A research team from MIT have developed a form of terahertz spectroscopy that could be key to non-invasive security screening. The group has been working in the terahertz region for more than 20 years, and after demonstrating THz quantum cascade laser (QCL) frequency combs in 2014 (

Figure 1. (a) Multiheterodyne spectroscopy (two frequency combs with somewhat different repetition rates). (b) Experimental setup (inset photo: laser frequency combs on copper mount – both silicon lens-coupled). (c) Spectrum of one device under comb operation.

Figure 2. Broadband THz spectroscopy facilitates detection of different explosives, which absorb strongly in the THz range, providing a THz fingerprint.
Yang says that terahertz is a unique field of great importance for spectroscopy, as it is a range where lots of molecules have fingerprints. “By conducting broadband spectroscopy at terahertz, one can not only elucidate those molecules but also identify their concentrations,” he says. “Frequency combs based on terahertz QCLs feature broadband coverage and high output powers in a compact package, making them an attractive option for broadband spectroscopy.”
According to Yang, multiheterodyne spectroscopy based on two frequency combs (
With its potential for detecting explosives and noxious gases (as well as screening mail and personnel), the new technique could play an important role in homeland security. “Explosive materials absorb THz light strongly at certain terahertz frequencies but not at others, and this ‘terahertz fingerprint’ can be used to identify an explosive, distinguishing it from clothing or other inert materials,” Yang says. “Because clothing is transparent at terahertz frequencies, the THz light can pass through several layers, including common garments and shoes, providing a less invasive screening method.” Manufacturer TeraView claims the technology is capable of detecting different types of plastic explosives, including PETN (pentaerythritol tetranitrate) – a material used in several terrorist bombings but notoriously difficult to detect.
- Y Yang et al., “Terahertz multiheterodyne spectroscopy using laser frequency combs”, Optica, 3, 499-502 (2016).
- S Schiller, “Spectrometry with frequency combs”, Optics Letters, 27, 766-158 (2002).
- F Keilmann et al, “Time-domain mid-infrared frequency-comb spectrometer”, Optics Letters, 29, 1542-1544 (2004).
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