Asymmetric optical-invisibility camouflage

A joint research team from RIKEN^* and Tokyo Institute of Technology^** has constructed the design theory of asymmetric invisibility camouflage devices (^*Dr. Masato Taki, Scientist, Interdisciplinary Fundamental Physics Team, Interdisciplinary Theoretical Science Research Group, RIKEN, ^**Assistant Professor Tomohiro Amemiya and Professor Shigehisa Arai, Quantum Nanoelectronics Research Center, Tokyo Institute of Technology).

Optical invisibility camouflage (or invisibility cloaking) is a technology to make an object seem invisible by causing incident light to avoid the object, flow around the object, and return undisturbed to its original trajectory. Such sophisticated manipulation of light will probably be realistic thanks to the recent progress in the research on metamaterials⁽¹⁾. To date several research institutes have carried out the theoretical and experimental study of invisibility camouflage devices, using the extraordinary optical properties of metamaterials and the technique of transformation optics⁽²⁾.

Optical camouflage devices designed using transformation optics have a closed region that incident light from every direction avoids. A person hiding in this region therefore seems invisible to external onlookers (Fig. 1a). However, no light can enter the cloaked region, and consequently the person hiding therein cannot be able to see outside. This is quite inconvenient for practical use. A practical camouflage device must have unidirectional transparency such that a person inside cannot be seen from the outside but can see the outside.

To overcome this problem, the research team has formulated a theory of asymmetric (or nonreciprocal) camouflage that can achieve unidirectional transparency in which "they cannot see us, but we can see them." This theory is unrelated to transformation optics but instead based on the concept of 'Lorentz/Coulomb-like forces for photons.' Unidirectional transparency needs a high-level nonreciprocity in the propagation of light. For example, as shown in Fig. 1b), rightward-propagating light have to avoid and circumvent the hiding person, whereas leftward-propagating light have to travel straight to enter the eyes of the hiding person. Such nonreciprocity can be achieved by controlling the movement of photons with two forces that are analogous to Lorentz force⁽³⁾ and Coulomb force⁽⁴⁾ for moving charged particles. These Lorentz-like and Coulomb-like forces can be generated with an optical resonator lattice consisting of metamaterials. Asymmetric camouflage can be achieved by surrounding a hiding person with the optical resonator lattice.

This research was published in IEEE Journal of Quantum Electronics, Vol. 51, Issue 3 (2015).

Figure1. Path of incident light around invisibility camouflage device. (a) Existing camouflage device with optical path independent of light direction. No light can enter into device, and therefore hiding person cannot see outside. (b) Asymmetric camouflage device. Rightward-propagating light avoids hiding person, whereas leftward-propagating light travels straight to enter hiding person's eyes. Hiding person cannot be seen from onlookers on right side but can see them.

Reference

Authors:	Tomohiro Amemiya, Masato Taki, Toru Kanazawa, Takuo Hiratani and Shigehisa Arai
Title of original paper:	Optical Lattice Model Toward Nonreciprocal Invisibility Cloaking
Journal:	IEEE Journal of Quantum Electronics, Vol. 51, No. 3, pp.6100110
DOI :	10.1109/JQE.2015.2389853

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