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 metamaterials1. 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 optics2.
Figure 1. 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.
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 force3 and Coulomb force4 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.
Explanations of Technical Terms
Artificial material consisting of multiple nanostructural elements such as minute resonators, arranged periodically with a pitch smaller than the wavelength of light. It can exhibit extraordinary permittivity and permeability values that are not found in nature. Using metamaterials enables to create a unique electromagnetic field surrounding an object we wish to hide, and it should therefore be possible to control the optical path around the object to make it appear invisible.
2. Transformation optics
A mathematical technique to design optical systems on the basis of the idea that a distorted space is equivalent in terms of the propagation of light to a flat space filled with a medium having an appropriate spatial distribution of refractive index. (Not used in this research of asymmetric invisibility camouflage.)
3. Lorentz force
A force that a moving charged particle experiences in a magnetic field. According to Fleming's rule, when the middle finger, index finger, and thumb of the left hand are stretched perpendicular to each other, if the direction of the middle finger represents the moving direction of the electric current due to the moving particle and the index finger represents the direction of the magnetic field, then the particle experiences the Lorentz force in the direction of the thumb. The direction of the Lorentz force is reversed if the moving direction of the particle is reversed.
4. Coulomb force
A force that a charged particle experiences in an electric field. The direction of the Coulomb force depends on the charge: it is in the direction of the electric field for a positive charge and in the opposite direction for a negative charge. The force is not dependent on the direction of particle movement.