Many materials of interest to the forensic and security services, such as explosives, drugs and biological agents, exhibit
characteristic spectral features in the terahertz (THz) frequency range. These spectral features originate from
inter-molecular interactions, involving collective motions of molecules. Broadband THz time-domain spectroscopy
(THz-TDS) system have been used to analyze a number of drugs-of-abuse and explosives that are of interest to the
forensic and security services. These samples ranged from crystalline powders, pressed into pellets, to thin sheets of
plastic explosives, and all being measured in transmission geometry in the frequency range 0.1 - 8 THz.
To well understand the nature of the observed spectral features and the effects of thermal broadening on these
far-infrared signatures, temperature-dependent THz-TDS measurements have also been performed at temperatures as
low as 4 K, especially for two types of cocaine. Well-resolved low-frequency absorption peaks were observed in the
frequency range 0.1 - 3 THz with high resolution. Some of absorption peaks were found clearly to become more intense
and shift to higher frequencies as the temperature was reduced. The results confirm that the low-frequency collective
modes are highly sensitive to the structural and spatial arrangement of molecules. Furthermore, a number of common postal packaging materials made from paper, cardboard, even several types of plastic,
have been tested with drug sample to assess the ability of THz-TDS in a hostile detection environment.
Broadband terahertz time-domain spectroscopy (THz-TDS) has been shown to be a valuable technique for the detection and analysis of explosives. In this paper, we present recent work on the use of this technique to analyse two pure explosives, 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) and 1,3-dinitrato-2,2-bis(nitratomethyl)propane (PETN) and three plastic explosives (Semtex, SX2 and Metabel). For each explosive, a clear and unique spectral absorption fingerprint is found, which can be attributed to oscillations of the molecular lattice. Temperature dependent terahertz absorption measurements of RDX show that the observed vibrational modes are anharmonic in nature. Terahertz spectra are compared with Raman spectra, and the similarities and differences between the spectra produced by the two techniques are discussed. A molecular mechanics calculation approach is used to calculate vibrational intensities and frequencies, and these results are compared with experimental results at both room temperature and 4 K.
We demonstrate that, through coherent measurement of the transmitted terahertz frequency electric fields, broadband (0.3 - 8 THz) time-domain spectroscopy can be used to measure far-infrared vibrational modes of a range of drugs-of-abuse and high explosives that are of interest to the forensic and security services. Our results indicate that absorption features in these materials are highly sensitive to the structural and spatial arrangement of the molecules. Terahertz frequency spectra are also compared with high-resolution low-frequency Raman spectra to assist in understanding the low-frequency inter- and intra-molecular vibrational modes of the molecules.
The on-chip detection of nanolitre volumes of dielectric material is demonstrated using terahertz (THz) pulses. Simultaneous analysis at different frequencies on separate, lithographically defined locations is shown to be possible using THz band-stop filter elements connected by a microstrip line. Integrated thin film low-temperature-grown GaAs photoconductive switches are used for THz pulse generation and detection within the microstrip interconnect. This technique is expected to be useful in sensing organic films such as DNA, and other biomolecular materials, in an on-chip environment.
We demonstrate that, through coherent measurement of the transmitted terahertz electric fields, broadband (0.3-8THz) time-domain spectroscopy can be used to measure far-infrared vibrational modes of a range of illegal drugs and high explosives that are of interest to the forensic and security services. Our results show that these absorption features are highly sensitive to the structural and spatial arrangement of the molecules. Terahertz frequency spectra are also compared with high-resolution low-frequency Raman spectra to assist in understanding the low frequency inter- and intra-molecular vibrational modes of the molecules.