ensure that all wavelengths in the visible are focused in the same spot, using a single metalens. This dramatically reduces thickness and design complexity compared to composite standard achromatic lenses.”

It doesn’t just reduce thickness and design complexity; it makes a more achromatic lens than ever before. In terms of marketability, applications of these metalenses first came along in fairly mundane devices, including:

• cameras,
• virtual reality headsets,
• microscopes,
• and other medicinal and augmentative technologies.

However, if we’re willing to dream big, we can imagine a possibility far grander than any of these meager advances. A longer-term fusion of the metalens/nanofin concept with metamaterials could be exactly the holy grail of technological combinations that would lead to true invisibility: a real-life cloaking device.

Through the power of a metalens, incoming light from across the spectrum along a wide area can be focused down to a point. If that light can then be bent around an object, de-focused, and sent off in its initial direction, we would have a true invisibility cloak.

The biggest challenge facing the construction of a real-life invisibility cloak has been the incorporation of a large variety of wavelengths, as the cloak’s material must vary from point-to-point to bend (and then unbend) the light by the proper amount. While metamaterials have managed an impressive range of coverage, it’s thus far excluded visible light, but the addition of a metalens layer to metamaterials might finally be the key step towards overcoming that critical obstacle. It’s possible, with the right type of metalens/metamaterial combination, that the full wavelength range from ultraviolet down to radio waves could eventually be included. As long as you don’t have enough energy-per-photon to ionize your electrons in the material, such as with X-rays, gamma-rays, or sufficiently short-wavelength ultraviolet light, a true cloaking device is theoretically within reach.

Based on the materials discovered so far, however, we haven’t yet managed to penetrate the visible light portion of the spectrum with a cloak. This recent advance in metalenses seems to come along with a remarkable implication: that if you can do it for a single, narrow wavelength, you can apply this nanofin technology to extend the wavelength covered tremendously. This first application to achromatic lenses covered nearly the full visible-light spectrum: from 470 to 670 nm, whereas human vision extends from 400 to 700 nm. The successful fusion of this metalens advance with concurrent advances in metamaterials would make visible-light cloaking devices a reality.

Bending light and focusing it to a point, regardless of wavelength or where it’s incident on your surface, is one key step toward a true cloaking device. The combination of metalenses and metamaterials could make this sci-fi dream a reality.

Just a few years ago, it was speculated that a real-life invisibility cloak could only be applied to a very narrow set of wavelengths for a few specific configurations. It was thought inconceivable that large, macroscopic objects could be cloaked across a wide variety of wavelengths. And the idea that even a composite device could yield cloaking in the:

• ultraviolet,
• visible,
• infrared,
• microwave,
• and radio,

portions of the spectrum seemed more like a science fantasy than anything that would ever become science fact.

And yet today, owing to considerable advances in the science of metalenses, by guiding light of various wavelengths to the proper location, we can get the distortion-free outcomes of background light that we so eagerly desire. By combining metalens and longer-wavelength metamaterial technology, that might be just the leap we need to herald the arrival of a true cloaking device.

As Star Trek first envisioned it, it took centuries for cloaking technology to be perfected by incredibly advanced civilizations. Here on Earth, another decade or two may be sufficient to bring this dream to fruition. If these recent metalens and metamaterial advances can be rapidly applied to form hybrid cloaks, then an optical, 3D cloaking device may finally become a reality. In the ideal scenario, it could arrive in humanity’s very near future.

This article was first published in April of 2022. It was updated and expanded in October of 2026.

This article The sci-fi inspired “cloaking device” makes invisibility possible is featured on Big Think.