Guided Wave THz Spectroscopy of Explosive Materials

Abstract

One of the important applications of THz time-domain spectroscopy (TDS) is the detection of explosive materials through identification of vibrational fingerprint spectra. Most recent THz spectroscopic measurements have been made using pellet samples, where disorder effects contribute to line broadening, which results in the merging of individual resonances into relatively broad absorption features. To address this issue, we used the technique of parallel plate waveguide (PPWG) THz-TDS to achieve sensitive characterization of three explosive materials: TNT, RDX, and HMX.The measurement method for PPWG THz-TDS used well-established ultrafast optoelectronic techniques to generate and detect sub-picosecond THz pulses. All materials were characterized as powder layers in 112 μm gaps in metal PPWG. To illustrate the PPWG THz-TDS method, we described our measurement by comparing the vibrational spectra of the materials, TNT, RDX, and HMX, applied as thin powder layers to a PPWG, or in conventional sample cell form, where all materials were placed in Teflon sample cells. The thin layer mass was estimated to be about 700 μg, whereas the mass in the sample cell was ～100 mg. In a laboratory environment, the absorption coefficient of an explosive material is essentially based on the mass of the material, which is given as:   ln     .In this paper, we show spectra of 3 different explosives from 0.2 to 2.4 THz measured using the PPWG THz-TDS. Ⅰ. IntroductionThe Terahertz (THz) range is typically defined as the frequency range from 0.1 to 10 THz. Wavelength of THz is 3 mm～30 μm. Due to the difficulties in generating and detecting THz, the THz range is veiwed as one of the most inaccessible. The recent development of nanomaterial technology and ultrafine processes has allowed the advent of new THz sources. The commercialization of techniques for ultrashort femtosecond laser pulses has also accelerated the development of techniques for generation and detection of THz [1].At present, THz applications are being intensively studied in various fields such as new materials, medicine, biology, security, defense, environment, space, and communication [2]. Terahertz time domain spectroscopy (THz-TDS) is one technology that is used for analysis of materials. This technology can be used to obtain the phase information of materials directly through the vibration spectrum, conformational change, and the E-field measurement of THz pulse. Complex refractive index, complex dielectric properties, and complex conductivity can also be obtained using these techniques. One important application of THz-TDS is the detection of explosive and prohibited materials through identification of vibrational fingerprint spectra [3～4].During the past ten years, THz waveguides, which could be applied in various fields including THz communication, have been reported in terms of a rectangular waveguide [5], a circular waveguide [6], a fiber [7], a single wire [8], a coaxial cable [9], planar transmission lines [10], and parallel plat...