Design of Invisibility Cloak Qu Tianyue Wu Yanjun Introduction - - PowerPoint PPT Presentation
EEE 016 Design 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
Qu Tianyue Wu Yanjun
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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▪ Internal Cloak ▪ External Cloak
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1) investigate the use of metamaterials to fabricate internal invisibility cloak 2) investigate the realisation of external cloak and illusion device 3) compare the strengths and constraints of both schemes
a light-bending cloaking device surrounding the object as an outer layer
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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
remains undetectable by eyes, making it “invisible”.
a cloak at a distance outside a cloaking shell, designed based on transformation optics
The cloak contains a dielectric core, as well as an “anti-object” embedded inside a negative index
field complementary to that of the
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 placed in medium air. Due to the conducting property
copper metal, the electric field of the incident wave is scattered. The wave no longer propagates in the
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Effect of Invisibility using Internal Cloak
By adding a layer
metamaterial with carefully manipulated permeability and permittivity around the copper ball, an illusion of free space is achieved. The electric field before and after passing through the copper ball follows relatively same pattern with the help of a cloaking shell.
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Effect of Invisibility using Diamond Cloak
When an object, which is a perfect electric conducting (PEC) cylinder is placed inside the diamond cloak region filled with material air (i.e. without any cloaking effect), the magnetic field of the incident wave is scattered as shown in the figure.
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Effect of Invisibility using Diamond Cloak
When a perfect electric conducting (PEC) cylinder is fitted into the cloaked region with designed cloaking effect, the figure shows the magnetic field distribution when a TM polarized plane wave exerts effect on the cloak from left to right. The wave fronts are bent around the cloaked area on the left, and then restored their original paths
transformation method.
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Effect of Invisibility using Diamond Cloak
Specifically, we only alter the value εxx , εxy , εyy and μzz for the eight separate components of the outer layer diamond cloak during the simulations (because of the symmetry
the material parameters tensor, we can derive εxy = εyx). As a result, the fields outside the cloak are almost undisturbed except tiny scattering due to the defect of the
detected by human eyes.
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Effect of Invisibility using External Cloak
When an object with a U shape is placed in an electric field, it can be detected by human eye.
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Effect of Invisibility using External Cloak
As shown in the graph, a dielectric core is placed in an electric field.
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Effect of Invisibility using External Cloak
By using a complementary medium layer with an embedded complementary “image”
the
surrounding space is
cancelled. Thereafter, dielectric core material can restore the
path. Then, an illusion of invisibility is achieved as the object becomes invisible to any incident waves.
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Effect of Illusion Using External Cloak
A solid ball is placed in medium air.
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Effect of Illusion using External Cloak
By 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 of placing a square in medium air. This has similar wave pattern as that in the previous result.
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Internal Cloak
It is independent of the object inside the cloak, because the light only passes through the cloak, but does not interact directly with the
freely as long as it has a layer of cloak outside it.
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Internal Cloak
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1. For internal cloaks, the person using the cloak is completely shielded off from incident light rays, he is unable to see the surroundings, thus making movement difficult for him.
Since the cloak is designed to create an illusion that light travels in the same direction after passing through it, the internal cloak needs to be divided into numerous small sections with changing parameters in order to bend light bit by bit. This design makes the cloaking material difficult to manufacture in real life, given that each small portion has different optical properties.
External Cloak
The cloak is at a distance away from the object, and not completely surrounding the object. When a person is using the cloak, the person is still able to receive light from surroundings, which means he is still able to see what is
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External Cloak
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Since the complementary medium utilised is calculated specifically based on the shape and size of the object, the
must stay exactly identical and still for the invisible effect to show. Also the object needs to stay in the exact same position in order for the ‘complementary medium’ to ‘cancel’ it.
Limitation for both schemes
Visible light has a spectrum of frequencies. Each colour has different properties, therefore making a single homogeneously designed cloak incapable of achieving invisibility at all of the frequencies, resulting in scattering and poor image.
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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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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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