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. While Star Trek brought the idea of a cloaking device into the popular consciousness, the closest we’ve come has been through the development of stealth technology. The invisibility to radar, which is long-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. Earlier this week, a novel material called a broadband achromatic metalens has covered the entire visible light spectrum for the first time. The fusion of this technology with metamaterial cloaking could enable the first visible-light cloaking device. Here’s the story.
Under normal circumstances, when you bombard any material with light of any wavelength, the typical behavior is either absorption or reflection. If the light is absorbed, then any background light and signals will be obscured, alerting you to its presence. (In other words, the object won’t be transparent.) If the light is reflected, any signal you send out will be bounced back to you, illuminating the object and allowing you to observe it directly. While stealth technology minimizes reflectivity, a true “cloaking device” would divert the light around an object from all directions, so that anyone, from any location, would simply see the background signals, as though the cloaked object weren’t there at all.
A special, multi-layered coating of a substance known as a metamaterial has been developed, enabling electromagnetic radiation 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 unperturbed in the same direction that it came in. 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 field, and in the right configuration, an interior object can be completely hidden. By bending and then un-bending light by the proper amount, objects can be cloaked to particular wavelengths of light. As of 2016, a 7-layer metamaterial cloak has extended the range from the infrared all the way through the radio portions of the spectrum.
Related to metamaterials is the field of metalenses. Most normal materials that you can create a lens out of has the same dispersive property as a prism: when you pass light through it, light slows down. But light of different wavelengths slows down by different amounts, which is why you 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 try and 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 it’s heavy, bulky, expensive, and not 100% successful.
A metalens, ideally, would shape the wavefronts regardless of wavelength, allowing focusing down to a single point on even the smallest of scales. A metalens can be very thin (on the order of a single wavelength of light), they’re easy to fabricate, and they can focus light of a variety of wavelength all onto the same point. The recent breakthrough, published in Nature Nanotechnology, is through the application of titanium-based nanofins. Based on the wavelength of the incident light, these nanofins will 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.
Immediately, this makes a cheaper, lighter, more effective lens. As Wei Ting Chen explains:
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.
While the immediate applications of these metalenses should include cameras, VR devices, microscopes, and other medicinal and augmentative technologies, a longer-term fusion of the metalens/nanofin concept with metamaterials could be exactly the holy grail that a cloaking device requires.
The biggest challenge facing a real-life 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. Based on the materials discovered so far, we haven’t yet managed to penetrate the visible light portion of the spectrum with a cloak. This new advance in metalenses, however, seems to indicate 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), and fusing this with advances in metamaterials would make visible-light cloaking devices 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 to a huge variety of wavelengths. Today, an advance in metalenses, by guiding light of various wavelengths to the proper location to get the distortion-free outcome we so keenly desire, might be just the discovery 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. Here on Earth, it may require a mere decade or two. If this latest metalens advance can be rapidly applied to metamaterial cloaks, an optical, 3D cloaking device may become a reality in humanity’s very near future.
