THE ART OF INVISIBLE CLOAKING

 

                                         

CHAPTER - I

INTRODUCTION

 

1.1        LIGHT AROUND US Light is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye (about 400–700 nm, or perhaps 380–750 nm[1]). 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 an important research area in modern physics.

                                        Figure 1.1 : Intensity of light

  1.2        FORMATION OF COLOURS ON VARIOUS OBJECTS Every object in the universe has its own colour and characteristics. Each and every object is identified by its colour and composure. All the living and non-living things in this universe absorb all the seven colours VIBGYOR and eventually reflect back one a single colour and this single colour becomes the primary 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 its own colour.

 

 

 

1.3        THE CONCEPT OF REFLECTION AND REFRACTION  

                                 Figure 1.3 : The concept of reflection and refraction

 

Reflection is the change in direction of a wave front at an interface between two different media so that the wave front returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves. The law of reflection says that for specular reflection the angle at which the wave is incident on the surface equals the angle at which it is reflected. Mirrors exhibit specular reflection.

 

 

Refraction is the change in direction of a wave due to a change in its speed. This is most commonly observed when a wave passes from one medium to another at an angle. Refraction of light is the most commonly 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 into water of a different depth. Refraction is described by Snell's law, which states that the angle of incidence θ1 is related to the angle of refraction θ2 by

Where v1 and v2 are the wave velocities in the respective media, and n1 and n2 the refractive indices.   Some metamaterials have been created which have a negative refractive index. With metamaterials, we can 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 water to magnify objects. First, as light is entering a drop of water, it slows down. If the water's surface is not flat, then the light will be bent into a new path. This round shape will bend the light outwards and as it spreads out, the image you see gets larger.  

 

 

 

 

 

CHAPTER - II

THE DENOTATION OF INVISIBLE MATERIALS

 

2.1 THE EVOLUTION OF METAMATERIALS

Meta materials are artificial materials engineered to provide properties which "may not be readily available in nature". These materials usually gain their properties from structure rather than composition, using the inclusion of small in homogeneities to enact effective macroscopic behavior. The primary research in Meta materials investigates materials with negative refractive index .Negative refractive index materials appear to permit the creation of 'super lenses' which can have a spatial resolution below that of the wavelength, and a form of 'invisibility' has been demonstrated at least over a narrow wave band.

Among the many tropes found in science fiction and fantasy, few are more popular than the cloaking device. In the real world, scientists have long engaged in research that would at least improve camouflaging technology, conceal aircraft from radar or further our knowledge of how light and electromagnetic waves work. In 2006, a group of scientists from Duke University demonstrated a simplified 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 device only masked an object from one wavelength of microwave light, it does provide more information that will 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, where an 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. Smith's team, compared the effect to "river water  

  Flowing around a smooth rock" Anything placed inside the cylinder is cloaked, or effectively invisible to the microwave light. The device isn't perfect. It creates some distortion and "shadowing of the microwaves"]. 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 Meta materials. The properties of Meta materials are based on their structure rather than their chemistry. For the cloaking device, researchers made mosaic-like constructions out of fiberglass sheets stamped with loops of wire, somewhat similar to a circuit board. The arrangement of the copper wires determines the way it interacts with electromagnetic fields. The unique advantage of meta materials is that they can be used to create objects with electromagnetic characteristics that can't be found in the natural world.  

                                      Figure 2.1 : Circular Rings of Metamaterials

     The key to the cloaking device is taking advantage of a concept known as the index of refraction. An object's index of refraction, or refractive index, determines how much light bends when passing through it. Most objects have a uniform index of refraction throughout, so light only bends when it crosses 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 one nanometer. That means that visible light waves, which are hundreds of nanometers in size, or longer wavelength waves cannot even come close to resolving the atomic structure. Although we know materials are formed from collections of atoms, we cannot see the individual atoms because the light we perceive is so much larger than the atomic scale. So, we are able to approximate the discrete atoms and molecules of a material as a continuous substance, whose properties derive not only from the individual 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; these vibrations in turn affect the speed of the light. A Meta material shell with the right gradient of metal elements should cause light of a particular wavelength to wrap around     interior.   Engineers David Schurig and David Smith of Duke University say they were concealing something themselves last May when they and their colleagues reported their proposal: "We had a cloak we liked pretty well in May, and it got better from there," Schurig reveals. In the group's current version a central copper ring--the object to be cloaked--is surrounded by concentric rings of Meta material standing one centimeter tall and spanning 12 centimeters. The rings are sandwiched between two plates so that Microwaves can only travel through the cloak in the plane of the rings, as described in a paper published 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 wasn't there. The researchers sampled the electric field component of the microwaves at many points in the apparatus to see how the radiation was affected, and the results match well with their simulations, they report. "We don't say anything quantitatively about how well this is cloaking, but we've reduced both the reflection and the shadow generated by the object, and those are the two essential features of the invisibility cloaking," Schurig says.

 

                                               Figure 2.2 : Passage of the microwave light.

 

 

 

 

                                   Figure 2.3 : Alignment inside the Metamaterial. 

 

                                                                 

 

 

 

 

 

CHAPTER - III

FORMATION OF METAMATERIALS

Diffusion of hydrogen in metal-doped glasses leads to the reduction of metals and to the growth of metallic nanoparticles in the glass body that allows the formation of metamaterials. The nanoparticles grow due to the super saturation of the glass matrix by neutral metals, whose solubility in glasses is low compared 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 diffusion accompanied by the inter diffusion of protons, metallic ions and neutral metals allowed us to study the temporal evolution of the average size of the metallic nanoparticles and their spatial distribution. The developed model of the formation of metallic nanoparticles defines range of parameters providing the formation of layered structures of metallic inclusions in silver and copper doped glasses. The layered structure arises at relatively low super saturation of the diffusion zone by a neutral metal as the result of the competition of the enrichment of the glass by neutral metal atoms via reducing of metal ions by diffusing hydrogen and the depletion of the glass by the metal atoms caused by their diffusion to the nanoparticles. The results of numerical calculations are compared with the data of optical spectroscopy of 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 its composition. Scientists found that the new material particularly adept at capturing light from any direction and focusing it in a single direction. Redirecting scattered light means none of it bounces off the 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 not only retransmit the color and brightness of what is behind, like squid or chameleons do, but also bend the light around, preserving the original phase information of the signal.  

CHAPTER - IV LIMITATIONS AND CONCLUSION

4.1 LIMITATIONS OF META MATERIALS AND CLOAKING

There has been some controversy surrounding some of the scientific concepts associated with Meta materials and cloaking. People have also questioned if invisibility cloak s really a possibility. Several years ago, some scientists claimed that it was possible to make Meta materials with a negative index of refraction. Initially, many experts claimed that a negative index of refraction was against the laws of physics, but most now accept that it is possible. Even so, it had proven difficult to make negative refraction Meta materials for visible light Experiments in negative refraction had been done with Meta materials affecting microwave light.) But this year scientists at Germany's Karlsruhe University and the Ames Laboratory in Iowa were able to produce Meta materials with a negative index of refraction for visible light.

  However, there's still a lot of work to be done before a working cloak is developed for more than one wavelength 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. They also don't yet know if it's even possible to cloak multiple wavelengths simultaneously.If a full invisibility is decades off or simply impossible, one other possibility seems intriguing, and it's not unlike what we've seen in some movies.   It may be possible in the future to create some sort of phasing cloaking device, in which each color of the 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, there's one

  other factor that limits the uses of a cloaking device that scientists say many people don't consider. People inside a cloaked area wouldn't be able to see out because all visible light would be bending around where they are positioned. They'd be invisible, but they'd be blind, too.  

4.2        CONCLUSION  The primary research in Meta materials investigates materials with negative refractive index. Negative refractive index materials appear to permit the creation of ‘super lenses' which can have a spatial resolution below that of the wavelength and a form of 'invisibility' has been demonstrated at least over a narrow wave band. Although the first Meta materials were electromagnetic, acoustic and seismic Meta materials are also areas of active research.  

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER V

APPLICATIONS AND FUTURE PROSPECTS

5.1   THE ART OF INVISIBILITY IN THE NEAR FUTURE

          Invisibility may now be a real possibility according to report published in the Journal of Science and Nature .The BBC has a nice breakdown of the article http://news.bbc.co.uk/2/hi/science/nature/7553061.stm which uncovers a new material which allows light to be bent around objects.

          A material that is able to reverse the effect of light refracting, thus rendering objects invisible has major implications for the world of creativity and technology. Whilst the first thought for most people will be impersonating Harry Potter and donning an invisibility cloak to become an instant spy, the practical 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 or simply reduce the visual impact of objects on demand is an exciting prospect.

                                        Figure 5.1 : The ideal invisible cloak

  5.2        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 BIBLIOGRAPHY

  BOOKS:

·       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

 

 

 

 

 

END

 

 

 

 

     2006-2010

 

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