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May 22, 2016

MIT researchers present new terahertz spectroscopy to detect explosives

Researchers from the Massachusetts Institute of Technology's (MIT's) Research Laboratory of Electronics have introduced a terahertz spectroscopy system that uses a quantum cascade laser to detect explosives.

By Lopamudra Roy

Researchers from the Massachusetts Institute of Technology’s (MIT’s) Research Laboratory of Electronics have introduced a terahertz spectroscopy system that uses a quantum cascade laser to detect explosives.

A quantum cascade laser is a source of terahertz radiation that is as small as a computer chip.

Terahertz spectroscopy uses electromagnetic radiation to extract the spectroscopic fingerprints of different materials, including the chemicals used in dangerous explosives.

The new system can derive the spectroscopic signature of any material in 100µs.

"QCL-based frequency combs are a great candidate for spectroscopy."

It emits terahertz radiation in a ‘frequency comb,’ which refers to several frequencies that are evenly spaced.

MIT Electrical Engineering and Computer Science graduate student and first author on the new paper Yang Yang said: "With this work, we answer the question, ‘What is the real application of quantum-cascade laser frequency combs?’

"Terahertz is such a unique region that spectroscopy is probably the best application. And QCL-based frequency combs are a great candidate for spectroscopy."

Terahertz radiation is absorbed differently by materials, thereby providing each one with a specific terahertz-absorption profile.

If the frequencies in a frequency comb are evenly spaced out, a material’s absorption fingerprint can be reconstructed from a few measurements, without the need for any mechanical adjustments.

In a bid to even out the lasers’ frequencies, the researchers use a gain medium that is though oddly shaped, have regular, symmetrical indentations in the sides that can change the medium’s refractive index, and thereby bring back uniformity to the distribution of the emitted frequencies.

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