“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 |
Terahertz radiation can penetrate numerous
materials—plastic, clothing, paper, and some biological tissues—making it an
attractive candidate for applications such as concealed weapons detection,
package inspection, and imaging skin tumors. However, to date there is no
standard method for measuring the absolute output power of terahertz lasers,
one source of this type of radiation. Now, researchers at NIST have found that
dense arrays of extra-long carbon nanotubes absorb nearly all light of long
wavelengths, and thus are promising coatings for prototype detectors intended
to measure terahertz laser power.
The research is part of NIST’s effort to develop the first
reference standards for calibrating lasers that operate in the terahertz range,
from the far infrared at wavelengths of 100 micrometers to the edge of the
microwave band at 1 mm.
“There is no measurement traceability for absolute
power for terahertz laser sources,” NIST project leader John Lehman says.
“We have customers asking for the calibrations. This coating looks viable
for terahertz laser power detectors.”
The coating, called a VANTA (vertically aligned carbon
nanotube array), has several desirable properties. Most obviously, it is easy
to handle. The nanotubes are tens of micrometers to over a millimeter long, so
a dense layer is visible without a microscope. A chunk of VANTA can be cut,
lifted, and carried like a piece of cake, making 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 micrometers and
1.5 mm and found that longer tubes reflect less light. The 1.5 mm version
reflects almost no light—just 1% at a wavelength of 394 micrometers. 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. NIST
researchers found that the material absorbs and releases heat quickly compared
to 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.
