Terahertz-based system could detect explosives in microseconds

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Researchers from MIT’s Research Laboratory of Electronics have developed a terahertz spectroscopy system that can extract the spectroscopic ‘fingerprints’ of materials in a matter of microseconds. The system, described in the latest issue of journal Optica, could be used for identifying explosives much more rapidly than existing methods.

Terahertz spectroscopy uses the band of electromagnetic radiation between microwaves and infrared light. Traditional terahertz spectroscopy requires a radiation source that’s heavy and about the size of a large suitcase, and it takes 15 to 30 minutes to analyse a sample.

The new terahertz spectroscopy system developed at MIT’s Research Laboratory of Electronics can extract a material’s spectroscopic signature in just 100 microseconds. In addition, the device's measurements have been found to be a good fit for the etalon’s terahertz-transmission profile, suggesting that it could be useful for detecting chemicals, such as those in explosives.

The device uses a quantum cascade laser - a source of terahertz radiation that’s the size of a computer chip. It is so efficient because it emits terahertz radiation in what’s known as a ‘frequency comb’, meaning a range of frequencies that are perfectly evenly spaced. ‘Terahertz is such a unique region that spectroscopy is probably the best application. And QCL-based frequency combs are a great candidate for spectroscopy.’ said Yang Yang, a graduate student in electrical engineering and computer science and first author on the new paper.

Different materials absorb different frequencies of terahertz radiation to different degrees, giving each of them a unique terahertz-absorption profile. Traditionally, however, terahertz spectroscopy has required measuring a material’s response to each frequency separately, a process that involves mechanically readjusting the spectroscopic apparatus. That’s why the method has been so time consuming.

Because the frequencies in a frequency comb are evenly spaced, however, it’s possible to mathematically reconstruct a material’s absorption fingerprint from just a few measurements, without any mechanical adjustments. This drastically reduces the time to analyse a sample, making the system an ideal security tool for detecting explosives, where time is a crucial element.