Nano ‘cupcakes’ may open door to new applications of THz radiation

Nano ‘cupcakes’ may open door to new applications of THz radiation

Researchers at the National Institute of Standards and Technology have found that dense arrays of extra-long carbon nanotubes (vertically aligned carbon nanotube arrays or VANTAs) can absorb nearly all light of long wavelengths, and thus are promising coatings for prototype detectors intended to measure terahertz laser power. Because THz waves can penetrate materials such as plastic, cloth, paper, and some biological tissue, terahertz lasers could suit applications such as concealed weapons detection, package inspection, and imaging skin tumors.

The problem with using such lasers today is that, “There is no measurement traceability for absolute power for terahertz laser sources,” NIST project leader John Lehman explains. “This coating looks viable for terahertz laser power detectors.” A VANTA has several desirable properties, among them handling ease, high light absorption, and thermal performance.

Because the nanotubes in the coating can measure over a millimeter long, not only is a dense layer visible without a microscope, but a chunk of VANTA can be cut, lifted, and carried like a piece of a cupcake (see photo ). This makes it easy to transfer the material from the silicon surface on which the tubes are grown to a laser power detector.

A chunk of VANTA, grown on silicon and looking like like cupcakes on a tray, can be sliced from the silicon with a razor blade and, using the blade as a spatula, easily “served” to the top of a laser power detector. The very dark nanotube coating absorbs terahertz laser light. (Photo credit: John Lehman/NIST.)

Since it is very dark, the coating absorbs laser light well. By evaluating three VANTA samples with nanotubes of varying average length 40 μm, 150 μm, and 1.5 mm the NIST team found that longer tubes reflect less light. In fact, the 1.5-mm version reflected almost no light — just 1% for a 394-μm laser. Since virtually all arriving laser light is absorbed, a detector using the material would provide highly accurate measurements of laser power. While it is clear that the 1.5-mm VANTA absorbs more light than comparable coatings, such as gold black, more work is needed to calculate uncertainties and determine the effects of such factors as light angle.

The NIST researchers also found that VANTA absorbs and releases heat quickly compared to other black coatings. This would make detectors using the material more responsive and quicker to produce signals. Otherwise, a coating thick enough to absorb long wavelengths would not transmit heat to the detector efficiently.

Researchers are using the VANTA to coat a prototype thermopile that will be used in a measurement system NIST is currently building. The system will consist of a terahertz laser designed for routine measurements and the thermopile/detector to measure the laser’s power, producing a voltage proportional to the energy when the thermopile is heated by the laser. Further research is planned to design detectors that might be used as reference standards. For further information, e-mail John Lehman at
or call 303-497-3654.

Richard Comerford