Invisibility Cloaks Aren't Just for Harry Potter

Before you don your invisibility cloak, grab your wand, and join the other muggles at the penultimate Harry Potter movie released this month, consider that cloaking (making an object invisible) has been around since before Lord Voldemort was called Tom Riddle. Invisibility figures prominently in Greek mythology, J. R. R. Tolkien's The Hobbit, H. G. Wells' The Invisible Man, and Star Trek's Romulan technology.

Theoretically, invisible objects do not reflect, scatter or absorb light, or produce a shadow.1 An invisibility cloak would work by directing light or other electromagnetic waves around the object, but would also obscure visibility from within the cloak.2 (No sneaking into the restricted section of the library.) None of this would exist outside the realms of theory or fantasy without the advent of metamaterials, which were first named in 2000.

A metamaterial is a non-naturally occurring composition that behaves differently as a whole compared to its component parts alone. Electromagnetic light waves interact with metamaterials according to electric and magnetic properties and refractive index, or the material's ability to bend light. An object appears invisible when the electric and magnetic properties of a cloaking device divert light around the object. Metamaterials make this possible because they are not limited by the atomic and molecular compositions found in nature.1

Viktor Veselago first proposed the concept of a negative refractive index in 1967.3 However, three decades passed before negative index metamaterials (NIMs) were envisioned. NIMs became highly visible after back-to-back publications in Science made cloaking and transformation optics top scientific breakthroughs of 2006.2,4

Since then, cloaking has advanced well beyond first-wave split ring resonators made of copper wires. Current technology uses woodpile photonic crystals with polymers, silicon, and other metals to take 2-dimensional cloaking into 3-D territory. Recent achievements include a cloak that can hide a µm-sized bump on a gold reflector and cylindrical cloaks made of glass challogenides5 such as cadmium sulfide (CAS Registry Number® (RN): 1306-23-6) or gallium arsenide (CAS RN: 1303-00-0). Perhaps soon casting invisibility spells will not be solely within the power of a wizard.

Contributed by
Kathryn J. Kitzmiller, Ph.D.
Public Relations Representative


References
  1. Litchinitser, N. M. and Shalaev, V. M. Photonic Metamaterials. Laser Phys. Lett. 2008, 5, 411-420. DOI: 10.1002/lapl.200810015. SciFinder users access here
  2. Pendry, J. B.; Schurig, D.; Smith, D. R. Controlling Electromagnetic Fields. Science 2006, 312, 1780-1782. DOI: 10.1126/science.1125907. SciFinder users access here Veselago, V. G. Properties of Substances with Simultaneously Negative and Dielectric (ε) and Magnetic (µ) Susceptibilities. Fizika Tverdogo Tela. 1966, 8, 3571-3573. SciFinder users access here
  3. Leonhardt, U. Optical Conformal Mapping. Science 2006, 312, 1777-1780. DOI: 10.1126/science.1126493. SciFinder users access here
  4. Semouchkina, E.; Werner, D. H.; Semouchkin, G. B.; Pantano, C. An infrared invisibility cloak composed of glass. Appl. Phys. Lett. 2010, 96, 233503/1-233503/3. DOI: 10.1063/1.3447794 SciFinder users access here


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