In this blog post, we’ll explore how the concepts of invisibility cloaks and invisible humans—once considered mere fantasies in movies and novels—are now being scientifically studied through metamaterials and transformational optics.
Invisibility cloaks and invisible humans have long been popular themes in novels and movies. For example, in the “Harry Potter” series, the protagonist Harry Potter uses an invisibility cloak inherited from his father to overcome various challenges. Additionally, movies centered on the theme of invisible humans are quite common. While watching these novels or movies, everyone has probably thought at least once about wanting an invisibility cloak or trying to become invisible. In fact, research on invisibility cloaks has been actively underway for a long time. Notably, in August 2011, a research team at the University of California, Berkeley, experimentally demonstrated a “carpet cloak” capable of concealing microscopic objects—about the size of a red blood cell—using visible light. This device worked by concealing the surface irregularities created by an object in the visible light spectrum—the range visible to the human eye—and making the light wavefront appear as if it were reflected from a flat surface. The device created by the research team was composed of silicon nitride and nanoporous silicon oxide, and it spatially varied the material’s effective refractive index through a precise arrangement of nanometer-scale holes. Although this research did not make people or large objects completely disappear, like an invisibility cloak in a movie, it is significant in that it demonstrated the principle of an actual invisibility cloak in the visible light spectrum. Metamaterials are the primary artificial materials used in such research.
Before exploring what metamaterials are, it is necessary to first understand the refractive index of a material. If the speed of light in a vacuum is c and the speed of light in a given material is v, the material’s refractive index n can be calculated as the ratio of these two speeds, c/v. The refractive index of most materials is generally greater than 1. Since the speed of light varies between two materials with different refractive indices, refraction occurs when light passes through the interface between them. When light travels from a material with a lower refractive index to one with a higher refractive index, it bends toward the normal at the interface. Conversely, when light travels from a material with a higher refractive index to one with a lower refractive index, it bends away from the normal.
If you place a straw in a glass of water and look at the side of the glass, the straw will appear bent. This is because the refractive indices of air and water differ, causing the light reflected from the straw to refract. The refractive index of air is approximately 1.0003, while that of water is approximately 1.33 in the visible light spectrum. But what would happen if a material’s refractive properties were the opposite of those of ordinary materials, exhibiting a negative refractive index? In that case, light would bend in the opposite direction from what is expected in ordinary materials. A negative refractive index is a unique electromagnetic property not found in ordinary materials in nature; it can be achieved through artificially designed metamaterials. However, metamaterials do not necessarily have to possess a negative refractive index. Metamaterials refer to artificial materials that control the propagation and interaction of electromagnetic waves in a desired manner through artificially designed microstructures within the material, rather than through the chemical properties of the constituent materials themselves. Therefore, metamaterials are significant in that they can realize electromagnetic properties rarely seen in conventional materials and, by utilizing these properties, precisely control the direction of light propagation.
Metamaterials with these unique properties can be artificially created by precisely arranging materials such as metals or silicon into microstructures, or by processing suitable materials—such as silicon nitride and silicon oxide—on the nanometer scale. The key starting point for research on invisibility cloaks using metamaterials dates back to 2006. At that time, researchers at Duke University and Imperial College London proposed a theoretical design for an invisibility cloak that uses a method called transformation optics to make electromagnetic waves bend around an object, and in the same year, a research team at Duke University actually fabricated an invisibility cloak operating in the microwave range based on this concept. In this experiment, the researchers created a structure to conceal a copper cylinder; the invisibility cloak utilized a concentric artificial structure to make microwaves bypass the hidden central region. However, this experiment targeted microwaves rather than visible light, which is visible to the human eye. Subsequently, in 2011, a research team at the University of California, Berkeley, demonstrated a carpet-shaped invisibility cloak that operated in the visible light spectrum. Therefore, the research at the University of California, Berkeley can be considered a significant example that experimentally demonstrated the principle of invisibility cloaks in the visible light spectrum.
So, what is the principle behind creating an invisibility cloak using metamaterials that control the path of light? Imagine arranging metamaterials in a specific structure around an object and then placing the object inside it. When light reflected from the background behind the object enters the metamaterial, its path is altered by the metamaterial’s spatially varying refractive properties. As a result, instead of striking the object directly, the light bends around it and continues on its path; from the observer’s perspective, the light from the background appears uninterrupted, as if the object were not there. The microwave invisibility cloak, experimentally demonstrated in 2006, was also a structure designed to allow electromagnetic waves to pass around the hidden region. The carpet-based invisibility cloak that emerged later evolved to conceal the surface irregularities created by the object from the light wavefront, making it appear as a flat surface. Ultimately, metamaterials and transformational optics do not simply block light; rather, they meticulously engineer the path of light to ensure that objects remain unseen.
There are various potential applications for cloaks created in this way. The first and most intuitive use is in the military. If metamaterial-based cloaks could be scaled up significantly, they could be used to create invisible fighter jets or tanks. However, if an invisibility cloak works by diverting specific electromagnetic waves around an object, a person inside the cloak might have difficulty seeing ahead using those incoming electromagnetic waves. As a solution, one could consider using light of a different wavelength—such as infrared, like in an infrared telescope—to see ahead. Alternatively, installing a micro-camera on the outside could be another option. Furthermore, since metamaterials can be designed to manipulate electromagnetic waves within specific frequency or wavelength ranges in a desired manner, structures that control not only visible light but also electromagnetic waves in the microwave range could potentially be used to reduce the likelihood of detection by radar. In fact, this potential application has been proposed since the early stages of research, as evidenced by the fact that the first metamaterial invisibility cloak, developed in 2006, was designed to operate in the microwave spectrum. However, this potential does not immediately imply the complete concealment of large objects such as fighter jets or tanks. Even today, there are fundamental limitations to extensively concealing large objects in the visible light spectrum using passive metamaterials alone.
Second, medical applications are also worth considering. Suppose we could create a metamaterial that is completely harmless to the human body. During major surgery, if the organ requiring surgery is obscured by an organ in front of it and is difficult to see, we could use a structure similar to an invisibility cloak to conceal the front organ, thereby making it easier to observe the organ behind it. This could potentially help improve the precision of the surgery. In addition, we can envision applications related to aesthetics or lighting, such as allowing light to enter areas where it is blocked by other structures or in places like basements where natural light does not penetrate well. Of course, when applying an invisibility cloak—which works by guiding or blocking light of specific wavelengths around an object—to a real-world space, it could block even the light and electromagnetic waves needed by people inside. Therefore, it would be more appropriate to apply this technology to structures where no people are present and where direct light is not necessary.
However, as with everything, there are both advantages and disadvantages. While invisibility cloaks using metamaterials can be utilized in many beneficial ways, if used for malicious purposes, there is ample potential for them to be exploited in various crimes. If such a scenario were to occur, invisibility cloaks could become a technology that is worse than having none at all. Therefore, if there is a possibility that this technology could actually be misused for criminal purposes, institutional mechanisms must be put in place to properly regulate and sanction such use. For example, one could consider a system that allows the use of specific forms of invisibility technology only by government-certified institutions or entities that meet predetermined conditions. Additionally, measures to thoroughly manage and monitor the manufacturers of invisibility devices and their distribution channels could also be considered.
However, as mentioned earlier, it is still difficult to freely utilize invisibility cloaks across various fields. The carpet-style invisibility cloak developed by a research team at the University of California, Berkeley, in 2011 was not a device designed to conceal large objects such as people or tanks, but rather an experiment targeting microscopic objects about the size of a red blood cell. Furthermore, this device was not a three-dimensional invisibility cloak capable of concealing objects from all directions, but rather an experimental setup that controlled the propagation of visible light under specific conditions and structures. The research paper confirmed experimental results at wavelengths such as 480 nm, 520 nm, and 700 nm, and while the device itself demonstrated the potential to operate across a broad spectrum within the visible light range, the technology to completely conceal large objects—such as humans—from visible light in all directions has not yet been commercialized. In particular, as objects grow larger, it becomes more difficult to suppress light scattering across a wide wavelength range, and there are fundamental physical limitations to making large objects—such as people or trams—virtually invisible in the visible light spectrum using a passive invisibility cloak. Therefore, metamaterials and the invisibility cloaks based on them remain technologies that require further research and development. However, research into controlling light and electromagnetic waves with greater precision using metamaterials and metasurfaces is ongoing, and the scope of this research is expanding beyond invisibility to various fields such as imaging, detection, and communications. We look forward to further technological advancements that will overcome current limitations, enabling metamaterials and invisibility cloaks to be widely utilized in ways that truly benefit people.
