Terahertz polarizer nears perfection

While Hauge and Pint were developing their nanotube arrays, Kono and his team were thinking about terahertz. Four years ago, they came across a semiconducting material, indium antimonide, that would stop or pass terahertz waves, but only in a strong magnetic field and at very low temperatures.

At about the same time, Kono’s lab began working with carbon nanotube arrays transferred onto a sapphire substrate by Pint and Hauge. Those aligned arrays — think of a field of wheat run over by a steamroller — turned out to be very effective at filtering terahertz waves, as Kono and his team reported in a 2009 paper.

“When the polarization of the terahertz wave was perpendicular to the nanotubes, there was absolutely no attenuation,” Kono recalled. “But when the polarization was parallel to the nanotubes, the thickness was not enough to completely kill the transmission, which was still at 30-50 percent.”

The answer was clear: Make the polarizer thicker. The current polarizer has three decks of aligned nanotubes on sapphire, enough to effectively absorb all of the incident terahertz radiation. “Our method is unique, and it’s simple,” he said.

Kono sees use for the device beyond spectroscopy by manipulating it with an electric field, but that will only become possible when all of the nanotubes in an array are of a semiconducting type. As they’re made now, batches of nanotubes are a random mix of semiconductors and metallics; recent work by Erik Hároz, a graduate student in Kono’s lab, detailed the reasons that nanotubes separated through ultracentrifugation have type-dependent colors. But finding a way to grow specific types of nanotubes is the focus of a great deal of research at Rice and elsewhere.

Co-authors are former Rice postdoctoral researcher Takashi Arikawa and research associate Iwao Kawayama and Professor Masayoshi Tonouchi of the Institute of Laser Engineering at Osaka University, Japan.

The Department of Energy, the National Science Foundation and the Robert A. Welch Foundation supported the research.

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Read the abstract at http://pubs.acs.org/doi/abs/10.1021/nl203783q

Related links:

“Nanotubes Take Flight”: http://www.media.rice.edu/media/NewsBot.asp?MODE=VIEW&ID=12868
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“A see-through surprise”: http://www.media.rice.edu/media/NewsBot.asp?MODE=VIEW&ID=13458&SnID=840080198
“Carbon Nanotube Terahertz Polarizer”: http://pubs.acs.org/doi/abs/10.1021/nl900815s
“Nano parfait a treat for scientists”: http://www.media.rice.edu/media/NewsBot.asp?MODE=VIEW&ID=14248
“Scientists solve mystery of colorful armchair nanotubes”: http://stage.media.rice.edu/media/NewsBot.asp?MODE=VIEW&ID=16606&SnID=1904501430

Image for download:

http://media.rice.edu/images/media/NewsRels/0201_%20terahertz.jpg

CAPTION:

A triple layer of carbon nanotube arrays on a sapphire base are the basis for a new type of terahertz polarizer invented at Rice University. The polarizer could lead to new security and communication devices, sensors and non-invasive medical imaging systems. (Credit: Lei Ren/Rice University)

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