Monday, November 30, 2015
SpectroscopyNOW-Last Month's Most Accessed Feature: Invisibility cloak: Hiding the microscopic
A microscopic invisibility cloak based on brick-like blocks of gold nanoantennae could pave the way to a flexible device for making an object invisible in visible light.
Invisibility cloaks have been a staple of science fiction and fantasy for decade, who, after all, has not mused on what they might get up to if they could make themselves invisible? For several years camouflage covers that resemble the background environment to help hide a person were the best option, leafy greens allowing a soldier to crawl through undergrowth undetected perhaps, except when night-vision or thermal imaging is in place. Aircraft can be made "stealth" to hide them from the reflective waves of a radar tower but they can still be spotted as they fly by at altitude with a decent set of binoculars and a good eye.
In recent years, however, meta materials have emerged that might take invisibility to the next level. There have been demonstrations of infrared and other forms of invisibility but now, researchers at the US Department of Energy's Lawrence Berkeley National Laboratory and the University of California Berkeley have come up with a microscopic invisibility cloak that can hide a three dimensional object from the microscope's viewfinder. The team suggest that the principle should work on the macroscopic level to once it is scaled up.
The team has worked with gold nanoantennae to fabricate a flexible skin a mere 80 nanometres in thickness that can be wrapped around a three-dimensional object of arbitrary shape the size of a clump of biological cells. Adopting the underlying lumps and bumps of the object. The meta-engineered surface of the skin cloak allows it to re-route reflected light waves so impinge on it so that the object's lumps and bumps are rendered invisible to optical detection when the cloak is activated.
"This is the first time a 3D object of arbitrary shape has been cloaked from visible light," explains meta materials expert Xiang Zhang, director of Berkeley Lab's Materials Sciences Division. "Our ultra-thin cloak now looks like a coat. It is easy to design and implement, and is potentially scalable for hiding macroscopic objects," he adds.
Zhang, working with Xingjie Ni, Zi Jing Wong, Michael Mrejen and Yuan Wang, point out that that it is the scattering of electromagnetic radiation, whether that is visible light, infrared, X-ray, or another band in the spectrum, and its interaction with matter that enables us to detect and observe objects. The researchers explains that the rules that govern these interactions in natural materials can be circumvented using meta materials whose optical properties arise from their physical structure rather than their chemical composition. The surface of a butterfly's wing has no coloured pigment, it's surface texture interacts with visible light to cause iridescence that gives rise to its beautiful colours and patterns and so might be thought of as a natural meta material.
For the past ten years, Zhang and his research group have been pushing the boundaries of how light interacts with fabricated meta materials. They have managed to curve the path of light or bend it backwards, creating negative refractive index meta materials, a phenomenon not seen before in nature, and to render objects optically undetectable. In the past, their meta material-based optical carpet cloaks were bulky and hard to scale-up, and entailed a phase difference between the cloaked region and the surrounding background that made the cloak itself detectable, although what it was concealing could not be detected, which defeats the object of being invisible one has to say.
"Creating a carpet cloak that works in air was so difficult we had to embed it in a dielectric prism that introduced an additional phase in the reflected light, which made the cloak visible by phase-sensitive detection," explains team member and co-lead author Ni, who has recently moved to Pennsylvania State University. "Recent developments in meta surfaces, however, allow us to manipulate the phase of a propagating wave directly through the use of sub-wavelength sized elements that locally tailor the electromagnetic response at the nanoscale, a response that is accompanied by dramatic light confinement."
In their experiments, the team shone red light struck on a sample object with an area of about 1300 square micrometres. When it was sheathed in the gold nanoantennae skin cloak, light reflected from the object's surface produced the same effect as if the light were simply reflecting from a plane mirror. The 3D object cloaked in this way is thus invisible even by phase-sensitive detection. The team points out that their cloaking device can be turned on or off simply by switching the polarization of the nanoantennae.
"A phase shift provided by each individual nanoantenna fully restores both the wavefront and the phase of the scattered light so that the object remains perfectly hidden," explains Wong. Ironically, this ability to manipulate the interactions of light and a meta material for invisibility hints at a future of high resolution optical microscopes and superfast optical computers and as a component of a future 3D display technology. Conversely, such a device could be used for security through obscurity applications allowing microscopic components to be hidden for privacy or security applications. purposes.