Guided-wave terahertz characterization of fingerprint lines in threat materials

One promising application of terahertz (THz) spectroscopy is detection and identification of threat materials. For explosives and related threat materials, vibrational resonances (or absorption lines) of molecules in crystalline environments can provide a unique ‘fingerprint.’ These resonances typically occur between approximately 0.5 and 10THz (17–333/cm). To develop a robust database of THz-fingerprint spectra for detection applications, we must achieve a fundamental understanding of the relevant vibrational-fingerprint resonances. For example, ambient THz-fingerprint spectra consist of a superposition of a large number of underlying resonances, which our experimental methods must resolve for full characterization.

Although much progress has been made towards measuring THz vibrational spectra of explosives,^1,2 some remaining issues must be addressed for THz sensing to reach its full potential. First, to date, most THz measurements have used pellet samples, where disorder effects contribute to line broadening so that individual resonances merge into relatively broad absorption features (even at cryogenic temperatures). Line broadening makes identification more difficult and impedes a full understanding of the vibrational spectrum. Second, THz vibrational resonances tend to be relatively weak compared to those in the mid-IR spectral regime. This emphasizes a need to develop methods that can boost detection sensitivity. To address these issues, we applied THz-waveguide spectroscopy to achieve sensitive characterization of explosives films with sharper vibrational linewidths.

Our application is based on our recently improved understanding of how subpicosecond THz pulses propagate through a metal parallel-plate waveguide (PPWG).^3,4 In PPWGs, THz waves are strongly confined within the narrow gap (20–100μm) between the metal plates while propagating over a relatively long pathlength of several centimeters (see Figure 1). We exploit this potential for high sensitivity by depositing an ordered polycrystalline thin film of a target threat material on one of the inner metal surfaces. (Line-broadening effects may be attenuated if the waveguide film exhibits sufficiently high crystallinity.) Our measurement method, waveguide THz time-domain spectroscopy (THz-TDS), uses well-established ultrafast opto-electronic techniques to detect subpicosecond THz pulses.

Figure 1. (left) Schematic overview of waveguide terahertz (THz) time-domain spectroscopy. He, Si: Helium, silicon. fs: Femtosecond. PPWG: Parallel-plate waveguide. (right) Schematic of the metal PPWG containing a thin film. Al, Cu, Au: Aluminum, copper, gold. Prep: Preparation. E_THz: THz energy.

We recently demonstrated the resolution of the underlying THz vibrational spectrum for explosives-related materials using waveguide THz-TDS at cryogenic temperatures.^5,6 Figure 2 compares the THz responses at 11K of 2,4-dinitrotoluene (2,4-DNT) as a film in an aluminum PPWG and in conventional pellet form (where 2,4-DNT was randomly dispersed in transparent polyethylene). The persistent ringing patterns of the signal waveforms result from THz absorption by the vibrational modes of the 2,4-DNT crystal. We note that the oscillatory pattern for the pellet sample decays to the noise floor within approximately 20ps, while oscillations for the film persist well beyond 50ps. We derived the corresponding vibrational-absorption spectra by Fourier transformation of the time-dependent signals (see Figure 2, bottom). The vibrational spectrum of the film shows much stronger line narrowing and resolves several of the relatively broad absorption features of the pellet spectrum. The waveguide film measurement resolves more than twice as many vibrational lines between 0.5 and 2.5THz than the pellet-based experiment. To put the narrow-line capability of waveguide THz-TDS in perspective, some of the vibrational lines in the film are sharper than 10GHz (0.3/cm) full width at half maximum (FWHM), which is about as narrow as a molecular rotational gas line under ambient conditions.

Figure 2. (top) Comparison of signal waveforms for pellet (red line) and waveguide THz time-domain-spectroscopy (THz-TDS: black line, using an aluminum PPWG) characterizations of 2,4-dinitrotoluene (DNT), where both experiments were performed at 11K. (bottom) Corresponding absorbance spectra (base e) for 2,4-DNT as film (black line) and in pellet form (red line).

Figure 3 shows that waveguide THz-TDS resolves previously unseen underlying THz vibrational spectra for the explosives trinitrotoluene (better known as TNT), pentaerythritol tetranitrate, and the explosives-related material 4-amino-DNT. In cases where the FWHM linewidth is <10GHz, we have shown that the line frequencies can be specified with a precision of ±1GHz.⁷ As an intuitive way to understand how this higher precision relates to spectral fingerprinting, consider that we have three spectral features in the THz range that identify a particular explosive. With conventional techniques, these lines are detected at 0.6, 1.4, and 2.3THz. In addition, consider that the line centers (peaks) are measured to a precision of ±0.1THz. The THz fingerprint is thus represented by the label 06-14-23. With our new technique, we can measure the line centers (peaks) to a precision of better than ±0.01THz (at cryogenic temperatures), yielding line positions of 0.62, 1.41, and 2.35THz. This more precise measurement is represented by the label 062-141-235, which leads to conclusive sample identification, even in the presence of other materials with similar spectral features in the THz range of the fingerprint.

Figure 3. Waveguide THz-TDS spectra of explosives thin films at cryogenic temperatures: (a) trinitrotoluene (commonly known as TNT) on aluminum, (b) pentaerythritol tetranitrate on gold, and (c) 4-amino-DNT on aluminum. The inset shows the lower-frequency portion using an expanded vertical scale.

Finally, the sensitivity enhancement provided by the PPWG is highlighted by the relatively small mass of the explosives films (≈150μg). This represents about 1% of the analyte mass typically needed in THz measurements of pellet samples to achieve similar absorbance levels.

In summary, we have demonstrated a waveguide-based method that has resulted in unprecedented narrow-line THz measurements of explosives and related materials at cryogenic temperatures. As one of our next steps, we will explore incorporating photonic-type elements within the PPWG gap for potential sensing of threat materials including gases.

This work was supported by the Office of Naval Research and the Defense Threat Reduction Agency.