Invisibility cloaks are becoming a reality thanks to advances in metamaterials. Metamaterials with negative refractive index have been able to bend light to hide objects, and the recent emergence of smart metamaterials and memory metamaterials are playing an important role in the commercialization of invisibility cloaks.
The best-selling Harry Potter series, which has also been turned into a movie, is a huge worldwide success. There is hardly a person reading this who hasn’t heard of Harry Potter, and if you’ve read the books, you’ve probably wondered what it would be like if the magical objects in the novels existed in real life. However, there’s one item that’s not a figment of our imagination, but has actually been developed: the invisibility cloak.
Research into invisibility cloaks started relatively recently. In 1967, Russian physicist Victor Georgievich Veselago demonstrated the possibility of the existence of a material with a negative refractive index. Refractive index is the degree to which the speed of light is bent as it changes at the boundary of two media. When the index of refraction is negative, light is bent in the opposite direction from the one we know. In the case of metamaterials, light that reaches the boundary of an object is not reflected, but bent and turned back. This work overturned the conventional optical theory that refractive index is always positive and marked a turning point in transparency research. Then, in 2005, Professor David R. Smith of Duke University in the United States successfully used metamaterials to make a copper cylinder undetectable to radar. However, there were limitations at the time, such as the object’s shape changing or losing its transparency easily when subjected to external stimuli. However, with the advent of new types of metamaterials, such as smart metamaterials and memory metamaterials, it is now possible to create a highly perfected invisibility cloak.
So, what are metamaterials, and how did invisibility cloaks become possible? Metamaterials are artificially engineered materials that do not exist in nature and exhibit properties that violate the normal laws of physics. They consist of composite assemblies of elements made from common materials like metals and plastics, whose structure or arrangement determines their properties. In particular, metamaterials designed to exhibit negative refractive index at certain wavelengths, such as visible light or microwaves, are being used to create invisibility cloaks. Among these, smart metamaterials and memory metamaterials can be seen as a step forward from existing metamaterials.
Smart metamaterials are metamaterials that can automatically adjust their refractive index to maintain certain functions in response to external stimuli, and have played an important role in the creation of large invisibility cloaks. They are made of spongy, elastic materials that are designed to increase in density in the stimulated area when folded or bent to maintain their refractive index. In addition, to realize the properties of a metamaterial that satisfies the negative Poisson’s ratio, a structure with four ladders of legs was adopted. Negative Poisson’s ratio refers to the property that when an object is stimulated, it simultaneously stretches and contracts both horizontally and vertically.
Memory metamaterials can remember their optical properties even after a momentary stimulus, allowing them to be transparent over a wide range of wavelengths without the need for a power supply. These metamaterials are made by combining graphene and ferroelectric polymers, and in particular, the ferroelectric polymers are designed to rotate according to the polarity of an external voltage. Graphene can also be used to improve memory performance, and logic computing metamaterials are being developed.
With the advent of metamaterials like these, science fiction stories like invisibility cloaks are becoming a reality. In the U.S., a single invisibility cloak can cost up to $1,000 and is approaching commercialization. Given that metamaterials are artificially engineered, it’s likely that we’ll continue to see new materials with properties we’ve never seen before. We’re excited to see what role metamaterials will play in our rapidly changing world.
