Electromagnetic Detection of a Perfect Invisibility Cloak

Abstract

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. DOI: 10.1103/PhysRevLett.103.243901 PACS numbers: 41.20.Jb, 41.60.Àm, 42.25.Fx Physical laws that describe nature govern the behavior of an object in space and time. For example, two sets of fundamental physical laws, Newton's laws of motion and Maxwell's equations, are both functions of space and time, but apply to different physical objects-one commonly applies to concrete objects with inertia, while the other applies to electromagnetic (EM) waves, or photons with zero stationary mass. In general, the space where these physical laws apply is flat if gravity is ignored. However, analogous to general relativity where space and time are curved, the new blooming field of transformation optics shows that the space for light can also be bent in an almost arbitrary way [1,2] by transforming an original EM space [3,4] in the absence of gravity.The most attractive prediction of transformation optics might be the possibility of perfect invisibility cloaking [3][4][5][6][7][8][9][10][11][12], where the cloak mimics the coordinate transformation, squeezing the originally flat EM space and guiding light smoothly along curved trajectories around a hidden object. Perfect cloaking is believed to be undetectable at its working frequency because a perfect cloak is equivalent to a curved but empty EM space. Although the first cloak models can achieve invisibility only in a very narrow frequency band [3,13], efforts to extend the bandwidth are currently proceeding [9][10][11][12]14]. While it is well known that nothing can hide from a pulse detection covering the whole frequency spectrum [15], the possibility of realizing invisibility cloaking within a nonzero bandwidth has great significance on both theoretical and practical levels and has attracted a lot of attention from scientific community. However, the other side of this issue-how to detect a truly ideal perfect invisibility cloak, or a curved EM space, within its working band-has not been solved in previous studies.It is the purpose of this Letter to develop a method to detect a perfect invisibility cloak electromagnetically, no matter what frequency band it works in. Our results demonstrate the asymmetry of coordinate transformation: Though this transformation of EM space can completely control the motion of photons, it is not directly applicable to the motion of charges. The consequence is that the curved EM space and the flat mechanical space overlap at the cloak's loca...

Negative propagation effect in nonparaxial Airy beams

“…On the other hand, the negative propagation effect could also be used as an effective tool in the detection of invisibility cloacks. This nonparaxial AiB could serve as an additional bridge from the flat mechanical space to the curved electromagnetic space by means of the interaction with a charged particle through a perfect invisibility cloak [33]. As a final question, one could ask why the negative propagation effect in AiBs was not previously reported.…”

Detection of a diffusive cloak via second-order statistics

“…However, unlike the long-range correlation employed in our work, scanning ultrasonic focus would be a serial process. Lastly, an entirely different kind of a "beacon" -Cherenkov radiation produced by the fast-moving charged particles -was proposed for detection of the ballistic cloak based on transformation optics [31].…”

Electrodynamics of transformation-based invisibility cloaking

“…67; when the space for Maxwell's equations (electromagnetic space) is compressed, the space for Newton's laws of motion (mechanical space) remains flat. Although the transformation of electromagnetic space can completely control the motion of photons, it is not directly applicable to the motion of a charged particle that can perceive both the electromagnetic space (because it has charge) and the mechanical space (because it has mass).…”

“…Electrodynamics of TB invisibility cloaking BL Zhang 5 space, respectively, and the radiation generated from the charged particle passing through an invisibility cloak. 67 …”