Harnessing darkness for practical use, researchers at the National
Institute of Standards and Technology (NIST) have developed a laser power
detector coated with the world's darkest material-a forest of carbon nanotubes
that reflects almost no light across the visible and part of the infrared spectrum.
NIST will use the new ultra-dark detector, described in a new paper in Nano
Letters,* to make precision laser power measurements for advanced technologies
such as optical communications, laser-based manufacturing, solar energy conversion,
and industrial and satellite-borne sensors.
Inspired by a 2008 paper by Rensselaer Polytechnic Institute (RPI) on "the
darkest man-made material ever,"** the NIST team used a sparse array of
fine nanotubes as a coating for a thermal detector, a device used to measure
laser power. A co-author at Stony Brook University in New York grew the nanotube
coating. The coating absorbs laser light and converts it to heat, which is registered
in pyroelectric material (lithium tantalate in this case). The rise in temperature
generates a current, which is measured to determine the power of the laser.
The blacker the coating, the more efficiently it absorbs light instead of reflecting
it, and the more accurate the measurements.
The new NIST detector uniformly reflects less than 0.1 percent of light at
wavelengths from deep violet at 400 nanometers (nm) to near infrared at 4 micrometers
(µm) and less than 1 percent of light in the infrared spectrum from 4
to 14 µm. The results are similar to those reported for the RPI material
and in a 2009 paper by a Japanese group. The NIST work is unique in that the
nanotubes were grown on pyroelectric material, whereas the other groups grew
them on silicon. NIST researchers plan to extend the calibrated operating range
of their device to 50 or even 100 micrometer wavelengths, to perhaps provide
a standard for terahertz radiation power.
NIST previously made detector coatings from a variety of materials, including
flat nanotube mats. The new coating is a vertical forest of multiwalled nanotubes,
each less than 10 nanometers in diameter and about 160 micrometers long. The
deep hollows may help trap light, and the random pattern diffuses any reflected
light in various directions. Measuring how much light was reflected across a
broad spectrum was technically demanding; the NIST team spent hundreds of hours
using five different methods to measure the vanishingly low reflectance with
adequate precision. Three of the five methods involved comparisons of the nanotube-coated
detector to a calibrated standard.
Carbon nanotubes offer ideal properties for thermal detector coatings, in part
because they are efficient heat conductors. Nickel phosphorous, for example,
reflects less light at some wavelengths, but does not conduct heat as well.
The new carbon nanotube materials also are darker than NIST's various Standard
Reference Materials for black color developed years ago to calibrate instruments.
* J. Lehman, A. Sanders, L. Hanssen, B. Wilthan and J. Zeng. 2010. A Very Black
Infrared Detector from Vertically Aligned Carbon Nanotubes and Electric-field
Poling of Lithium Tantalate. Nano Letters. Posted online Aug. 3, 2010.
** Z.P. Yang, L. Ci, J.A. Bur, S.Y. Lin and P.M. Ajayan. Experimental observation
of an extremely dark material made by a low-density nanotube array. Nano Letters.
Vol. 8, No. 2, 446-451.