Scientific Method / Science & Exploration

Is it possible to make a real invisibility cloak?

In a special issue of the New Journal of Physics, researchers examine the …

We have been reporting on invisibility cloaks since starry-eyed theorists managed to show that it was mathematically possible to design a structure that prevented electromagnetic waves from penetrating a central spherical volume. We were all very impressed until we realized that it only worked for one color of light, and only then if you used materials that didn't exist. Nevertheless, even an imperfect form of invisibility is quite useful—ask the USAF if they want to give up the stealth bomber—so this seems worth pursuing. It is natural to ask if invisibility can be extended to more than a single color. The answer, provided by a special issue of the New Journal of Physics, is yes, but then again no.

The researchers took a long hard look at the theory behind cloaking with passive materials and discovered that there are fundamental issues that limit the range of invisibility cloaks. An invisibility cloak for a single frequency is based on the structured material, which requires that the motion of electrons in its metallic elements are resonant with the frequency of the light field. This strongly absorbs light, and then remits it in a direction that is controlled by the spacing of all the metallic parts (and therefore the spacing of the material in between, of course). Now, the need for the resonance requirement means that only a very tiny range of the electromagnetic spectrum can be effectively guided around the inner volume.

One can also consider multilayered structures, where an inner structure is designed to cloak one color while the outer layer cloaks a different color. This works, but neither layer winds up structures resonant with the particular color they cloak; rather, the interaction between the wave and the two layers acts to guide two wavelengths of light. One color lies between the two resonances of the metallic structures, while the second is bluer than both resonances, but redder than the plasma frequency (the natural frequency of the electrons in the bulk metal).

One might naively think that this means that one can indefinitely extend the invisibility range by increasing the number of layers. Unfortunately, reality stomps pretty hard on that idea. Basically, the resonances become broader, and the properties of the materials become increasingly unrealistic. Now, some of you will argue that unrealistic materials have been created before, so I shouldn't be such a party-pooper. However, there are good reasons to think that this is not going to work here. The materials must be designed such that the inner layers of the invisibility cloak respond to the light before the outer layers do so—everyone repeat after me: deliberately violating causality in any context other than Star Trek will get your fingers jammed in the desk.

Even with a limited number of layers, the invisibility becomes less perfect for any individual color—one can choose between perfect invisibility at one color or imperfect invisibility at a few different colors. The method isn't very flexible, either. The structural shapes used in each layer creates a fixed set of material properties that limits what you can do with the rest of the cloak. In other words, choosing the color that's handled within one layer fixes the remaining colors for the entire cloak, no matter how many layers are designed into it.

This sounds a bit pessimistic, but I should note that these are passive materials. Active materials respond to incoming light in a way that provides much more design freedom. Harry Potter, all hope is not lost.

New Journal of Physics, 2008, DOI: 10.1088/1367-2630/10/11/115036

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