“Cakes” of Carbon Nanotubes
Could Measure Terahertz Power
To use far-infrared and terahertz light effectively, scientists need to measure its power properly. Researchers at
the U.S. National Institute of Standards
and Technology (NIST; Boulder, Colo.,
U.S.A.) created arrays of extra-long
carbon nanotubes that absorb virtually
all radiation at these wavelengths—an
important step in designing appropriate
radiometric detectors (Appl. Opt. 50,
4099; doi: 10.1364/AO. 50.004099).
John H. Lehman and his colleagues
grew three different lengths of multiwalled carbon nanotubes, all aligned
perpendicular to their silicon substrates.
These tiny tubes were 40 µm, 150 µm
and 1. 5 mm ( 1,500 µm) tall. Multiwalled tubes have more uniform radiance properties than the single-walled
Next, the NIST scientists mea- sured how much light these arrays reflected from a 394-µm gas laser beam, close to the middle of the terahertz part
of the spectrum. As they expected, the
longer the nanotubes, the less radiation
they reflected—about 0.01 percent for
the 1.5-mm-tall array—with much higher
absorption in the far-infrared than comparable proprietary coatings. According to
the NIST group, the thermal decay time
of the nanotube arrays is also relatively
low compared to these black coatings.
On a practical level, the height of the
1.5-mm nanotube “forest” made it easy
for humans to see the arrays without a
microscope and to move them around
with such “macro” tools as a razor blade.
“Arrays of large area can be segmented
and lifted from the silicon substrate and
John H. Lehman/NIST
“Cupcakes” of vertically aligned carbon
nanotube arrays grown on silicon substrate, which appears blue in the photo.
transferred to the detector, not unlike
serving pieces of a birthday cake,” the
Lehman and his team built upon
their earlier work on shorter carbon-nanotube arrays and their absorptance of
infrared light at wavelengths of less than
14 µm (OPN 21, 8, 2010). Scientists still
need to measure other properties of the
nanotube arrays, such as the dependence
of the reflectance on polarization and
angle of incidence of the impinging far-infrared and terahertz radiation.