THz Laser Power Detector Is a Piece of Cake

Dense arrays of extra-long carbon nanotubes absorb nearly all light of long wavelengths and are promising coatings for prototype detectors intended to measure terahertz laser power. This type of radiation can penetrate numerous materials, making it ideal for applications including weapons detection, package inspection and the imaging of skin tumors.

The research is part of the National Institute of Standards and Technology's (NIST) effort to develop the first reference standards for calibrating lasers that operate in the terahertz range — from the far-infrared (100 µm) to the edge of the microwave band (1 mm).

"There is no measurement traceability for absolute power for terahertz laser sources," NIST project leader John Lehman said. "We have customers asking for the calibrations. This coating looks viable for terahertz laser power detectors."

"Cupcakes" of vertically aligned carbon nanotube arrays (VANTAs) grown on silicon, which appears blue in the photo. A chunk of vANTA can be sliced from the silicon with a razor blade and, using the blade as a spatula, easily moved to the top of a laser power detector. The very dark nanotube coating absorbs terahertz laser light. (Image: Lehman/NIST)

The coating, called a VANTA (vertically aligned carbon nanotube array), consists of nanotubes that are tens of microns to more than 1 mm long, so a dense layer is visible to the naked eye and can be lifted and carried like a piece of cake, which makes it easy to transfer from a silicon surface where the tubes are grown to a laser power detector.

Most importantly, the coating is very dark. The NIST team evaluated three VANTA samples with average lengths of 40 and 150 µm and 1.5 mm and found that longer tubes reflect less light. The 1.5-mm version reflects almost no light — just 1 percent at a wavelength of 394 µm. This result, the first-ever evaluation of a VANTA's reflectance at that terahertz wavelength, indicates that virtually all arriving laser light is absorbed, which would enable highly accurate measurements of laser power.

The 1.5-mm VANTA absorbs more light than comparable coatings such as gold black, but more work is needed to calculate uncertainties and determine effects of factors such as light angle. The project extends NIST's long history in laser power measurements and Lehman's recent advances in ultradark nanotube coatings.

VANTAs also have desirable thermal properties — the material absorbs and releases heat quickly compared with other black coatings, which will make the detectors more responsive and quicker to produce signals. Otherwise, a coating thick enough to absorb long wavelengths of light would not efficiently transmit heat to the detector.

In developing the capability for terahertz laser radiometry, NIST is building a terahertz laser designed for routine measurements and a detector called a thermopile to measure the laser's power. This simple detector design produces a voltage when heat is applied to a junction of two dissimilar metals. NIST researchers used the VANTA to coat a prototype thermopile. Further research is planned to design detectors that might be used as reference standards.

For more information, visit: www.nist.gov