“Nanotubes on a Chip” Modernize Optical Power Measurements

by Rachel Nuwer

Materials Research Society | Published: 01 February 2013

CNCR-220 The circular patch of carbon nanotubes on a pink silicon backing is one component of NIST’s new cryogenic radiometer, shown with a quarter for scale. Gold coating and metal wiring has yet to be added to the chip. The radiometer will simplify and lower the cost of disseminating measurements of laser power .Image credit: Tomlin/NIST. Click image to enlarge. 

A newly designed penny-sized chip may revolutionize our ability to measure absolute laser power by improving upon the precision, scalability, versatility and efficiency of current models. The chip, a carbon nanotube cryogenic radiometer, relies upon a forest of tiny, ultradark carbon nanotubes as its key to performance. The new radiometer, described in Optics Letters, could improve laser power and energy measurements for sectors ranging from communications to space to biochemistry.

“A conventional radiometer is built by hand; it’s big and complicated,” says John Lehman, a physicist at the National Institute of Standards and Technology and the study’s co-author. “We’ve reduced all of that into a single chip—that’s the cool thing.” 

To quantify laser power, scientists must convert it into electrical units. Radiometers work by absorbing energy from the laser, converting it to heat and measuring the equivalent electrical power needed to cause the same temperature increase. To perform this function, conventional cryogenic radiometers require a mishmash of materials that must be painstakingly hand-assembled. About the size of a coffee can, these older models often involve manual fine-tuning and can only work as single units. 

Measuring absolute optical power, whether 100 kilowatts or 1 picowatt, requires three basic components: some sort of absorber, heater and sensor. Standard radiometers derive each of these measurements from discreet components, but the nanotubes accomplish all three requirements in one tidy assembly. 

“The most exciting thing is that resistance as a function of temperature changes sufficiently to make a thermistor,” Lehman says. “That means we can do temperature sensing, optical absorption and heating all on the same chip with the same nanotubes.” 

To create the carbon nanotube cryogenic radiometer, Lehman and co-author Nathan Tomlin grow vertically aligned multiwall carbon nanotube arrays at the end of the chip’s micromachined silicon, which acts as a weak thermal link that maximizes the temperature change for a given optical input. They attach electrodes on opposite sides of the nanotube patch. As they cool the chip, experiments showed, the resistance changes—the colder the temperature, the less the conductivity. When they add light from a laser to the nanotube forest, they can measure the change it causes in resistance. Lehman and Tomlin verified that their carbon nanotube cryogenic radiometer responds to both electrical and optical power inputs equivalently at 3.9 K and with a telecom wavelength of 1550 nanometers. 

In contrast to the older versions, the new chip radiometer is simpler, easily reproduced and orders of magnitude faster than the original radiometers pioneered at the National Physical Laboratory in the U.K. In addition, it can operate over a wide range of wavelengths, meaning it may potentially serve a number of purposes that conventional radiometers cannot. Many chips can also be manufactured on a single wafer, so researchers can experiment with different designs that optimize an array of functions. For now, telecommunications networks are likely to benefit first, though military uses, night vision and information technology could eventually be equally well served. “With collaborators, we hope we can eventually reestablish how people define their primary standards for optical power,” Lehman says. 

For now, the researchers plan to experiment with higher temperatures, perhaps designing a chip that can operate at 100 K, for example, as compared to its current operational standard of 4 K. In the coming months, they also plan to build a model that can be used in NIST’s low background infrared radiometer facility. 

NIST is currently considering a provisional patent for the chip-scale radiometer. In the meantime, Lehman and Tomlin look forward to collaborating with other National Metrology Institutes—such as the National Physical Laboratory and the Physikalisch Technische Bundesanstalt in Germany—in efforts to improve the product, whether from an applications or design point of view. 

“We’ll also continue to work with nanotube growers to make improvements,” Lehman said. “I’m just fascinated by nanotubes—thermal, optical, electrical properties, you name it.” 

Read the abstract in Optics Letters  here.



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