Nanotubes on a chip promise to ease optical power measurements

A team of researchers at the National Institute of Standards and Technology has recently announced a chip-scale instrument made of carbon nanotubes that promises to simplify laser power measurements such as the light signals that are transmitted by optical fibers in telecom networks.The device — a mini version of a cryogenic radiometer — is a silicon chip topped with circulars mats of carbon nanotubes standing on end (see figure). It is based on NIST's past work to make a highly efficient optical power detector.

  
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. Photo credit: Tomlin/NIST
“This is our play for leadership in laser power measurements,” says John Lehman, the project leader of this research. “This is arguably the coolest thing we've done with carbon nanotubes. They're not just black, but they also have the temperature properties needed to make components like electrical heaters truly multifunctional.”

Laser power is measured by tracing it to fundamental electrical units. Radiometers absorb energy from light and convert it to heat. Then the electrical power needed to cause the same temperature increase is measured. The team demonstrated that the mini-radiometer accurately measures both laser power (brought to it by an optical fiber) and the equivalent electrical power within the limitations of the imperfect experimental setup. The testing was done with a temperature of 3.9 K, using light at the telecom wavelength of 1,550 nm.

The thin “vertically aligned nanotube arrays,” called VANTAs, uniformly absorb light over a broad range of wavelengths, and their electrical resistance depends on temperature. The versatile nanotubes perform three different functions in the radiometer: One VANTA mat serves as both a light absorber and an electrical heater, and a second VANTA mat serves as a thermistor. The VANTA mats are grown on the micro-machined silicon chip, an instrument design that is easy to modify and duplicate. In this application, the individual nanotubes are about 10 nm in diameter and 150 μm long.

Traditionally, cryogenic radiometer use many more materials and are also more difficult to make. They can be hand assembled using a cavity painted with carbon as the light absorber, an electrical wire as the heater, and a semiconductor as the thermistor. They also need to be modeled and characterized extensively to adjust their sensitivity. The NIST device is easily patterened in the silicon.

NIST is working on getting a patent on this mini-radiometer. They also expect to make some minor changes to improve temperature stability. In the future, they hope to extend the laser power range into the far infrared. For further information, contact Laura Ost at laura.ost@nist.gov or call 303-497-4880.