Terahertz frequency radiation possesses a unique combination of desirable properties for noninvasive imaging and spectroscopy of materials. This includes the ability to obtain chemical and structural information about substances concealed within dry packaging, such as paper, plastics, and cardboard. As a result, the application of terahertz frequency spectroscopy for the sensing and identification of materials of security interest, such as explosives and, to a lesser extent, drugs-of-abuse, has caught the attention of a number of researchers and security agencies. We describe terahertz time-domain spectroscopy and examine the terahertz spectra of a wide range of drugs-of-abuse, pure explosives, and plastic explosives.

Broadband terahertz time-domain spectroscopy (THz-TDS) has been used to measure the far-infrared (FIR) vibrational spectra of several commonly used pure explosives, including 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), 1,3-dinitrato-2,2-bis(nitratomethyl)-propane (PETN), and two types of plastic explosive, SEMTEX and SX2. A number of distinct absorption peaks, originating from FIR-active vibrational modes of these polycrystalline energetic materials, were observed in the frequency range 0.3–7.5 THz (10–250 cm⁻¹). In addition, the temperature-dependent FIR vibrational spectra of PETN were measured between 4 K and 296 K with several well-resolved absorption peaks observed across this temperature range. We find that as the temperature is reduced, the observed absorption peaks resolve into narrower features and shift towards higher frequencies. The temperature dependence of the spectra is explained in terms of the anharmonicity of the vibrational potentials of crystalline compounds, and an empirical fit is given to describe the peak shift with temperature.

Terahertz frequency time-domain spectroscopy has been used to analyse a wide range of samples containing cocaine hydrochloride, heroin and ecstasy – common drugs-of-abuse. We investigated real-world samples seized by law enforcement agencies, together with pure drugs-of-abuse, and pure drugs-of-abuse systematically adulterated in the laboratory to emulate real-world samples. In order to investigate the feasibility of automatic spectral recognition of such illicit materials by terahertz spectroscopy, principal component analysis was employed to cluster spectra of similar compounds.

We demonstrate a significant enhancement in the sensitivity of split ring resonator terahertz metamaterial dielectric sensors by the introduction of etched trenches into their inductive-capacitive gap area, both through finite element simulations and in experiments performed using terahertz time-domain spectroscopy. The enhanced sensitivity is demonstrated by observation of an increased frequency shift in response to overlaid dielectric material of thicknesses up to 18 µm deposited on to the sensor surface. We show that sensitivity to the dielectric is enhanced by a factor of up to ∼2.7 times by the incorporation of locally etched trenches with a depth of ∼3.4 µm, for example, and discuss the effect of the etching on the electrical properties of the sensors. Our experimental findings are in good agreement with simulations of the sensors obtained using finite element methods.

In this paper, resin-based photocurable polymer materials for stereolithography (SLA), digital-light-processing (DLP) and polymer-jetting (PJ) additive manufacturing techniques were characterized from 0.2 – 1.4 terahertz (THz) for their comprehensive dielectric properties, e.g. refractive index, absorption coefficient, dielectric constant and loss tangent, by using laser-based THz time-domain spectroscopy (THz-TDS). Fourteen photocurable 3D-printing polymers were chosen, owing to their suitability, in terms of printing resolution, material characteristics and etc., for millimeter-wave (mm-wave) and THz applications. From the measurement results from 0.2-1.4THz, the dielectric constants of all photopolymer samples under test are between 2.00-3.10 while the loss tangents are from 0.008-0.102, which are quite useful for many applications, e.g. 3D printed antennas and THz transmission lines, which were demonstrated by an asymptotically quasi-single-mode Bragg fiber microfabricated by DLP micromanufacturing technique using HTM140-V2 photopolymer previously reported at the nominal frequencies from 0.246 to 0.276 THz.