Gaithersburg, MD--Although there is no standard method for measuring the absolute output power of terahertz lasers, the National Institute of Standards and Technology (NIST) is making progress towards using dense arrays of extra-long carbon nanotubes as a promising standards material for the THz region. A vertically aligned carbon nanotube array (VANTA) coating has several desirable properties: it absorbs nearly all light of long wavelengths, and a chunk of VANTA is easily cut, lifted, and carried from a silicon surface where the tubes are grown to a laser power detector.
The research is part of NIST's effort to develop the first U.S. reference standards for calibrating lasers that operate in the terahertz range, from the far infrared at wavelengths of 100 micrometers to the edge of the microwave band at 1 millimeter. NIST project leader John Lehman says, "We have customers asking for the calibrations. This coating looks viable for terahertz laser power detectors."
The coating is very dark in color. The NIST team evaluated three VANTA samples with average lengths of 40 and 150 micrometers and 1.5 mm and found that longer tubes reflect less light. The 1.5 mm version reflects almost no light—just 1% at a wavelength of 394 micrometers. This result, the first-ever evaluation of a VANTA's reflectance at that terahertz wavelength, indicates that virtually all arriving laser light is absorbed, which would enable highly accurate measurements of laser power compared to other coatings such as gold black.
VANTAs also have desirable thermal properties. NIST researchers found that the material absorbs and releases heat quickly compared to other black coatings, which will make the detectors more responsive and quicker to produce signals. Otherwise, a coating thick enough to absorb long wavelengths of light would not efficiently transmit heat to the detector.
Currently, NIST is building a terahertz laser designed for routine measurements and a detector called a thermopile to measure the laser's power. This simple detector design produces a voltage when heat is applied to a junction of two dissimilar metals. NIST researchers used the VANTA to coat a prototype thermopile. Further research is planned to design detectors that might be used as reference standards.
SOURCE: NIST; www.nist.gov/pml/optoelectronics/laser-071911.cfm
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