Electromagnetic Detection of a Perfect Invisibility Cloak

A perfect invisibility cloak is commonly believed to be undetectable from electromagnetic (EM) detection because it is equivalent to a curved but empty EM space created from coordinate transformation. Based on the intrinsic asymmetry of coordinate transformation applied to motions of photons and charges, we propose a method to detect this curved EM space by shooting a fast-moving charged particle through it. A broadband radiation generated in this process makes a cloak visible. Our method is the only known EM mechanism so far to detect an ideal perfect cloak (curved EM space) within its working band.

Figure 1

Trajectory of a fast-moving charged particle inside the cloak compared with the trajectory of a photon in (a) the physical space and (b) the virtual space. $B$ is the midpoint of the trajectory. Background is the mesh of EM space. Mechanical space is not shown.

Figure 2

$|E_{\mathrm{tot}}(\overline{r},t)|$ during the radiation from a charged particle going through a spherical invisibility cloak. The dotted line represents the trajectory of the particle. The small arrow indicates the exact position of the particle’s center along its trajectory. The inner radius and outer radius of the cloak are $1$ and $2\mu \mathrm{m}$, respectively. The net charge corresponds to 1000 electrons. $v=0.9c$.

Figure 3

Far field spatial radiation pattern at 500 THz from the fast-moving charged particle through the invisibility cloak. The inner radius and outer radius of the cloak are $1$ and $2\mu \mathrm{m}$, respectively.