Friday, May 06, 2011

#MATERIALS: "Seeing Is Believing When Cloaks Disappear"

The world's first visible-light cloak recently converted nonbelievers by making a tiny object disappear—Harry Potter style. Transformational optics will enable a future where what you see is not always what is really there, according to the Karlsruhe Institute of Technology, which recently pulled off the first disappearance act to cloak a normal visible object.

The internal structure of the carpet cloak arrays pillars at the spacing of a photonic crystal which matches the wavelength of light to be cloaked, thereby bending it around hidden objects. Credit: KIT

Invisibility cloaks have been shown for infrared or microwave wavelengths, and Northwestern and Oklahoma State universities showed the world's first terahertz (which lies between infrared and microwave) cloaking at a recent conference. KIT, however, claims to have brought the magic of transformational optics to the normal visible-light spectrum.

By sculpting a "woodpile" of parallel pillars into a layered polymer that acts as a photonic crystal, the resulting "carpet cloak" shields objects illuminated with normal, unpolarized, red light—albeit only in an area half the size of a human hair. By making the spacings of the pillars the same as the light being transmitted through it, the material can bend light in almost any direction. Transformation optics works by continuously adjusting this spacing to guide light around objects, enabling invisibility cloaks.

The cloak works by adjusting the local phase velocity of light with the pillars’ spacing, but the beauty of the process is that it can be accomplished with direct-write lasers. The process works by first laying down a pattern of nanoscale-spaced pillars by etching a single layer of polymer. Then a filler is spread into the etched-out areas, a new layer of polymer is laid down, and the process repeats. After the material has been built up to whatever size is needed for the cloaked area, the filler is dissolved leaving just the woodpile of light-bending pillars. Even though the cloak only worked for red light (700-nanometer wavelength), KIT nevertheless claimed it was the first demonstration for normal unpolarized visible light.

KIT's lead researcher, Joachim Fischer, explained at CLEO that the key to achieving the nanoscale spacings necessary to cloak visible light was adapting diffraction-unlimited microscopy techniques to the laser direct-writing process. Those adjustments allowed the researchers to dramatically increase their etching resolution to the nanometer scale, thus achieving a visible light cloak where others were at longer wavelengths of infrared (micrometer), terahertz (millimeter) or microwaves (centimeter).

Next, KIT is aiming to half the spacing between its pillars again, to shrink their size enough that the carpet cloak will work at all visible wavelengths—not just red—finally realizing the Harry Potter dream. The researchers are also looking for ways to increase the area concealed.

Besides invisibility cloaks, the researchers believe their technique will enable flat, aberration-free lenses that can be integrated onto future optical microchips. Fischer's team is also working on what KIT calls optical "black holes" that could improve the efficiency of solar cells by letting them harvest a broader swath of wavelengths than today's solar cells, which must be tuned to a region's typical spectrum.
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