Jan 07, 2014 |
Inverse design: New route to design a practical invisibility cloak
|
(Nanowerk News) With the emergence of metamaterials and transformation optics in the past few years, invisibility has become a scientific possibility that has attracted sustainable research interest. Recently, a review paper published in SCIENCE CHINA Information Sciences ("Invisibility cloaks from forward design to inverse design") reviewed design methodologies and experimental developments of the invisibility cloak from a practical perspective. The recent transition from a forward cloaking design to inverse cloaking design was also addressed. The paper pointed out that the combination of "forward designs" and "inverse designs" rather than using a single cloaking strategy is very likely to make invisibility cloaks far more realistic.
|
There are three popular design methodologies: the use of transformation optics, conformal mapping, and scattering cancellation. The key point of cloaking techniques is to use materials with specific constitutive parameters and refractive index. A competent candidate is a metamaterial, which can be realized with a collection of artificial "atoms" with subwavelength size and spacing. As artificial "atoms" can be tuned, electromagnetic properties of metamaterials can be engineered almost at will.
|
Although these three design methods differ sharply in specific means and were developed almost independently without overlap, they share the same goal of reducing the total scattering cross section to a minimum. The three invisibility strategies can be categorized as "forward designs", where the properties of the cloak can be worked out only after completing the design process. From a perspective of experimental demonstration, the practical development of an invisibility cloak based on forward design has several bottlenecks.
|
The paper focused on the review of an alternative target-oriented invisibility strategy, which is referred to as an "inverse design" and profoundly different from the forward design. From a practical point of view and with reverse thinking, the inverse design allows the possibility of integrating the technical advantages of forward strategies; e.g., (i) the central concept of invisibility is to minimize the total scattering cross section, (ii) anisotropic materials ensure invisibility without violating the uniqueness theorem of the inverse problem, and (iii) non-superluminal propagation provides cloaking performance with a relatively broad bandwidth. With these advantages, the backward cloaking strategy may eventually be implemented for a practical large-scale freestanding cloak in free space with relatively broad bandwidth.
|
Because a target-oriented approach is able to combine all the significant properties, inverse designs integrating desired properties might provide an alternative to current cloaking technology and solve the bottlenecks of individual strategies.
|