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 invisibility, such as using active camouflage, and using metamaterials to fabricate cloaking devices, and using complementary medium to 'cancel' a piece of space optically. 2

Types of Cloaking Schemes ▪ Internal Cloak ▪ External Cloak 3

Aims and Objectives 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 4

1 . Internal Cloak a light-bending cloaking device surrounding the object as an outer layer

Working Principle 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”. 6

2 . External Cloak a cloak at a distance outside a cloaking shell, designed based on transformation optics

Working Principle The 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. 8

Our simulation results 9

Effect of Invisibility using Internal Cloak When a solid copper ball is placed in medium air. Due to the conducting property of copper metal, the electric field of the incident wave is scattered. The wave no longer propagates in the original direction and path. 10

Effect of Invisibility using Internal Cloak By adding a layer of 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. 11

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. 12

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 on the right, which uses the coordinate transformation method. 13

Effect of Invisibility using Diamond Cloak Specifically, we only alter the value ε xx , ε xy , ε yy and μ zz for the eight separate components of the outer layer diamond cloak during the simulations (because of the symmetry of the material parameters tensor, we can derive ε xy = ε yx). As a result, the fields outside the cloak are almost undisturbed except tiny scattering due to the defect of the cloak. Therefore, the object cannot be detected by human eyes. 14

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. 15

Effect of Invisibility using External Cloak As shown in the graph, a dielectric core is placed in an electric field. 16

Effect of Invisibility using External Cloak By using a complementary medium layer with an embedded complementary “image” of the object, the object as well as the surrounding space is optically cancelled. Thereafter, dielectric core material can restore the original optical path. Then, an illusion of invisibility is achieved as the object becomes invisible to any 17 incident waves.

Effect of Illusion Using External Cloak A solid ball is placed in medium air. 18

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’. 19

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. 20

Strength and Limitations 21

Internal Cloak ● Strength It is independent of the object inside the cloak, because the light only passes through the cloak, but does not interact directly with the object. The object can also change shape, size, and position and move freely as long as it has a layer of cloak outside it. 22

Internal Cloak ● Limitation 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. 2. There is difficulty in the manufacturing of the designed cloaks. 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. 23

External Cloak ● Strength 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 outside the cloak. 24

External Cloak ● Limitation Since the complementary medium utilised is calculated specifically based on the shape and size of the object, the object 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. 25

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. 26

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. 27

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. 28

Thanks! 29