Invisibility cloaks have been the object of human fantasy since ancient times. Ever since Harry Potter got one for Christmas, every child (and every adult) wanted to find one under their own Christmas tree. No one has found one yet, not because it is invisible, but because physicists could not design one. However, invisibility cloaks may become a reality in the future due to the development of metamaterials.
What are metamaterials?
Metamaterials are specially designed and fabricated materials. Their composition and structure can be designed so that it has properties that no natural material has. One property that has recently sparked a lot of interest is the negative refraction index. The refraction index is a dimensionless quantity that describes how the speed of light changes as it spreads through a medium (such as air, water or wood). For example, light travels faster in a vacuum than it does in the water so the refractive index of water is the ratio of the speeds of light in measured in vacuum and water respectively. Hence, the refractive indices of all naturally occurring materials are greater than or equal to one. This is not true of the specially engineered negative index metamaterials also known as NIMs.
The amazing thing about NIMs is that their constituent parts, such as wires and dielectrics, are all naturally occurring materials but once these are assembled together in a special way, they produce a completely new material! NIMs can manipulate the paths of light rays so that they bend around the object instead of scattering off it, hence causing it to appear invisible. What a physicist understands by “light” is different to what it means in everyday life. In physics the electromagnetic spectrum is referred to as light. A very narrow band in this spectrum is called visible light and this is the light that we see. However there are many wavelengths of light in the spectrum that we cannot see. At the moment, the NIMs are not perfect and have limitations and it is very difficult to design NIMs that can manipulate visible light. The best results with NIMs were achieved at the infrared wavelengths.
In just November of last year, researchers at the University of Texas at Austin devised an invisibility cloak which would be able to cover a broad range of frequencies: everything from microwaves to visible light. Currently, scientists have only managed to engineer metamaterials to cloak a few frequencies from the microwave section part of the electromagnetic spectrum – a huge accomplishment in its own right, but not much use if you want to keep hidden from muggles and the like. “Our group was the first to theoretically show that metamaterials could provide a realistic route towards invisibility and cloaking,” Alú says. “We are now working to experimentally prove broadband cloaks for radio waves and cloaked antennas. These possibilities are in reach within the coming months.”
However Alu’s team at UT also has plans for a proposed design which will be able to completely cloak the user from a broad spectrum of frequencies (many of which are visible light) under certain conditions of weather and distance. Alú’s design will involve a metamaterial base but with CMOS squares placed on top. CMOS – or Complementary Metal Oxide Semiconductors are also an important part of the recipe for invisibility, and are often sought as transceivers and communication devices. These small electrical devices are use in almost every microprocessor and laptop computers since 1976.
These invisibility cloaks sound like really good fun, but they may also have very important applications. Silk-based cloaking devices that use split-ring resonators can be used in mammalian surgery. Silk is not rejected by the human body so surgeons can use these materials to coat the internal organs of a patient to see what lies behind them and check for obstructions, tumours and perhaps bleeding. The business of a negative refractive index is also a spark for theoretical physicists to reconsider the interpretation of basic physical laws.
It will take a long time before anyone will be able to find their own invisibility cloak under the Christmas tree. They will remain in the movies, books and imaginations for now. However, the idea is no longer considered a fairy tale, but a worthwhile research topic that will revolutionise physics and our lives.
Illustration: Sarah Morel