The Science of Metamaterials for Invisibility Cloaks
The divinity of engineering is expressed in the infusing power that it has over the electromagnetic spectrum-from radio waves, to the visible part, and up to the exploding energy of gamma rays. but how does all this work?
Cloaks for invisibility work because they bend light waves around an object; unfortunately, that is all the experimental development to date has achieved for a particular type of the wave.
What are Metamaterials?
Metamaterials are artificially synthesized materials having some extrinsic features concerning electromagnetism. These peculiar properties arise owing to the structure or special arrangement of this artificial media which defines the way light and other electromagnetic waves interact with them. This is something passive or somewhat difficult to reproduce by natural materials.
The name refers to a broad class of artificial materials known as metamaterials, each one with specific electromagnetic properties that make it stand out. The magic is in the composition-how does it get configured, what size is it, what shape does it take-but it is a magic that gives birth to these non-natural characteristics.
For example, the refraction of light occurs as it travels from air into glass, changing speed and bending per verse the refractive indices between the two substances. Physicists have now engineered materials that create this effect at the other end of the spectrum: without bouncing waves off an object, they redirect them back onto itself.
They usually consist of a grid of tiny metallic elements spaced distances smaller than the wavelengths they are meant to cloak, so their currents interact similarly to the way electric and magnetic fields of atoms and molecules interact in natural materials, effectively suppressing any reflective waves while allowing passage as nature would.
The first metamaterials were experimentally discovered at microwave frequencies, ending up sparking lots of interests across the common engineering electromagnetics community. Engheta and Ziolkowski organized special sessions on metamaterials during the 2002 IEEE International Symposium on Antennas and Propagation/USNC/URSI National Radio Science Meeting in San Antonio; they furthermore served as guest editors of an Optics Express special issue dedicated to artificial magnetism and negative index metamaterials.
How do Metamaterials Work?
Metamaterials do not present in nature; however, physicists can use nanotechnology to create such remarkable metamaterial properties in materials that bend electromagnetic waves in quite unexpected ways. They may even create those materials that have a negative index of refraction to certain wavelengths of electromagnetic radiation such as light or radio waves-these materials referred to as possible negative-refractive-index metamaterials.
NRMs can cause light to bend around an object, thereby creating an invisibility cloak that makes objects of particular sizes and shapes invisible to lookers. This was accomplished by incorporating an NRM in a shell structure surrounding the object to be hidden.
Cloaks may impede light waves around an object that is covered by redirecting, altering speed and energy-given the shape or through a means referred as tunable metamaterials.
Metamaterials use different types of effects for example, they can be used in transformation cloaking or they can cause change in optical dispersion. Igor Smolyaninov of University of Pennsylvania developed a metamaterial that will deflect sound waves from an object.
Active metamaterials are capable of affecting all electromagnetic waves and can be put to innumerable applications, starting from shielding of various military equipment and improving cooking through microwave technology and computing. People have started experimenting with bizarre applications of this technology.
What Are Metamaterial Cloaks?
Some of the hardest challenges for scientists in producing an invisibility cloak involves the scattering of light of all wavelengths. Cloak materials must produce a pattern in spacelike grid patterns by which they distort an electromagnetic field such as to allow wavelengths to pass through unobtrusively while deflecting other wavelengths outside of their path.
The answer is through transformation optics-an engineering concept for the control of electromagnetic fields with nanometer-level precision. This would allow the shaping of metamaterials with unique electromagnetic properties to form cloak structures.
In his studies, David Smith, an electrical engineer from Duke University, was the first to exhibit negative indices of refraction through electromagnetic metamaterials. He was even able to build a cloak that negated the effects of electromagnetic metamaterials for making objects covered by it invisible to people's eyes.
Another step toward the development of an invisible cloak was achieved in 2007, under the guidance of physicist Igor Smolyaninov from the University of Maryland. They designed a device to deflect microwave radiation, at well over a long wavelength (around centimeters), around an inner copper cylinder inside the device.
It could also cloak a fairly wide region in two dimensions, even though the object it concealed remained detectable by radar and optical imaging systems. The next part of the project will relate to developing an invisibility cloak to conceal objects in three dimensions and cover different wavelengths of visible and invisible light.
What Are Future Applications of Metamaterial Cloaks?
Metamaterials promise the possibilities for future invisibility cloaks that will modulate electromagnetic waves of all different wavelengths. For such a task to succeed, though, materials are needed to have a quite specific structure that will bend light to hide things from sight without interfering with other forms of electromagnetic radiation.
Different engineers could achieve successfully cloaking via metamaterials at microwave and infrared frequencies through varying dielectric properties and thicknesses; optimizations were conducted to minimize scattering while also ensuring no transmission of wave through any of the cloaked objects using these metamaterials.
