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.

A planar terahertz metamaterial sensor consisting of a corrugated metal stripe perforated by three rectangular grooves is proposed and investigated numerically. Due to the formation of Fabry-Perot resonance of the spoof surface plasmons mode on the corrugated metal stripe, the extremely sharp resonance in transmission spectrum associated with strong local field enhancement and high quality factor can be realized and exploited for ultrasensitive sensing. Since the intense interaction between electromagnetic waves and analyte materials, the frequency sensitivity of 1.966 THz per refractive index unit and the figure of merit of 19.86 can be achieved. Meanwhile, the film thickness sensitivity of this metamaterial sensor is higher than 52.5 GHz/μm when the analyte thickness is thinner than 4 μm. More interestingly, we find that the metal thickness has a great effect on the sensor performance. These findings open up opportunities for planar metamaterial structures to be developed into practical sensors in terahertz regime.

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.

By arranging two pairs of high-index dielectric disks into a unit cell, a novel, to the best of our knowledge, terahertz metamaterials sensor integrated with a microfluidic channel is proposed. With the introduction of a new way of symmetry breaking in the unit cell, the strong toroidal dipole response with ultrahigh-$Q$ is excited and investigated, which is related to the existence of the trapped mode. The simulation results show that the calculated quality factor and the corresponding figure of merit (FoM) of this sensor can reach 3189 and 515, respectively. These advantages allow for the proposed structure to have potential applications in high-performance gases, liquids, and biological materials sensing.

We have used femtosecond pump-probe spectroscopy to study the ultrafast nonlinear response of AlGaAs two-dimensional photonic crystal waveguides in the near-infrared spectral region. The modulation of the reflectivity spectra due to the refractive index change produced by photogenerated carriers was measured. We observed an instantaneous pump-induced shift in the wavelength of a photonic resonance at 882 nm with a fast decay time of ≈8 ps. The magnitude of the reflectivity change was very large at wavelengths close to the photonic resonance, with a maximum value of $\mathrm{\Delta R/R}$ >30% at 877 nm. These results confirm the excellent potential of photonic crystal waveguides in ultrafast nonlinear switching applications.