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Carbon nanotube forest enhances NIST detector

Photonics Spectra
Nov 2010
BOULDER, Colo. – A new ultradark detector that reflects almost no light could lead to vastly improved laser power measurement.

Thermal detectors measure laser power by converting photons into heat, and National Institute of Standards and Technology (NIST) physicists found that applying a black coating made of carbon nanotubes optimizes the performance of these detectors.

This breakthrough will improve the measurement accuracy of solar cells, satellite-based sensors and optical fiber communications, said Dr. John H. Lehman, a physicist in the optoelectronics division of NIST Boulder Laboratories. Motivated by their own coating work – spanning decades – and by the 2008 research out of Rensselaer Polytechnic Institute that claimed to have created a material so black that it has a total reflective index of 0.045 percent, the NIST group looked to harness these results for practical uses.


NIST’s new detector will improve the measurement accuracy of solar cells, satellite-based sensors and optical fiber communications, say researchers. Courtesy of John H. Lehman.


The coating reflects almost no light across the visible and part of the IR spectrum. It absorbs laser light and converts it to heat, which is registered in a pyroelectric material (in this case, lithium tantalate). In turn, the rise in temperature generates a current that is measured to determine the power of the laser. The blacker the coating, the more efficiently the detector absorbs light instead of reflecting it, and the more accurate the measurements, say Lehman and colleagues.

The coating used is a vertical forest of multiwalled carbon nanotubes, each less than 10 nm in diameter and roughly 160 μm long. The deep hollows may help trap light, and the random pattern diffuses and reflects light in various directions.


This colorized micrograph shows the world’s darkest material, a laser power detector coating designed at the National Institute of Standards and Technology (NIST). The sparse “forest” of fine carbon nanotubes represents a region approximately 25 μm across. Courtesy of Aric Sanders of NIST.


The detector evenly reflects less than 0.1 percent of light at wavelengths from deep violet at 400 nm to near-IR at 4 μm, and less than 1 percent of light in the IR spectrum from 4 to 14 μm.

Although the ideal absorbing black material would take in colors of all wavelengths and reflect none, developing such a material that is compatible with the detector platform has proved to be a challenge.

“It’s a difficult measurement to do because we’re essentially measuring what’s not there,” Lehman said. “I think that, as a laser-based measurement, we could get the total reflectance down to one part per 10,000 – that’s as close as we could get to a total reflectance of zero.”

The NIST researchers plan to extend the calibrated operating range of their device to 50- or even 100-μm wavelengths, to perhaps provide a standard for terahertz radiation power.

Spectrum Detector Inc. of Lake Oswego, Ore., has begun talks with NIST about laser sources and detectors for terahertz calibration services. The two entities already have had several successful technology transfer partnerships, including the 2009 awarding of a Phase II NIST contract to develop large-area, thin-film, domain-engineered pyroelectric detectors for the terahertz gap. That project, which also included Spectrum Detector’s research partner Srico Inc. of Columbus, Ohio, aimed at creating thin-film lithium tantalate material and manufacturing techniques to handle and fabricate a variety of single- and multiple-channel terahertz sensors. (In 2008, NIST and Spectrum Detector received a Federal Laboratory Consortium Excellence in Technology Transfer award for their collaboration in bringing precision optical trap detector technology out of the labs.)

“We seek to extend the range of absolute laser power measurements into the terahertz (free-space, IR wavelengths 10 to 50 μm and longer),” Lehman said. “A pyroelectric detector having a spectrally uniform, highly efficient absorber (such as vertically aligned carbon nanotubes) is an important technology for us to achieve this goal.

“It’s not Teflon – it’s not going to change the world, but as a detector coating, it has unprecedented promise for reproducibility and scalability.”


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