Terahertz (THz) radiation offers innovative sensing technologies that can provide information unavailable through other conventional electromagnetic techniques. With the advancement of THz technologies, THz sensing will impact a broad range of areas. This chapter focuses on the use of THz spectroscopy for sensing applications in three aspects: explosives detection, pharmaceutical identification, and biological characterization. A THz spectral database in the 0.1-20 THz range was established using both THz time-domain spectroscopy (THz-TDS) and Fourier transform far-infrared spectroscopy. The calculated spectra based on density functional theory and the experimental results are in good agreement in the 3-20 THz range, but not in the 0.1-3 THz range. It is also demonstrated that THz spectroscopy is capable of detecting and identifying the explosive Royal Demolition Explosive (RDX) in diffuse reflection geometry. THz-TDS was applied successfully for pharmaceutical identification, such as identifying hydrated and anhydrous drugs, probing the reaction kinetics of dehydrations, and solid-state reactions of pharmaceutical materials. It was found that most solid-state amino acids, purines, and other small biocompounds have THz absorption features in the 0.1-3 THz range. Solid-state proteins and bioactive protein microsuspensions in organic media also exhibit THz absorption features which may reflect their collective vibrational modes and could be used to probe their functional 3D conformation states. Additionally, owing to the high sensitivity of differential THz-TDS, it was successfully used to sense the minute change of biological cell monolayers. These studies have pointed to new ways for using THz spectroscopy in pharmaceutical and biological sensing.
Liu, H.-B., & Zhang, X.-C. (2007). Terahertz Spectroscopy for Explosive, Pharmaceutical, and Biological Sensing Applications. In Terahertz Frequency Detection and Identification of Materials and Objects (pp. 251–323). Springer Netherlands. https://doi.org/10.1007/978-1-4020-6503-3_17