太赫兹时域光谱在邮件隐匿危险品检测中的应用

Terahertz Time-Domain Spectroscopy for Identification of Hazardous Substances in Mail

随着电子商务的快速发展,邮件数量剧增,在邮件中隐匿危险品已经成为犯罪分子重要的犯罪手段,威胁公共安全和社会稳定。邮件的安全检查变得尤为重要,而常规的检测技术不能准确识别危险品。太赫兹波是介于红外和微波之间的电磁波,邮件中隐匿的爆炸物、毒品和有害生物因子等在太赫兹波段存在特征吸收光谱,而邮件常用的非极性包装材料可以被太赫兹波穿透。太赫兹波还具有低能性、相干性等特性,这些特性使得太赫兹技术可以实现邮件隐匿危险品高灵敏度的无损检测。文章介绍了太赫兹技术的特性,太赫兹时域光谱系统的组成和获取光学常数的菲涅尔公式解析法。该方法通过样品透射或反射信号和参考信号来获取包括吸收光谱在内的材料参数。将样品的太赫兹特征吸收光谱和已建立的各种危险品的光谱特征数据库进行比对,可以判断是否为危险品以及危险品种类。对爆炸物、毒品在太赫兹波段的特征吸收光谱的研究成果,及在各种非极性材料遮挡下吸收光谱的特异性的研究进展进行了总结。获取吸收光谱的解析法适用于较厚样品,针对薄样品,还介绍了一种P-谱法。该方法不需要参考信号就能准确获取覆盖物下样品的吸收光谱。除直接利用吸收光谱做检测外,近些年还提出了很多其他识别危险品的方法,如光谱动力学分析法,化学计量学方法和基于太赫兹时域光谱的成像分析法等。其中,光谱动力学分析法可以很好的区分吸收频率有重叠的物质;化学计量学方法可以对混合物进行成分的定性和定量分析;光谱成像法可以完成较大面积的隐匿危险品识别。分析了太赫兹时域光谱技术识别有害生物因子的可行性,以及针对有害生物因子携带量小的特点,总结了太赫兹时域光谱技术在提高生物因子检测灵敏度方面的研究进展。探讨了太赫兹技术在邮件安检应用中一些有待解决的问题,如太赫兹功率有限、受环境因素影响较大、缺乏统一的标准等,展望了未来的发展趋势。

With the rapid development of e-commerce,the number of mails has increased dramatically,and the hazardous substances in the mail have become an important criminal means for criminals,which threatens public safety and social stability.The security check of emails becomes especially important,but the conventional detection techniques can not accurately identify hazardous substances.Terahertz waves occupy the region between microwaves and the infrared.The explosives,illicit drugs and harmful biological factors concealed in the mail have a characteristic absorption spectrum in the terahertz band,and THz waves can penetrate the non-polar packaging materials commonly used in mail.Terahertz radiation also has characteristics of low energy,coherence and so on,making it possible to achieve high-sensitivity and non-destructive detection of hazardous substances by using terahertz technology.The present paper introduces the characteristics of terahertz technologies,the composition of the terahertz time-domain spectroscopy system and the Fresnel formula analytic method for obtaining optical constants.The method obtains material parameters including absorption spectra by sample transmission or reflection signals and reference signals.The terahertz characteristic absorption spectra of samples were compared with the established spectral characteristic databases of various dangerous substances to determine whether the samples were dangerous and the types of hazardous substances.The research achievements of the characteristic absorption spectra of explosives and drugs in the terahertz band and the research progress of the absorption spectra under various non-polar materials were summarized.The analytical method for obtaining the absorption spectrum is suitable for thicker samples.For the thin sample article,a P-spectrum method is also introduced,which can accurately obtain the absorption spectrum of the sample under the cover without the reference signal.In addition to directly using absorption spectroscopy for detection,in recent years,many methods for terahertz spectroscopy have been proposed,such as the spectral dynamics analysis method which can distinguish the substances with overlapping absorption frequencies well,chemometrics method which can achieve qualitative and quantitative analyses of terahertz spectra,and imaging analysis based on terahertz time domain spectroscopy which can complete the identification of hidden dangers of large areas.The feasibility of terahertz time-domain spectroscopy to identify harmful biological factors,and the characteristics of for small carrying amount of harmful biological factors were also analyzed.Meanwhile,the progress of terahertz time domain spectroscopy in improving the detection sensitivity of biological factors was summarized.Finally,the existing technique difficulties,such as limited power of terahertz,large influence by environmental factors,lack of unified standards,were discussed and the future development trend was analyzed.