Optical Camouflage

Orlando J. Camacho Vega 

 Metamaterial membrane, hold by tweezers.

 The first time I watched Harry Potter use his invisibility cloak, I wanted one. Surely, I was a small kid then and the idea of just using such device seemed marvelous, yet I knew it was science fiction, just a fantasy. Nowadays methods of hiding an object are possible with the use of what is called “metamaterials”, which are exotic composite materials that display properties beyond those available in natural occurring materials and are constructed at the macroscopic level. The metamaterials can be engineered to yield both negative electric permittivity and magnetic permeability. By using metamaterials the flow of light can be interrupted and channeled at the fundamental level, in other words they manipulate the material such that it can interact with light and achieve almost any desired result, for example better optics or perceived invisibility. These materials are better for light manipulation than normal or natural materials, for example if you took a cup of water and submerged a pencil, the portion below the water would appear to be at a different angle than the portion above the water, thus showing that its refractive index is positive. This happens because of the difference in the indices of refraction of air and water, an index of refraction has permittivity, its electrical component, and permeability, its magnetic component; due to the property of metamaterials of having a negative refractive index, light can be manipulated. Researchers tested the electromagnetic response of samples of such materials by defining two plasmonic structures and then they were described and characterized optically with a polarization-controlled white light source and their light transmission curves were analyzed both numerically and experimentally. Also, the material needed to be characterized in such a way that the material’s optical properties were tested and changed as it was bent and folded. So far, the technology that has been developed just works on the nano-scale with a plasmonic resonance down to a wavelength of 620 nm.

In order for the light to be manipulated, several fishnet-like patterns need to be created with metamaterials and stacked layer by layer as it can achieve a tailored response, for example bending light around a fixed object, effectively making it invisible. Nevertheless, restrictions are imposed by the fabrication method, i.e. electron beam lithography. For example, a full-fledged cloaking device needs to be mobile and needs to have the properties of being deformable and most importantly be large enough to actually cloak an object. Already the military uses cloaking technology in the Terahertz and Near Infrared waves, its next logical step would be to implement the use of metamaterials created in a single layer as optical cloaking components of their equipment. Of course, limitations are present because the structures can only be created in the nano-scale and they need to have certain shapes in order to be effective. This discovery provides a huge leap over what was considered to be just fiction and now is a reality, and provides the steps in the right direction of research beyond what is considered normal. Imagine what breakthroughs could be had in the next 10 years in this field, by then a child’s dream of having his own invisibility cloak could come true.

Sources:
http://iopscience.iop.org/1367-2630/12/11/113006/fulltext
http://www.physorg.com/tags/metamaterials/