Using terahertz laser techniques to detect bombs on-the-go

When it comes to bomb-detection, one usable method has been terahertz spectroscopy — a technique that uses the band of electromagnetic radiation between microwaves and infrared light to determine if there are chemicals, like the ones used in explosives, present.

Luggage awaiting loading at the airport. (Image Credit: Billy Hathorn)
Luggage awaiting loading at the airport. (Image Credit: Billy Hathorn)

While the method is solid from a scientific and engineering perspective, for traditional terahertz spectroscopy to be effective, it requires a radiation source that’s heavy and about the size of a large suitcase. In addition, it can take anywhere from 15 to 30 minutes to analyze a single sample, which means its not the most efficient option.

Now, researchers from MIT’s Research Laboratory of Electronics have developed a new kind of terahertz spectroscopy system that uses a quantum cascade laser, which is a good source of terahertz radiation — and is only the size of a computer chip. The system can determine if a material is dangerous in just 100 microseconds.

How does it work?

The device emits terahertz radiation in what’s known as a “frequency comb,”which means that the range of frequencies are evenly spaced.

Different materials absorb different frequencies of terahertz radiation to different degrees, so each material has a unique profile. giving each of them a unique terahertz-absorption profile. Instead of measuring each material’s response to the frequencies emitted (which involved a lot of manual, time-consuming re-adjustments) the team was able to use math to reconstruct a material’s absorption fingerprint from just a few measurements.

 

In order for this method to prove effective, the spacing in the comb needs to be perfectly even, which the team achieved by making adjustments to the space where the lasers bounce electromagnetic radiation back and forth.

“With this work, we answer the question, ‘What is the real application of quantum-cascade laser frequency combs?’” said Yang Yang, a graduate student in electrical engineering and computer science and first author on the new paper. “Terahertz is such a unique region that spectroscopy is probably the best application. And QCL-based frequency combs are a great candidate for spectroscopy.”

The researchers tested their new system to measure the spectral signature an optical device called an etalon, which is made from a wafer of gallium arsenide, a material whose spectral properties could be calculated in advance, just so they had a good comparison comparison. The system’s measurements matched their previous calculations for the etalon’s terahertz-transmission profile, which suggests that it could be useful for detecting chemicals.

 

“This paper is a breakthrough, because these kinds of sources were not available in terahertz,” said Gerard Wysocki, an assistant professor of electrical engineering at Princeton University. “Qing Hu is the first to actually present terahertz frequency combs that are semiconductor devices, all integrated, which promise very compact broadband terahertz spectrometers.”

Story via MIT.

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