1.5-mm-thick layer of aligned nanotubes is a 99% efficient absorber for terahertz photodetectors
Boulder, CO--Researchers at the National Institute of Standards and Technology (NIST) have created macroscopic disks of densely packed carbon nanotubes that look like little hockey pucks but are efficient absorbers of terahertz radiation.1 They found that if the nanotubes are long enough, the pucks absorb all but 1% of the terahertz radiation hitting them. The 1.5-mm-thick nanotube layers are potentially excellent coatings for terahertz photodetectors.
The researchers created pucks with thicknesses of 40 microns, 150 microns, and 1.5 mm and found that the reflection of radiation at a 0.76 THz wavelength off the pucks was 38%, 23%, and 1% respectively. The responsivity of a nanotube-puck-coated thermopile detector due to optical heating by terahertz radiation was equal to a control run of electrical heating within the uncertainty of the measurement; the thermal decay period was 500 ms. The nanotube layer is, after fabrication, easily transferable to a detector surface of choice.
The research is part of NIST's effort to develop the first reference standards for calibrating lasers that operate in the terahertz range. "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 vertically aligned carbon-nanotube array (VANTA), 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.
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.
REFERENCE:
1. John H. Lehman et al., Applied Optics, Vol. 50, No. 21, p. 4099 (2011); doi:10.1364/AO.50.004099.
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John Wallace | Senior Technical Editor (1998-2022)
John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.