Closing on reality - Invisibility cloaks for Harry Potter wannabes

The Idea Invisibility cloak is a popular topic that generated interest and curiosity among all age groups, mainly after the advent of the famous Harry Potter Series. A subject that was considered for fiction has gained wide interest because of its possible realization after scientists discovered the first invisibility cloak in 2010. It worked on three dimensional objects against light nearly visible to human eyes but the cloaked area was little, only 30 microns wide which is only one third the width of a human hair. Recently, the concept has shown steady progress with the development of an invisibility cloak that could hide three dimensional objects against red, blue and green lasers and ordinary white light. Devices like invisibility cloaks smoothly guide rays of light around objects making it proceed along their original path as if nothing was there. When light falls on something, be it an object or a person, it bends. When light falls on a cloaking device, it guides light along its usual path to make the object invisible. Potential Gainers If developed in the proper way, this technology will work wonders for the defense mechanism of a nation. The first beneficiaries of invisibility cloak will be the military force as this technology can guide quite a lot of soldiers out of the harm’s way. If used in the army combat uniform to make the person wearing it invisible, the enemy soldiers will find it quite hard to make the kill thus saving several soldiers and eventually winning the war. Invisibility cloaks can also be used in planes or to hide submarines in the sea floor. Brainchild Two teams from different parts of the world made a break-through in the field of cloaking devices. One team from the Singapore-MIT Alliance for Research and Technology (SMART Center) and the other team with researchers from the University of Birmingham, Imperial College, London and Technical University of Denmark, demonstrated their find – an invisibility cloak that can make a three dimensional object invisible in the broad range of wavelengths visible to the human eye. The scientists showed that cloaks developed by them could hide bigger objects than the cloaks of the other teams. The making • Rather than using artificially engineered materials like the ‘metamaterials’, scientists have used a naturally occurring crystal called calcite in the making of the latest invisibility cloak. The optical property of calcite known as birefringence or double-refraction splits a ray of light into two rays of polarizations traveling at different speeds and in different directions, causing the object viewed through the crystal to look doubled. • Research team from the University of Birmingham used two pieces of calcite with differing optical paths to create their invisibility cloak. Both crystals were glued together and placed on a mirror in order to perform demonstrations in air and a container of liquid. • A similar method was used by the SMART team as well. In order to create the invisibility cloak, two pieces of calcite were glued together to form a small wedge that measured 38mm x 10mm x 2mm and was placed over an intended object. When light entered the device, the intended object became invisible to the viewer from the left and right sides of the wedge, but remains visible when viewed from other angles. • Although the invisibility cloaks developed by both teams can hide only small objects such as a pin or paperclip, this discovery has opened new avenues for further research in the field of cloaking devices.

What’s new? The first cloaking devices that researchers made were effective only on microwave rays and were useless against the human eye, while the latest invisibility cloaks can hide three dimensional objects against red, blue and green lasers and ordinary white light. What’s refreshing? In the year 2010, Researchers at the University of St. Andrews developed a technology called Metaflex that can manipulate visible light and hide tiny scale objects. But the recent developments made by the research team of the University of Birmingham and other similar findings show that the naturally occurring crystal called calcite can hide three dimensional objects from human eye. The invisibility cloaks made using calcite can actually hide things like pins and paper clips, which is a breakthrough in the field of cloaking devices. Scientists take another step toward an invisibility cloak • New kind of cloaking devices that can render objects in the terahertz range have been developed by the researchers at Northwestern University. The Terahertz Invisibility Cloak is designed by Cheng Sun, Assistant Professor of Mechanical Engineering at Northwestern's McCormick School of Engineering and Applied Science. It uses microfabricated gradient-index materials to manipulate the reflection and refraction of light. • Another cloaking device, Sun’s tiny was developed using a technique called electronic transfer microstereolithography, where researchers use a data projector to project an image on a liquid polymer; then use light to transform the liquid layer into a thin solid layer. The device is prism-shaped and less than 10 millimeters long. • Each of the prism's 220 layers has tiny holes, smaller than terahertz wavelengths, which means they can vary the refraction index of the light and render invisible anything located beneath a bump on the prism's bottom surface. The light then appears to be reflected by a flat surface. • The purpose of the cloak is not to hide items but to get a better understanding of how to design materials that can manipulate light propagation. Applications Instead of using the usual meta-materials, scientists have used calcite in the invisibility cloak which has extraordinary light bending abilities. When placed over an object, it bounces light around it, hiding the object completely from human eye. It is absolutely a breakthrough in the field of cloaking devices, as the size of the cloaking area is limited only by the size of the calcite crystal. Quotes: Dr Shuang Zhang, lead investigator from the University of Birmingham’s School of Physics and Astronomy, said:

This is a huge step forward as, for the first time, the cloaking area is rendered at a size that is big enough for the observer to ‘see’ the invisible object with the naked eye. By using natural crystals for the first time, rather than artificial meta-materials, we have been able to scale up the size of the cloak and can hide larger objects, thousands of times bigger than the wavelength of the light... We believe that by using calcite, we can start to develop a cloak of significant size that will open avenues for future applications of cloaking devices.