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  • Invisibility Cloaks - Now You See Me, Now You Don't

    Horatio I. Davis

    With advancements in science and technology, we are getting closer to the famous quote from science fiction writer Arthur C. Clarke, where “any sufficiently advanced technology is indistinguishable from magic.” We have seen prototypes and experiments in many fields that would have been considered magic or superpowers in previous eras and cultures. Scientists have made incredible progress towards realizing super hero abilities like flying, levitation, invisibility, teleportation, and even time travel.


    Among science fiction fans, invisibility is one of the most sought-after powers. One of the most famous sci fi examples of invisibility was in “Harry Potter and the Sorcerer's Stone.” Harry Potter is given a garment as a Christmas gift by Professor Dumbledore, Headmaster of Hogwarts School of Witchcraft and Wizardry. The cloak renders whoever it covers invisible.


    Modern science has made such a cloak not entirely inconceivable. Cloaking techniques use specially designed materials, called metamaterials, to bend waves (such as thermal, electromagnetic, acoustic, or mechanical) around an object in order to hide it from detection. While electromagnetic cloaks can be used to make things invisible to the human eye, mechanical cloaks can hide an object from stress and mechanical vibrations. Traditional attempts at invisibility cloaking have mostly focused on electromagnetic emissions and light reflection from objects. For example, some fighter jets are coated with special paint to absorb radio waves. They also have mechanisms to reduce heat exhaust, allowing them to hide from radars [1]. These planes still cast shadows and are open to other types of detection.


    In the traditional sense, invisibility cloaking is mostly associated with light. This is called optical cloaking. The basic idea behind optical cloaking is to manipulate light. Scientists once again use metamaterials, exotic materials with unusual properties, to force light waves to behave differently, cloaking the objects from view. Initial attempts at invisibility cloaking worked with microwave frequencies, but since then, researchers have found ways to cloak visible light, sound, and even ocean waves. Scientists at Rochester University used physical lenses to hide objects [2] even though the viewer may be nearby. This technique could be used in vehicles to eliminate blind spots and in medical fields to improve visibility during operations.


    There is a somewhat primitive form of optical camouflage that involves capturing a scene behind a person or object. The captured scene is projected onto a special fabric covering an object, thus deceiving the viewer. This fabric is made from retro-reflective material [3], which is covered with thousands of small beads. This material reflects light rays back exactly in the same direction from which they came. This technique gives the illusion of seeing through the object, and making the object semitransparent. It is not perfect, but provides a good level of camouflage for basic targets. Researchers have used this technique to hide vehicles, people, and small objects from view.


    Metamaterials are engineered to produce properties that do not occur in nature. At the nanoscale, scientists control the structure of the material and do things once thought impossible. This includes cloaking to the point of near invisibility, superlenses that can see details at the limits of optical resolution, and illusions that can make an object look like another.


    Light is made up of vibrations of electric and magnetic fields. While natural materials can affect only the electric field, as in optics, metamaterials can affect both. Metamaterials can be created with spacing between elements in a material less than the wavelength of the light. When the gap width gets smaller, the light wave will be more diffracted, change direction and opens the door to invisibility cloaking. Metamaterials have been used in many cloaking applications to reroute light around an object.


    A lesser known type of cloaking is time cloaking, which involves hiding events rather than objects. An event cloak masks the evidence of the existence of an activity by controlling the flow of photons streaming from a laser source [4]. Cloaking leaves the light unperturbed during an event, but makes it invisible to time, as if it never happened.


    Cloaking is not limited to the scales of light and time. Scientists have worked on techniques for manipulating sound and water waves, too. Researchers at Duke University used metamaterials to interact with sound waves and create a 3D acoustic cloak [5]. The cloak is constructed using several sheets of plastic plates with patterns of dots. It alters the trajectory of sound waves, causing them to reflect as if from an unobstructed, flat surface. This technique can be used by submarines to avoid sonar in the water and in concert hall construction to improve acoustics by hiding structural objects from sound waves.


    A group of researchers from France and the UK have built an invisibility cloak for water waves [6]. This cloak directs the waves around an object, as if it is not there. It can be used to protect coastal areas from disasters like tsunamis by hiding the infrastructure from incoming waves.


    We might be years away from creating a large-scale cloaking device like the one used by Harry Potter – and we may never reach that level of sophistication. However, we have seen significant progress on all types of cloaking devices, and humans can now effectively cloak light, sound, and electromagnetic and water waves. These developments could have profound effects on human life moving forward, and could even protect cities from earthquakes, floods, and tsunamis.


     


    References

    [1] M. Selvanayagam and G. V. Eleftheriades, Experimental Demonstration of Active Electromagnetic Cloaking, Phys. Rev. X 3, 041011 (2013).

    [2] Joseph S. Choi and John C. Howell, “Paraxial ray optics cloaking”, Opt. Express 22, 29465-29478 (2014)

    [3] M. Inami, N. Kawakami, and S. Tachi, “Optical camouflage using retro-reflective projection technology”, in Proceedings of ISMAR 2003 (ISMAR, 2003), pp. 348-349.

    [4] M. Fridman, A. Farsi, Y. Okawachi & A.L. Gaeta, “Demonstration of temporal cloaking”, Nature 481, 62–65, 2012.

    [5] Z. L., Popa, B., Cummer, S.A. , “Three-dimensional broadband omnidirectional acoustic ground cloak”, Nature Materials, March 9, 2014.

    [6]  M. Farhat, S. Enoch, S. Guenneau, A.B. Movchan, “Broadband cylindrical acoustic cloak for linear surface waves in a fluid”, Phys. Rev. Lett. 101, 134501 (2008).

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