There are objects we cannot see within the range of the visible but which we can with imaging systems that use the terahertz (THz) wavelength. Within this range we can detect, for example, not only a foreign body hidden under clothing, but also determine what material it is made of. David Etayo, a telecommunications engineer and PhD holder of the NUP/UPNA-Public University of Navarre, has been able to identify explosive components not only in their pure state, but also, and for the first time, after the detonation has taken place. What is more, he has worked on other terahertz applications for agriculture and the food industry. His PhD thesis is entitled "New developments in the THz field for imaging applications".
Characterising (identifying) a material means finding out its distinctive features so that when the substance is subsequently subjected to a detector system, the system will indicate what it is. As this researcher pointed out, "what we have done is gone a step further in the imaging system: besides detecting that an object is there, we have characterised different materials to see how they react within the THz range. We have characterised explosives and, for the first time, a type of explosive like gunpowder, which was a material present, for example, in the March 11 attack". He has also characterised other materials like TNT, hexogen and pentrite.
One of the achievements of this PhD thesis has been the characterising also of explosives that have already been detonated. "The normal thing is to characterise explosives in their recently produced laboratory form, when they are safe, but what happens, for example, after an attack, is that only a few remains are left behind and are totally different from the original materials."
In the course of his research and in collaboration with the Guardia Civil (Spanish Gendarmerie) samples were taken before and after detonation. Furthermore, the materials were characterised in different forms: pure, commercial and homemade explosive materials, for example. That way, it has been possible to detect explosives in minimum sample quantities of between 5 and 10 milligrams. Blends of different explosives were also analysed and in all the cases it was possible to identify each of the components.
"Using the remains of a detonation as a sample, we can find out almost immediately what kind of explosive has been detonated. In the end, it is a chemical process that modifies the initial product, but the good thing is that in the pure as well as in the detonated state is it possible in the terahertz range to characterise, determine and find out what it is." The use of this technology could also allow these systems to be incorporated into crawler robots employed to dispose of devices, and thus enable them to detect the explosive involved.
Another part of the thesis focussed on THz technology applications in the fields of agriculture and the food industry. In the first case, work was done on vines since THz are very sensitive to the water content of a sample: "Although at first sight no variations can be detected, if you analyse the imaging of a vine leaf in terahertz you can see perfectly how the water content varies from one day to the next. This allows one to exercise greater control over the plants, cut irrigation costs and that way improve the quality of the wine, etc."
As regards the food industry, work was done in collaboration with a chorizo producing plant. On the one hand, the amount of water in the product was measured during the drying process which enables one to estimate how much longer the chorizo curing process will take. "The good thing about THz technology is that it is non-destructive; you don't need to cut the chorizo to carry out the measuring, all you have to do is bring the sensor up to the product," explained David Etayo. Furthermore, the most direct application they have found is the use of the system to detect remains or foreign bodies that may have ended up in the production chain of sliced chorizo.
Finally, during the work a double band was designed; it allows two different frequency ranges (infrared and terahertz) to be combined into a single measurement so that hidden objects can be detected and identified. Within the electromagnetic spectrum, THz radiation is located between microwaves and infrared light waves. The infrared range works at a higher frequency and provides resolution and greater imaging quality, while the THz part is the one that is used to identify and characterise the materials. "The idea is that a single detector can provide us with the resolution of the imaging and the identification of the material at the same time," pointed out the researcher. In this thesis we have designed and manufactured a detector that enables us to make this measurement. What is more, the use of Fresnel zones has enabled us to achieve a gain increase in the infrared band."