EEE 016Design of Invisibility Cloak Qu Tianyue

Wu Yanjun

Introduction

The concept of invisibility cloak has long been fascinating to the crowd since its first appearance in ancient Greek myths.

Many methods can be employed to achieve invisibility, such as using active camouflage, and using metamaterials to fabricate cloaking devices, and using complementary medium to 'cancel' a piece of space optically.

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Types of Cloaking Schemes

▪ Internal Cloak

▪ External Cloak

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1) investigate the use of metamaterials to fabricate internalinvisibility cloak

2) investigate the realisation of external cloak and illusion device

3) compare the strengths and constraints of both schemes

Aims and Objectives

Internal Cloaka light-bending cloaking device surrounding

the object as an outer layer

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Working Principle

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When an object is placed inside the

cloaking shell.The cloak surrounds

the object wholly. When light is

incident to the object, the cloak

prevents the reflection of light by

the object, and at the same time

directs the light in the original

pathway. In this way the object

remains undetectable by eyes,

making it “invisible”.

External Cloaka cloak at a distance outside a cloaking shell,

designed based on transformation optics

2.

Working PrincipleThe cloak contains a dielectric core, as well as an “anti-object” embedded inside a negative index shell. When light is incident to the object, the cloak with an electric field complementary to that of the object allows the light to continue the original pathway. Therefore, an illusion of free space is achieved.

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Our simulation results

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Effect of Invisibility using Internal Cloak

When a solid copper ball is placedin medium air. Due to theconducting property of coppermetal, the electric field of theincident wave is scattered. Thewave no longer propagates in theoriginal direction and path.

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Effect of Invisibility using Internal CloakBy adding a layer of metamaterialwith carefully manipulatedpermeability and permittivity aroundthe copper ball, an illusion of freespace is achieved. The electric fieldbefore and after passing through thecopper ball follows relatively samepattern with the help of a cloakingshell.

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Effect of Invisibility using Diamond Cloak

When an object, which is a perfectelectric conducting (PEC) cylinderis placed inside the diamond cloakregion filled with material air (i.e.without any cloaking effect), themagnetic field of the incident waveis scattered as shown in the figure.

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Effect of Invisibility using Diamond CloakWhen a perfect electric conducting(PEC) cylinder is fitted into the cloakedregion with designed cloaking effect,the figure shows the magnetic fielddistribution when a TM polarized planewave exerts effect on the cloak fromleft to right. The wave fronts are bentaround the cloaked area on the left,and then restored their original pathson the right, which uses the coordinatetransformation method.

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Effect of Invisibility using Diamond CloakSpecifically, we only alter the value εxx ,εxy , εyy and μzz for the eight separatecomponents of the outer layer diamondcloak during the simulations (because ofthe symmetry of the materialparameters tensor, we can derive εxy =εyx). As a result, the fields outside thecloak are almost undisturbed excepttiny scattering due to the defect of thecloak. Therefore, the object cannot bedetected by human eyes.

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Effect of Invisibility using External Cloak

When an object with a U shape isplaced in an electric field, it canbe detected by human eye.

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Effect of Invisibility using External Cloak

As shown in the graph, adielectric core is placed in anelectric field.

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Effect of Invisibility using External CloakBy using a complementary mediumlayer with an embeddedcomplementary “image” of theobject, the object as well as thesurrounding space is opticallycancelled. Thereafter, dielectriccore material can restore theoriginal optical path. Then, anillusion of invisibility is achieved asthe object becomes invisible to anyincident waves. 17

Effect of Illusion Using External Cloak

A solid ball is placed in medium air.

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Effect of Illusion using External CloakBy using a complementary medium with calculated values for basic properties, the part of circle can be ‘cancelled’ (appear invisible). The illusion of a square can then be achieved by using a ‘restoring medium’.

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Effect of Illusion using External Cloak

This figure shows the effect ofplacing a square in medium air.This has similar wave pattern asthat in the previous result.

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Strength and Limitations

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Internal Cloak

It is independent of the object inside the cloak, because the light onlypasses through the cloak, but does not interact directly with theobject. The object can also change shape, size, and position and movefreely as long as it has a layer of cloak outside it.

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● Strength

Internal Cloak

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1. For internal cloaks, the person using the cloak is completelyshielded off from incident light rays, he is unable to see thesurroundings, thus making movement difficult for him.

2. There is difficulty in the manufacturing of the designed cloaks.Since the cloak is designed to create an illusion that light travelsin the same direction after passing through it, the internal cloakneeds to be divided into numerous small sections with changingparameters in order to bend light bit by bit. This design makes thecloaking material difficult to manufacture in real life, given thateach small portion has different optical properties.

● Limitation

External Cloak

The cloak is at a distance away from the object, and notcompletely surrounding the object. When a person is usingthe cloak, the person is still able to receive light fromsurroundings, which means he is still able to see what isoutside the cloak.

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● Strength

External Cloak

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Since the complementary medium utilised is calculatedspecifically based on the shape and size of the object, theobject must stay exactly identical and still for theinvisible effect to show. Also the object needs to stay inthe exact same position in order for the ‘complementarymedium’ to ‘cancel’ it.

● Limitation

Limitation for both schemes

Visible light has a spectrum of frequencies. Each colour hasdifferent properties, therefore making a single homogeneouslydesigned cloak incapable of achieving invisibility at all of thefrequencies, resulting in scattering and poor image.

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Conclusion

By performing simulations on COMSOL Multiphysics, we investigate the use of

metamaterials to fabricate internal invisibility cloak and the realisation of illusion

using the principle from external cloak. We managed to simulate the effect of

invisibility using both principles. However, both cloaking schemes have certain

weaknesses which would show constraints in their real-life applications. Also, the

difficulty in manufacturing the cloaks also limits the feasibility of them.

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Acknowledgement

We would like to express our sincere thanks to Professor Luo Yu from Nanyang

Technological University for introducing us to the new concept of invisibility and

guiding us through the process of this project. We would also like to thank Huang

Yao, for the time and efforts she spent guiding us through the simulation process.

Without her invaluable advice, we would not be able to complete this project.

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Thanks!

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