Nonlinear Optics Group - University of St Andrews

The novel source of Terahertz waves that we have developed has the potential for identifying and characterising illegal drugs that are, for example, concealed about the person, have been sent through the mail, or have contaminated surfaces. The lack of non-destructive and non-invasive inspection techniques for the routine detection and identification of illicit drugs, including those cases where they are concealed in some way, is a major current concern in forensic science. Although x-ray scanners, canine detection, and external swiping with a trace detection system are currently in use, the former approach is invasive and unable to identify the type of drug, while the latter two are ineffective in the absence of detectable traces outside the enclosure containing the drugs. The use of THz waves as a means for drug detection and recognition has two particular advantages. Firstly, they are able to penetrate a wide range of materials such as paper, wood, plastic, fabrics, ceramics, bone, and tissue (both human and animal), and, importantly, in a non-invasive way unlike the case with x-rays. Secondly, they are capable of discriminating between different heavy organic molecules (such as drugs) through frequency-dependent changes in their absorption by these molecules that differ from one type of molecule to another . This combination makes it possible both to associate a spectral fingerprint with a particular drug so enabling it to be identified, but importantly to do so even when that drug is concealed within a container made of materials of the type described above.

Our recently-developed source of THz radiation based on parametric generation is compact, efficient, operates at room temperature, and delivers from a single device narrow-linewidth THz radiation that can be widely tuned to target particular spectral features, important in the context of spectral fingerprinting. Earlier sources based on parametric generation have been bulky and not easily transportable, hence making their in-field usage difficult. Based on over 15 years of experience in developing parametric generators for the ultraviolet, visible and infrared spectral ranges, we have exploited novel ideas, now patented, that have allowed us to build a compact, efficient and readily-transportable parametric generator of THz radiation (see THz device page)

Figure 1 | Simple THz spectrometer.

A simple THz spectrometer has been constructed, as illustrated in figure 1. The THz beam from the nonlinear crystal (NL) has a low divergence in the plane of the figure but a high divergence in the orthogonal plane. The beam is therefore first collimated in the high divergence plane with the cylindrical lens, C, before being focused with the off-axis parabolic mirror, P, into the bolometer. At the inlet port of the bolometer, B, the diameter of the THz beam is of the order of 5mm, and it is here that the sample cell, SS, is located. The procedure is to measure the THz transmission with an empty and then a filled sample cell at each wavelength covered. By way of illustration, figure 2 is a spectrum of glutamic acid obtained with the spectrometer.

In collaboration with the Forensic Alliance, who have supplied simulants of drugs to the University, a programme of in-house testing of the device is being carried out, including the production of characteristic spectra using only small amounts of simulants.

Figure 2 | THz spectroscopy of L-glutamic acid at room temperature using the simple spectrometer arrangement in figure 1. Larger Image

Evaluation to date is extremely encouraging, and, following the construction of a prototype version of our source, the technology will eventually be tested on real drugs at the Culham laboratories of the Forensic Alliance. In collaboration with the Forensic Alliance the aim is to identify effective protocols for the in-field detection of illicit and concealed drugs. Since features in the spectra of drugs (and substances likely to be accompanying drugs) are more likely to be associated with inter- rather than intra-molecular interactions, there is much still to be learnt about the effects of sample preparation, composition and structure on the THz spectra obtained before the approach can become a useful tool for drug identification.