SEMINAR REPORT ON INVISIBITY CLOAKING POOJA MOL GIRISH B14
CONTENTS 1 ABSTRACT
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2. INTRODUCTION
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3. METAMATERIAL CLOAKING
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4. METAMATERIALS AND TRANSFORMATION OPTICS
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5. SCIENCE OF CLOAKING DEVICES
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6. FIRST DEMONSTRATION 7. LIMITATIONS
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8. FIRST PRACTICAL INVISIBILITY CLOAK
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9. CONCLUSION
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10. REFERENCE
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ABSTRACT Class of materials that are engineered to produce properties that don’t occur in nature are called metameterials. Light is an electromagnetic radiation, made of perpendicular vibrations of both electric and magnetic fields. Natural materials usually affect the electric component only. Metamaterials can affect the magnetic components too. They are used to bend electromagnetic radiations such as light around an object, giving the appearance that it isn’t there at all. This process is known as invisibility cloaking.
INTRODUCTION Invisibility has long been employed in works of science fiction and fantasy, from “cloaking devices” on spaceships in various Star Trek series to Harry Potter’s magic cloak. But physicists are beginning to think they can actually make devices with just these properties. To achieve the feat of “cloaking” an object, they have developed what are known as metamaterials, some of which can blend electromagnetic radiations, such as light around an object and making it “invisible”. The first example only worked with long wavelength radiation such as microwaves.
METAMATERIAL CLOAKING Metamaterial cloaking is the usage of metamaterials in an invisibility cloaking. This is accomplished by manipulating the paths traversed by light through a novel optical material. Metamaterials direct and control the propagation and transmission of specified parts of the light spectrum and demonstrate the potential to render an object seemingly invisible. Metamaterial cloaking, based on transformation optics, describes the process of shielding something from view by controlling electromagnetic radiation. Objects in the defined location are still present, but incident waves are guided around them without being affected by the object itself.
METAMATERIALS AND TRANSFORMATION OPTICS The field of transformation optics is founded on the effects produced by metamaterials. Transformation optics has its beginnings in the conclusions of two research endeavors. They were published on May 25, 2006, in the same issue of Science, a peer reviewed journal. The two papers are tenable theories on bending or distorting light to electromagnetically conceal an object. Both papers notably map the initial configuration of the electromagnetic fields on to aCartesian mesh. Twisting the Cartesian mesh, in essence, transforms the coordinates of the electromagnetic fields, which in turn conceal a given object. Hence, with these two papers, transformation optics is born.
[Left: the cross section of a PEC cylinder subject to a plane wave(only electric field component of the wave is shown). The field is scattered. Right: a circular cloak, designed using transformation optics method, is used to cloak the cylinder. In this case the field remains unchanged outside the cloak and the cylinder is invisible electromagnetically. Note the special distortion pattern of the field inside the cloak.] Transformation optics subscribes to the capability of bending light, or electromagnetic wavesand energy, in any preferred or desired fashion, for a desired application. Maxwell's equationsdo not vary even though coordinatestransform. Instead it is the values of the chosen parameters of the materials which "transform", or alter, during a certain time period. So, transformation optics developed from the capability to choose the parameters for a given material. Hence, since Maxwell's equations retain the same form, it is the successive values of the parameters, permittivityand permeability, which change over time. Furthermore, permittivity and permeability are in a sense responses to theelectric and magnetic fieldsof a radiated light source respectively, among other descriptions. The precise degree of electric and magnetic response can be controlled in a metamaterial, point by point. Since so much control can be maintained over the responses of the material, this leads to an enhanced and highly flexible gradient-index material. Conventionally predetermined refractive index of ordinary materials instead, become independent spatial gradients in a metamaterial, which can be controlled at will. Therefore, transformation optics is a new method for creating novel and unique optical devices.
SCIENCE OF CLOAKING DEVICES The purpose of a cloaking device is to hide something, so that a defined region of space is invisibly isolated from passing electromagnetic fields, as with metamaterial cloaking The object or subject doesn’t really disappear. The vanishing is an illusion. With the same goal, researchers employ metamaterials to create directed blind spots by deflecting certain parts of light spectrum.it is the light spectrum, as the transmission medium, that determines what the human eye can see. In other words, the light is refracted or reflected determining the view, color, or illusion that is seen. The visible extent of light is seen in a chromatic spectrum such as rainbow. However visible light is only part of a broad spectrum, which extends beyond sense of sight. Electromagnetic radiations interacts with matter. Cloaking applications which employ metamaterials alter this interactions. The guiding vision for the metamaterial cloak is a device that direct the flow of light smoothly around an object, like water flowing past a rock in a stream, without reflection, rendering the object invisible. In reality the simple cloaking devices at the present are imperfect,and have limitations. One challenge up to the present is the inability of metamaterials and cloaking devices to interact at frequencies or avelengths within the visible light spectrum.
FIRST DEMONSTRAION The principle of cloaking, with a cloaking device, was first proved at frequencies in the microwave radiation band on October 19, 2006 by a group of scientists at Duke University’s Pratt School of Engineering. The demonstration used a small cloaking device. Its height was less than one half inch (<13mm) and its diameter five inches (125mm), and it successfully diverted microwaves around itself. The object to be hidden from view, a small cylinder, was placed in the center of the device. The invisibility cloak deflected microwave beams so they flowed around the cylinder inside with only minor distortion, making it appear almost as if there is nothing were
there at all. The researchers manufactured the cloak using "metamaterials" precisely arranged in a series of concentric circles that confer specific electromagnetic properties The team produced the cloak according to electromagnetic specifications determined by a new design theory proposed by Sir John Pendry of Imperial College London, in collaboration with the Duke scientists.
LIMITATIONS Although a successful demonstration, three notable limitations can be shown. First, since its effectiveness was only in the microwave spectrum the small object is somewhat invisible only at microwave frequencies. This means invisibility had not been achieved for the human eye, which sees only within the visible spectrum. This is because the wavelengths of the visible spectrum are tangibly shorter than microwaves. However, this was considered the first step toward a cloaking device for visible light, although more advanced nanotechnology-related techniques would be needed due to light's short wavelengths. Second, only small objects can be made to appear as the surrounding air. In the case of the 2006 proof of cloaking demonstration, the hidden from view object, a copper cylinder, would have to be less than five inches in diameter, and less than one half inch tall. Third, cloaking can only occur over a narrow frequency band, for any given demonstration. This means that a broad band cloak, which works across the electromagnetic spectrum, from radio frequencies to microwave to the visible spectrum, and to x-ray, is not available at this time. This is due to the dispersive nature of present-day metamaterials. The coordinate transformation (transformation optics) requires extraordinary material parameters that are only approachable through the use of resonant elements, which are inherently narrow band, and dispersive at resonance.
THE FIRST PRACTICAL INVISIBILITY CLOAK Physicists at the University of Rochester have created an incredibly versatile cloaking deviceAll it takes is four lenses, an optics bench for holding the lenses in place, and an insatiable appetite for making things disappear.
The four lenses, when aligned just right, will bend light around the object you place between them, cloaking the object in the process.
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Obtain two sets of two lenses with different focal lengths. The first set will have one focal length while the other set will have a different focal length. You will have four lenses in total.
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Using an optics bench, select one lens with the first focal length and a second lens with the second focal length. Separate them by a distance that is the sum of their focal length.
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Now, do the same with your remaining two lenses.
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Lastly, you need to know how far apart to separate your two sets. This will take a little math, but here's an example using the same measurements in Step 2: D=[2 (3) (5+ 3) ]/ (5- 3) = 12cm should be the distance between your two lenses with the focal length of 3 cm They used lasers to show how each of the four lenses bends light rays to recreate the image at the back of the set up, even if something's in the way between lens 1 and 2. You can see a great example of the laser rays converging at the focal point after exiting lens 1 located at the far left.
To make an object disappear, the physicists place it in between the first two lenses on the left side of the optics bench in the image. Below, they have placed a silver ruler between these two lenses.
The ruler then disappears when you look through the front lens. This is the first cloaking device that can make an object appear to vanish at multiple angles. So, if you move your eye from looking straight down the center of the lens to looking slightly from the left, right, top, or down, you still won't see the ruler in the image above.The device does have its limits, though. The device only cloaks at 15 degrees in either direction from where you deviate your eye from the central axis of the lens. But that's 15 degrees more than any former cloaking device.
CONCLUSION As shown, the invisibility is indeed physically possible. The idea of electromagnetic invisibility comes from the fact that Maxwell equations has the same form in all coordinate system. Since the same is true for all other laws of physics, it lead us to believe that similar cloaking device is possible in other wave systems as well. Indeed there has been proposition for an acoustic cloaking shell. The biggest obstacle in making invisibility cloak in practice is satisfying the complex material properties needed in such a cloak and realizing them on a scale that could be used with visible light. The problem still remains unsolved, but interest shown inn topic could mean that invisibility will soon become reality.
REFERENCE 1. 2. 3. 4.
Metamaterial cloaking – Wikipedia Cloak of invisibility- Wikipedia http://pratt.duke.edu/about/news/first-demonstration-working-invisibility-cloak Cloaking and invisibility: Fact and Fiction by Professor David R Smith-Electrical and
computer engineering- Duke University (May 28, 2006) 5. http://www.rochester.edu