In this blog post, we explore how transparent cloaks, a product of scientific imagination, could be realized in reality through metamaterials.
The best-selling Harry Potter series has been adapted into films and achieved huge global success. It is unlikely that anyone reading this article has never heard of Harry Potter. Those who have seen Harry Potter may have imagined what it would be like if the magical objects in the novels existed in reality. However, there is one object that has been developed in reality rather than existing only in imagination, and that is the invisible cloak.
Research on the invisibility cloak began relatively recently. In 1967, Russian physicist Victor Georgievich Veselago proved the possibility of a material with a negative refractive index. Refractive index refers to the degree to which light bends when passing from one medium to another. When the refractive index is negative, light bends in the opposite direction from what we are accustomed to. In the case of metamaterials, light that reaches the boundary of an object does not reflect but instead bends and returns in the opposite direction. This research overturned the existing optical theory that the refractive index is always positive and marked a turning point in the field of transparency research. In 2005, Professor David R. Smith of Duke University in the United States successfully conducted an experiment using metamaterials to make a copper cylinder undetectable by radar. However, at the time, there were limitations such as the object losing its transparency function easily when its shape changed or when exposed to external stimuli. Recently, however, the emergence of new concepts such as smart metamaterials and memory metamaterials has made it possible to create highly sophisticated invisible cloaks.
So what exactly are metamaterials, and how did they make invisible cloaks possible? Metamaterials are artificially designed substances that do not exist in nature and exhibit properties that violate the laws of physics. They are composed of a collection of composite elements made from common materials like metals and plastics, and their properties are determined by their structure and arrangement. In particular, metamaterials designed to exhibit negative refractive indices at specific wavelengths, such as visible light or microwaves, are used in the creation of invisible cloaks. Among these, smart metamaterials and memory metamaterials represent a further advancement over conventional metamaterials.
Smart metamaterials are metamaterials that can automatically adjust their refractive index in response to external stimuli to maintain specific functions, and they have played an important role in the production of large transparent cloaks. This material is made of an elastic material similar to a sponge, designed so that when it is folded or bent, the density of the stimulated area increases to maintain the refractive index. Additionally, to achieve the property of a negative Poisson’s ratio in metamaterials, a structure was adopted where four-legged ladders are arranged face-to-face. A negative Poisson’s ratio refers to the property where a material expands or contracts in both the horizontal and vertical directions simultaneously when subjected to a stimulus.
Memory metamaterials retain their optical properties even after receiving momentary stimuli, enabling them to exhibit transparency across a wide wavelength range without power supply. This metamaterial, created by combining graphene and ferroelectric polymers, is specifically designed so that the ferroelectric polymers can rotate depending on the polarity of an external voltage. Graphene plays a role in enhancing memory performance, and metamaterials for logic operations utilizing this property are also under development.
With the emergence of metamaterials, stories that once seemed possible only in science fiction, such as invisible cloaks, are now becoming a reality. In the United States, the cost of producing a single invisible cloak has reached $1,000, bringing commercialization closer than ever. Considering that metamaterials are artificially designed substances, there is a high likelihood that new materials with characteristics we have never seen before will continue to emerge in the future. It is exciting to anticipate the role metamaterials will play in this rapidly changing world.
