An artistic rendering of the gain medium used to produce the terahertz frequency comb. Photo by Yan Liang/L2Molecule.com

BOSTON, May 20 (UPI) -- Researchers at MIT and Princeton have built a new laser-powered terahertz spectroscopy system capable of detecting chemicals used in explosives.

Terahertz spectroscopy is the measurement of electromagnetic radiation between the frequencies of microwaves and infrared. Scientists have long realized the radiation's potential for bomb detection, but traditional terahertz spectroscopy systems are bulky and use lots of power. They also take a long time to analyze sample materials.


The new system uses a computer chip-size quantum cascade laser, and can detect terahertz signatures in just 100 microseconds.

The device produces a laser-powered frequency comb -- a spectrum made up of a series of equally spaced frequencies. The variety of frequencies allows the device to create a unique terahertz-absorption profile in just a few measurements.

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The frequency comb is created by bouncing a laser back and forth through a gain medium composed of several hundred alternating layers of gallium arsenide and aluminum gallium arsenide. The layers are calibrated to propel the laser with just the right amount of energy to break through the gain medium.

Scientists tested their system by measuring the emissions frequency of an etalon, a double-mirrored wafer of gallium arsenide. Researchers were able to theoretically calculate the wafer's transmission spectrum prior to their experiment.

Their calculations and the experimental results matched up neatly, proving their spectroscopy system reliable.

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Cascade lasers must be cooled to very low temperatures, a task that often requires rather bulky refrigeration systems. But because the quantum cascade laser frequency comb is so small, and because the laser uses very little energy, the new spectroscopy device doesn't need a big cooling system.

"We used to consume 10 watts, but my laser turns on only 1 percent of the time, which significantly reduces the refrigeration constraints," Yang Yang, a graduate student in electrical engineering and computer science at MIT, explained in a news release. "So we can use compact-sized cooling."

Yang is the first author of a new paper on the system, published this week in the journal Optica.

"With this work, we answer the question, 'What is the real application of quantum-cascade laser frequency combs?'" said Yang. "Terahertz is such a unique region that spectroscopy is probably the best application. And QCL-based frequency combs are a great candidate for spectroscopy."