THE ART OF INVISIBLE CLOAKING CHAPTER - I INTRODUCTION1.1 LIGHT AROUND USLight is electromagnetic radiation, particularly radiation of a wavelength that is visible to the humaneye (about 400–700 nm, or perhaps 380–750 nm ). In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.Four primary properties of light are: • Intensity • Frequency or wavelength • Polarization • Phase Light, which exists in tiny "packets" called photons, exhibits properties of both waves and particles.This property is referred to as the wave–particle duality. The study of light, known as optics, is animportant research area in modern physics.
Figure 1.1 : Intensity of light1.2 FORMATION OF COLOURS ON VARIOUS OBJECTSEvery object in the universe has its own colour and characteristics. Each and every object is identifiedby its colour and composure. All the living and non-living things in this universe absorb all the sevencolours VIBGYOR and eventually reflect back one a single colour and this single colour becomes theprimary colour of that thing. Figure 1.2 : Formation of VIBGYOR
For instance, the flora of the universe absorb all the seven colours of the nature and reflect only green,thereby they appear green. In the same way everything has its own ways of reflection and thereby itsown colour. 1.3 THE CONCEPT OF REFLECTION AND REFRACTION Figure 1.3 : The concept of reflection and refractionReflection is the change in direction of a wave front at an interface between two different media so thatthe wave front returns into the medium from which it originated. Common examples include thereflection of light, sound and water waves. The law of reflection says that for specular reflection theangle at which the wave is incident on the surface equals the angle at which it is reflected. Mirrorsexhibit specular reflection.
Refraction is the change in direction of a wave due to a change in its speed. This is most commonlyobserved when a wave passes from one medium to another at an angle. Refraction of light is the mostcommonly observed phenomenon, but any type of wave can refract when it interacts with a medium,for example when sound waves pass from one medium into another or when water waves move intowater of a different depth. Refraction is described by Snells law, which states that the angle ofincidence θ is related to the angle of refraction θ by 1 2Where v and v are the wave velocities in the respective media, and n and n the refractive indices. 1 2 1 2Some metamaterials have been created which have a negative refractive index. With metamaterials, wecan also obtain total refraction phenomena when the wave impedances of the two media are matched.There is then no reflected wave.Also, since refraction can make objects appear closer than they are, it is responsible for allowing waterto magnify objects. First, as light is entering a drop of water, it slows down. If the waters surface is notflat, then the light will be bent into a new path. This round shape will bend the light outwards and as itspreads out, the image you see gets larger. CHAPTER - II THE DENOTATION OF INVISIBLE MATERIALS2.1 THE EVOLUTION OF METAMATERIALSMeta materials are artificial materials engineered to provide properties which "may not be readilyavailable in nature". These materials usually gain their properties from structure rather thancomposition, using the inclusion of small in homogeneities to enact effective macroscopic behavior.The primary research in Meta materials investigates materials with negative refractive index .Negativerefractive index materials appear to permit the creation of super lenses which can have a spatialresolution below that of the wavelength, and a form of invisibility has been demonstrated at least overa narrow wave band.
Among the many tropes found in science fiction and fantasy, few are more popular than the cloakingdevice. In the real world, scientists have long engaged in research that would at least improvecamouflaging technology, conceal aircraft from radar or further our knowledge of how light andelectromagnetic waves work. In 2006, a group of scientists from Duke University demonstrated asimplified cloaking device. In October 2006, a research team from Duke, led by Dr. David R. Smith,published a study in the journal "Science" describing a simplified cloaking device. While their deviceonly masked an object from one wavelength of microwave light, it does provide more information thatwill help us to consider if a real-life cloaking device is possible.This cloaking device was made from a group of concentric circles with a cylinder in the middle, wherean object could be placed. When researchers directed microwave light at the device, the wave split,flowing around the device and rejoining on the other side. Dave Schurig, a researcher on Dr. Smithsteam, compared the effect to "river waterFlowing around a smooth rock" Anything placed inside the cylinder is cloaked, or effectively invisibleto the microwave light. The device isnt perfect. It creates some distortion and "shadowing of themicrowaves"]. It also works for only one wavelength of microwave light.To achieve their cloaking effect, the Duke team used a relatively new class of materials called Metamaterials. The properties of Meta materials are based on their structure rather than their chemistry. Forthe cloaking device, researchers made mosaic-like constructions out of fiberglass sheets stamped withloops of wire, somewhat similar to a circuit board. The arrangement of the copper wires determines theway it interacts with electromagnetic fields. The unique advantage of meta materials is that they can beused to create objects with electromagnetic characteristics that cant be found in the natural world.
Figure 2.1 : Circular Rings of MetamaterialsThe key to the cloaking device is taking advantage of a concept known as the index of refraction. Anobjects index of refraction, or refractive index, determines how much light bends when passingthrough it. Most objects have a uniform index of refraction throughout, so light only bends when itcrosses the boundary into the material. This occurs, for example, when light passes from air into water.The size and typical spacing of atoms within a material are on the order of angstroms, or tenths of onenanometer. That means that visible light waves, which are hundreds of nanometers in size, or longerwavelength waves cannot even come close to resolving the atomic structure. Although we knowmaterials are formed from collections of atoms, we cannot see the individual atoms because the lightwe perceive is so much larger than the atomic scale. So, we are able to approximate the discrete atomsand molecules of a material as a continuous substance, whose properties derive not only from theindividual atoms and molecules, but also their interactions.2.2 THE ACTION INSIDE When light strikes a Meta material it causes the electrons in the metal pieces to vibrate; thesevibrations in turn affect the speed of the light. A Meta material shell with the right gradient of metalelements should cause light of a particular wavelength to wrap around interior.Engineers David Schurig and David Smith of Duke University say they were concealing somethingthemselves last May when they and their colleagues reported their proposal: "We had a cloak we likedpretty well in May, and it got better from there," Schurig reveals. In the groups current version acentral copper ring--the object to be cloaked--is surrounded by concentric rings of Meta materialstanding one centimeter tall and spanning 12 centimeters. The rings are sandwiched between two platesso that Microwaves can only travel through the cloak in the plane of the rings, as described in a paperpublished online October 19 by Science.When the microwaves strike the shell they interact with its C-shaped copper wires and, theoretically,should be absorbed and reflected less by the enclosed object than if the shell wasnt there. Theresearchers sampled the electric field component of the microwaves at many points in the apparatus tosee how the radiation was affected, and the results match well with their simulations, they report. "Wedont say anything quantitatively about how well this is cloaking, but weve reduced both the reflectionand the shadow generated by the object, and those are the two essential features of the invisibilitycloaking," Schurig says.
Figure 2.3 : Alignment inside the Metamaterial. CHAPTER - III
FORMATION OF METAMATERIALSDiffusion of hydrogen in metal-doped glasses leads to the reduction of metals and to the growth ofmetallic nanoparticles in the glass body that allows the formation of metamaterials. The nanoparticlesgrow due to the super saturation of the glass matrix by neutral metals, whose solubility in glasses is lowcompared to initial concentration of metal ions. In some cases, these metallic nanoparticles are self-arranging to quasi-periodic layered structure. A theoretical analysis of the reactive hydrogen diffusionaccompanied by the inter diffusion of protons, metallic ions and neutral metals allowed us to study thetemporal evolution of the average size of the metallic nanoparticles and their spatial distribution. Thedeveloped model of the formation of metallic nanoparticles defines range of parameters providing theformation of layered structures of metallic inclusions in silver and copper doped glasses. The layeredstructure arises at relatively low super saturation of the diffusion zone by a neutral metal as the result ofthe competition of the enrichment of the glass by neutral metal atoms via reducing of metal ions bydiffusing hydrogen and the depletion of the glass by the metal atoms caused by their diffusion to thenanoparticles. The results of numerical calculations are compared with the data of optical spectroscopyof the glass-metal metamaterials containing silver and copper nanoparticles.In other words, a metamaterial, is a substance that gets its properties from its structure and not itscomposition. Scientists found that the new material particularly adept at capturing light from anydirection and focusing it in a single direction. Redirecting scattered light means none of it bounces offthe metamaterial back into the eye of an observer. That essentially makes the material invisible."Ideally, one should see exactly what is behind an object," says a scientist."The material should notonly retransmit the color and brightness of what is behind, like squid or chameleons do, but also bendthe light around, preserving the original phase information of the signal. CHAPTER - IV LIMITATIONS AND CONCLUSION4.1 LIMITATIONS OF META MATERIALS AND CLOAKINGThere has been some controversy surrounding some of the scientific concepts associated with Metamaterials and cloaking. People have also questioned if invisibility cloak s really a possibility. Severalyears ago, some scientists claimed that it was possible to make Meta materials with a negative index ofrefraction. Initially, many experts claimed that a negative index of refraction was against the laws ofphysics, but most now accept that it is possible. Even so, it had proven difficult to make negativerefraction Meta materials for visible light Experiments in negative refraction had been done with Metamaterials affecting microwave light.) But this year scientists at Germanys Karlsruhe University and theAmes Laboratory in Iowa were able to produce Meta materials with a negative index of refraction forvisible light.However, theres still a lot of work to be done before a working cloak is developed for more than onewavelength of the visible spectrum, much less the sort seen in science-fiction movies. At the moment,making a device that works on all wavelengths of visible light is beyond scientists capabilities. Theyalso dont yet know if its even possible to cloak multiple wavelengths simultaneously.If a fullinvisibility is decades off or simply impossible, one other possibility seems intriguing, and its notunlike what weve seen in some movies.
It may be possible in the future to create some sort of phasing cloaking device, in which each color ofthe spectrum of visible light is cloaked for a fraction of a second. If accomplished at sufficient speed,an object would likely appear translucent, though not quite invisible. Think of the alien villain in the"Predator" movies, who is barely perceptible when he moves but is otherwise essentially invisible.Finally, theres oneother factor that limits the uses of a cloaking device that scientists say many people dont consider.People inside a cloaked area wouldnt be able to see out because all visible light would be bendingaround where they are positioned. Theyd be invisible, but theyd be blind, too. 4.2 CONCLUSIONThe primary research in Meta materials investigates materials with negative refractive index. Negativerefractive index materials appear to permit the creation of ‘super lenses which can have a spatialresolution below that of the wavelength and a form of invisibility has been demonstrated at least overa narrow wave band. Although the first Meta materials were electromagnetic, acoustic and seismicMeta materials are also areas of active research. CHAPTER V APPLICATIONS AND FUTURE PROSPECTS5.1 THE ART OF INVISIBILITY IN THE NEAR FUTURE Invisibility may now be a real possibility according to report published in the Journal of Scienceand Nature .The BBC has a nice breakdown of the articlehttp://news.bbc.co.uk/2/hi/science/nature/7553061.stm which uncovers a new material which allowslight to be bent around objects.
A material that is able to reverse the effect of light refracting, thus rendering objects invisible hasmajor implications for the world of creativity and technology. Whilst the first thought for most peoplewill be impersonating Harry Potter and donning an invisibility cloak to become an instant spy, thepractical and creative uses could create major changes aesthetically in the world around us.We are all aware of the benefits of Glass and other transparent materials but the ability to hide orsimply reduce the visual impact of objects on demand is an exciting prospect. Figure 5.1 : The ideal invisible cloak5.2 APPLICATIONS Potential applications of Meta materials are diverse and include : • Remote Aerospace applications. • Sensor detection and infrastructure monitoring smart solar power management, • Public safety. • Radomes. • High frequency battle field communication and lenses for high-gain antennas, • Improving ultrasonic sensors and even shielding structures from earth quakes. • The research in meta materials is interdisciplinary and involves such fields as electrical engineering electromagnetic, solid state physics, microwave and antennae engineering, opto electronics, classic optics, material sciences,, semiconductor engineering, nanoscience and others.5.3 IDEAS FOR USE OF INVISIBLE MATERIALS IN THE REAL WORLD • Chairs and Tables without legs. • Buildings that appear to float by hiding structural elements • Lighting (e.g.: streetlights) that defy gravity. • Hiding of ugly infrastructure (e.g.: pylons). • Amazing visual transitions for live theatre. • General visual bulk reduction of any object.
• “Soft glass” - practical applications would be enormous. • Curtains that trap heat, but still allow light in. • Invisible ropes would have many uses. • Art. Insane sculptures that appear to defy physics. REFERENCES AND BIBLIOGRAPHYBOOKS: • Meta materials and Plasmonics: Fundamentals, Modelling, Applications Proceedings of the NATO Advanced Research Workshop on Meta materials for Secure Information and Communication Technologies Marrakech, Morocco 7-10 May 2008 • Electromagnetic Meta materials: Physics and Engineering Explorations (Hardcover) ~ Nader Engheta (Editor), Richard W Ziolkowski (Editor)WEBSITES:IEEE EXPLORE • www.sciam.com • www.livescience.com • www.howstuffworks.com