Detecting a Sample Using its Spectral Fingerprint

By Laura ThomsonMay 13 2022

Terahertz waves fall between the microwave and infrared regions of the electromagnetic spectrum. Biomedicine, homeland security, information and communication technology, pharmaceutical science, and various other fields benefit from these waves.

Image Credit: Ken stocker/Shutterstock.com

The non-destructive sensing and imaging of explosives and illicit drugs depending on their spectral fingerprints are well-known applications of THz waves.

Terahertz time-domain spectroscopy has been used in most terahertz wave measurements (THz-TDS). Different non-destructive testing and validation applications have used the transmission mode THz-TDS system. However, measuring highly absorbing and thick samples in transmission mode can be difficult.

Attenuated total reflection terahertz time-domain spectroscopy (ATR THz-TDS) could be used to solve such problems. In this technique, a sample is kept on a prism. The THz wave encounters total internal reflection, and the evanescent wave is produced at the interface between the prism. The sample allows THz properties to be determined.

Apart from the benefits of the ATR THz-TDS system, THz wave non-destructive testing is restricted to transmission and reflection mode measurement systems. Using ATR THz-TDS, the current study presents a method for identifying a sample sealed in a plastic bag without opening it.

Methodology

The experiment’s terahertz time-domain spectroscopy is shown in Figure 1. A beam splitter divides a femtosecond laser beam into two equal halves. The system’s maximum dynamic range is around 65 dB at around 200 GHz, and it covers a frequency range of 100 GHz to 3 THz. The frequency resolution of this system is 24 GHz.

Figure 1. (a) Schematic diagram of the THz time-domain spectrometer. The dotted line shows the position of the Silicon prism in the spectrometer. (b) Schematic diagram of a Silicon prism. Image Credit: Hashimoto and Tripathi, 2022

A Silicon (Si) prism with a refractive index of 3.41 was used because of its high resistivity and low terahertz absorption loss. The total internal reflection of the THz wave at the boundary between the Si prism surface and air generates an evanescent wave that infiltrates the sample mounted in contact with the prism. The frequency of the THz wave and the sample’s refractive index influence the depth of penetration.

The dependence of the penetration depth on the refractive index (n₂) and frequency are shown in Figure 2. This demonstrates that as the frequency decreases, the depth of penetration increases. This means that even if the sample is packaged in a plastic bag, the THz properties can be evaluated as long as the bag thickness is less than the evanescent wave’s penetration depth.

Figure 2. The penetration depth dependency on the frequency of THz wave and the refractive index of the sample. Here, a Silicon prism (n = 3.41) is used as a medium for total internal reflection. Image Credit: Hashimoto and Tripathi, 2022

The researchers used α-lactose monohydrate as a test sample as it has absorption features at 0.53 THz, 1.19 THz, and 1.39 THz. The sample has a particle size of less than 100 µm. As shown in Figure 3a, the lactose powder was sealed in a polyethylene plastic bag having a thickness of 10 µm ± 3 µm. To obtain spectroscopic information, the sample was placed on a silicon prism (Figure 3b).

Figure 3. (a) Lactose sample packaged in a plastic bag (b) Sample placed on the Silicon prism. Image Credit: Hashimoto and Tripathi, 2022

Results

The scientists started by measuring the lactose powder’s attenuated total reflection, refractive index, and absorption coefficient. Figure 4 reveals the time-domain THz pulse with and without placing the sample on a Silicon prism. They performed the measurement five times to increase the signal-to-noise ratio and averaged the results.

Figure 4. (a) The THz time domain reference and sample pulses, (b) their respective intensity spectra, (c) refractive index, (d) absorption coefficient of lactose sample. Image Credit: Hashimoto and Tripathi, 2022

The lactose sample's refractive index and absorption coefficient are shown in Figure 4c, d. Lactose’s distinct absorption characteristics at 0.53 THz and 1.39 THz are immediately evident.

The absorption coefficient and refractive index of the lactose sample, which was wrapped in a polyethylene bag, were determined in the next step. The sample’s refractive index and absorption coefficient were calculated and depicted in Figure 5.

Figure 5. (a) The refractive index and (b) the absorption coefficient of the packaged sample. The absorption peak at 0.53 THz is clearly visible, as shown by the arrowhead. Since the penetration depth of the evanescent wave is smaller than the thickness of the plastic bag at high frequencies, the absorption features of the lactose sample at high frequency cannot be measured reliably. Therefore, the absorption coefficient is shown up to 0.8 THz. Image Credit: Hashimoto and Tripathi, 2022

Although lactose has distinct absorption peaks, only the first absorption peak at 0.53 THz was identified when the sample was packaged in the polyethylene bag. The lactose sample’s absorption characteristics cannot be reliably measured at high frequency.

Overall, the results show that attenuated total reflection terahertz time-domain spectroscopy can be used to identify packaged samples with spectral fingerprints in the sub-terahertz frequency range. To further broaden the application of this system, the terahertz properties of other packaging materials, such as papers and fabrics, must be investigated.

Conclusion

One of the promising applications of terahertz waves is non-destructive testing. According to the findings, terahertz time-domain attenuated total reflection spectroscopy could be used to non-destructively detect samples packaged in a thin polyethylene bag based on their spectral fingerprints.

The penetration depth of the evanescent wave in the sub-terahertz frequency region is greater than the thickness of a typical plastic bag, enabling the absorption features of the sample to be obtained. This technique, according to the researchers, can be used to determine samples like medicine sachets in pharmacies to prevent medication errors.

Journal Reference

Hashimoto, K. & Tripathi, S. R. (2022) Non-Destructive Identification of Drugs in Plastic Packaging Using Attenuated Total Reflection Terahertz Time Domain Spectroscopy. Optics, 3(2), pp. 99–106. Available Online: https://www.mdpi.com/2673-3269/3/2/12/htm.

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