'Invisibility Cloak' Realized in Theory

Apr 18, 2008
Tetsuo Nozawa, Nikkei Electronics
Fig.1: Cross sectional view of columnar invisible cloak: The yellow lines are the trajectories of light. The lines in other colors represent the boundaries between five media that constitute the invisible cloak. The region outside the boundary of radius 1 is the medium w1 (refractive index n > 0), the region surrounded by the red and blue lines and the x-axis is the medium R1 (refractive index n > 0), the region surrounded by the blue and brown lines is the medium R2 (refractive index n < 0), the region surrounded by the brown and green lines is R3 (refractive index n > 0) and the region surrounded by the purple and green lines and the x-axis is the medium R4 (refractive index n < 0). Diagram courtesy of Ochiai, Toyama Prefectural University
Fig.1: Cross sectional view of columnar invisible cloak: The yellow lines are the trajectories of light. The lines in other colors represent the boundaries between five media that constitute the invisible cloak. The region outside the boundary of radius 1 is the medium w1 (refractive index n > 0), the region surrounded by the red and blue lines and the x-axis is the medium R1 (refractive index n > 0), the region surrounded by the blue and brown lines is the medium R2 (refractive index n < 0), the region surrounded by the brown and green lines is R3 (refractive index n > 0) and the region surrounded by the purple and green lines and the x-axis is the medium R4 (refractive index n < 0). Diagram courtesy of Ochiai, Toyama Prefectural University
[Click to enlarge image]
Fig.2: Enlarged view of central portion: Only the trajectories of light are shown in different colors. The central portion impenetrable to light is where an object can be hidden.
Fig.2: Enlarged view of central portion: Only the trajectories of light are shown in different colors. The central portion impenetrable to light is where an object can be hidden.
[Click to enlarge image]

Researchers at Toyama Prefectural University and other institutions announced that they theoretically formulated a "perfect invisible cloak."

The invisible cloak generates no reflection or phase delay at all even when an electromagnetic wave passes through it. It was developed with the use of an artificial dielectric material called "left-handed metamaterial," which has a negative refractive index n. It can be said that the researchers created a sort of blueprint of the perfect invisible cloak.

It is predicted that the use of left-handed metamaterials makes electromagnetic control devices available. Some examples of such devices are a lens that reflects no light and a lens that can provide a perfect focal point. They are believed to be difficult to produce with the existing materials. The latest development relates to one of these control devices.

The invisible cloak was designed by Tomoshiro Ochiai, a lecturer of Department of Information Systems Engineering of Faculty of Engineering at Toyama Prefectural University and two other researchers from University of St Andrews in Scotland, UK and Future University-Hakodate. A coauthored paper titled "A Novel Design of Dielectric Perfect Invisibility Devices" was published in a theoretical physics journal, "Journal of Mathematical Physics."

Here, an invisible cloak refers to a columnar or block object with a void in the core or center, which is designed such that a plane electromagnetic wave with a certain frequency irradiated at this object goes around the void and reaches behind the object. In particular, the object may be called "the perfect visible cloak" when the electromagnetic wavefront becomes planar again after passing through the object and the amplitude and the phase of the resultant plane wave completely coincide with those of the wavefront obtained when there is no object.

The cloak is named after the fact that it generates no reflection or phase delay when an electromagnetic wave with a given frequency is irradiated and the view behind the cloak can be seen as it is. Thus, taking into account an electromagnetic wave with a specific frequency, it appears as though anything hidden in the void portion of such an object will disappear together with the invisible cloak.

Five media attached together

The following describes the design of the latest invisible cloak (hereafter referred to as "the cloak"). The cloak has is substantially columnar in shape. Fig. 1 is a cross sectional view of the cloak. Note that yellow lines represent the trajectories of light that pass through the cloak, not the shape of the cloak itself. The scale indicates the space coordinates from the center. The unit of distance is arbitrary.

The cloak is composed of five metamaterials (artificial dielectric materials) with different values of refractive indices n and different index distributions, which are combined with one another like building blocks. In Fig.1, red purple green and blue lines illustrate the boundaries between these media.

The outermost medium w1 extends outside of the circle indicated by the red and purple lines. As a matter of fact, the outer side of this cloak cannot be clearly defined because the cloak extends towards infinity like gas. In Fig. 1, however, "it is appropriate to regard the vicinity of radius 5 as the practical outer borer," said Ochiai.

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