Based on Transform Optics: Realizing an Ideal Omnidirectional Invisible Cloak in Free Space

A team led by Professor Ye Dexin and Professor Chen Hongsheng from Zhejiang University, as well as Professor Yu Luo from Nanyang University of Technology, conducted practical research on full parameter transformation optical devices. The research team has designed and implemented an all parameter omnidirectional invisibility cloak based on the theory of linear transformation optics and omnidirectional matching transparent metamaterials, which can hide large objects in free space.

The research findings were published in the National Science Review under the title of "Omnidirectional Transformation Optical Devices with All Parameters". Dr. Yuan Gao from Zhejiang University was the first author, and Professor Yu Luo, Professor Chen Hongsheng, and Professor Ye Dexin were the corresponding authors.

In 2006, Professor Pendry from Imperial College London, UK, proposed transformation optics, which describes the correspondence between electromagnetic wave propagation paths and material composition parameters, providing a universal and powerful method for controlling electromagnetic waves.

In the past decade, transformation optics has developed rapidly, and various new optical devices have been designed through transformation optics, such as invisibility cloaks, electromagnetic illusion devices, and concentrators. However, the composition parameters of optical media transformation are anisotropic and often uneven or have singular values, making it difficult to achieve.

For example, the omnidirectional invisibility cloak achieved through experiments so far has always simplified the material parameters. Simplified design sacrifices impedance matching, thereby reducing the performance of transformation optical devices.

To address these issues, the research team designed a two-dimensional all parameter omnidirectional planar invisibility cloak based on linear transformation optics, which is composed of only two homogeneous materials. The composition parameters of the first material are anisotropic, with both zero and extreme values, and electromagnetic waves propagating along the optical direction have infinite phase velocities.

Design an ideal omnidirectional cloak in free space. (a) Stealth design based on linear transformation optical elements. (b) A schematic diagram of the actual cloak. (c) Simulate (I, II, III) and measure (IV, V, VI) stealth performance.

By using this material, electromagnetic waves can bypass the invisible region, achieving omnidirectional impedance matching and zero phase delay. The second material also has anisotropic composition parameters, which can achieve phase compensation under omnidirectional impedance matching, and electromagnetic waves propagating in the optical direction have sub cavity phase velocity.

In the experimental verification, researchers used these two materials with TM polarization wave full parameter composition parameters.
The first material is achieved using a subwavelength metal patch array with Fabry Perot resonance, while the second material is achieved using a structure composed of traditional I-type electric resonators and split ring resonators.

Finally, the researchers measured the magnetic field around the omnidirectional cloak composed of the first two materials under different angles of TM polarization wave incidence, and the results showed that it has excellent stealth performance.

This study presents for the first time a fully parametric omnidirectional invisibility cloak in free space, which can hide large objects under any incident light. The achieved invisibility cloak can be immediately used to suppress the scattering cross-section of targets in radar communication and bistable detection.

The method proposed in this study also has a profound impact on the practical application of other full parameter transformation optical devices.

Source: Physicist Organization Network