Friday, May 22, 2009

"MATERIALS: "Invisibility cloaks spread out"

Invisibility cloaks, made famous by Harry Potter but now possible in the lab, promise to hide our submarines from sonar, our missiles from radar and maybe someday even our soldiers wearing Potter-like capes. Despite the obvious hype, the scientific feasibility of invisibility cloaks as spawned a new field of science called transformation optics, which seeks to harness exotic metamaterials, nanophotonics and plasmonics to build super lenses for powerful microscopes.

Transfomation optics were used recently to develop an improved invisibility cloak with a 100-fold increase in area compared to previous visible-wavelength cloaks. The new design from Purdue University uses a relatively inexpensive glass and gold waveguide that achieved a more economical design using transformation optics. An inexpensive waveguide directs light around cloaked objects (center) so that even a laser bends around it to emerge on the other side with no shadow cast. Other invisibility cloaks use optical diffraction gratings that tune into specific wavelengths with a negative index of refraction that allows the cloak to provide invisibility, but only at those wavelengths. The Purdue device was formed from two gold-coated surfaces, one a curved lens, the other a flat sheet. The result was a broadband cloak, working at a wide swath of wavelengths simultaneously, enabling it to shield an area covering the entire spectrum of visible light. The demonstration showed how a laser benda around the cloaked area, leaving no shadow, even though the area was 100 times larger than the wavelength of the laser.