Realization of ideal omnidirectional invisibility cloak in free space

A team led by Professor Ye Dexin and Professor Chen Hongsheng of Zhejiang University and Professor Luo Yu of Nanyang Technological University conducted research on the actual implementation of full-parameter transformation optical elements. Based on the theory of linear transformation optics and omnidirectional matching transparent metamaterials, the research team designed and implemented a full-parameter omnidirectional invisibility cloak that can hide large objects in free space.

This work has been published in national science review Titled “Full-parameter omnidirectional transformation optics”, Dr. Gao Yuan of Zhejiang University is the first author, and Professor Luo Yu, Professor Chen Hongsheng and Professor Ye Dexin are the corresponding authors.

In 2006, Professor Pendry of Imperial College London proposed transformation optics, which describes the correspondence between the propagation path of electromagnetic waves and the constitutive parameters of materials, providing a universal and powerful method for controlling electromagnetic waves.

The past decade has witnessed the rapid development of transformation optics, through which various new optical components such as invisibility cloaks, electromagnetic illusion devices, and light collectors have been designed. However, the constitutive parameters of transformed optical media are anisotropic and often non-uniform or have singular values, making them difficult to implement.

For example, the omnidirectional invisibility cloaks that have been experimentally implemented so far always require some simplifications in material parameters. Simplified designs sacrifice impedance matching, thereby reducing the performance of transformation optics.

In order to solve these problems, the research team designed a two-dimensional full-parameter omnidirectional planar invisibility cloak based on linear transformation optics composed of only two homogeneous materials. The constitutive parameters of the first material are anisotropic, with both zero and extreme values, and electromagnetic waves propagating along the optical direction have infinite phase velocity.

The material is used to enable electromagnetic waves to bypass stealth zones and has omnidirectional impedance matching and zero phase delay. The second material also has anisotropic constitutive parameters to achieve phase compensation for omnidirectional impedance matching, and electromagnetic waves propagating along the optical direction have sub-light phase velocities.

In the experimental verification, the researchers realized the full-parameter constitutive parameters of TM polarized waves for these two materials.

The first was achieved using an array of subwavelength metallic patches with Fabry-Perot resonance, while the second was achieved with a structure consisting of a conventional I-shaped electrical resonator and an open-loop resonator.

Finally, the researchers measured the magnetic field around the full-parameter omnidirectional cloak composed of the first two materials under the incidence of TM polarized waves at different angles, showing excellent stealth performance.

This research demonstrates the first fully parametric omnidirectional invisibility cloak in free space that can hide large objects from any incident illumination. The implemented cloak can be immediately used to suppress the scatter cross section of targets in radar communications and bistatic detection.

The method proposed in this work also has profound implications for the practical implementation of other fully parametric transformation optical elements.