Friday, May 6, 2011

Latest news from CLEO 2011 on terahertz “invisibility cloak"

Scatterings image
Perspective view of the terahertz cloaking structure made by researchers from Northwestern University and Oklahoma State University.

Patricia Daukantas
Scientists from two U.S. universities have created a three-dimensional terahertz-spectrum “invisibility cloak” that hides a bump from a wide range of radiation frequencies.
The six-member group, led by Cheng Sun of Northwestern University, reported its results at the CLEO:2011 conference in Baltimore, Md. (U.S.A.). Three of the researchers are from Oklahoma State University.
The terahertz cloak is reminiscent of the “carpet bump” cloak devised in the near-infrared region by a German team (see OPN, June 2010, p. 7). From a distance, the device looks almost like a polymer resin near-cube cut in half diagonally.
Sun and his colleagues used a technique called “projection microstereolithography” to fabricate a varying array of holes through the resin block. The size of the holes changes around the negative “bump” on the bottom side of the block, so that the refractive index of the cloak increases in the vicinity of the bump.
The researchers coated the side containing the bump with a 200-nm-thick layer of gold to provide a reflecting surface. Then they shone broadband terahertz radiation into the block at an incident angle of 45 degrees with respect to the reflecting surface. Underneath the bump, they placed a block of pressed lactose powder, which has a strong absorption feature at 0.53 THz. They performed similar tests with a flat reflecting surface, a chunk of lactose covered by a non-cloaking reflecting surface and a bare chunk of lactose.
Imaging of the reflected terahertz light showed that the cloaked block of lactose showed the same spectroscopic signature as the flat reflecting surface—no scattering or absorption of the rays. According to the researchers, the result demonstrates that the cloak has hidden both the geometrical and spectroscopic signatures of the concealed material.
Previous cloaking studies have focused on the microwave and optical regions of the spectrum, leaving a gap at terahertz frequencies—a spectroscopically interesting band that researchers have been studying intensely for potential security and industrial applications.

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