Identification of Liquids by Hilbert Spectroscopy for Security Screening M. Lyatti 1,2 , I. Gundareva 1,2 , A. Snezhko 1,2 , V. Pavlovskiy 1 , V. Gubankov 1 , U. Poppe 2 , K. Urban 2 , Y. Divin 2 1 Institute of Radio Engineering and Electronics of RAS, 125009 Moscow, Russia 2 PGI, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany After uncovered terrorist plots in 2006 involving the mid-flight detonation of liquid explosives, it became clear that the security measures at airports and other public places should be extended. Specific screening of passenger’s hand luggage and checked baggage should be involved with the goal to find and identify particular sorts of liquids, which might be dangerous themselves or could be used as components for fabrication of explosives on board or on site in public areas. Among various ways of explosive detection discussed, the techniques using electromagnetic radiation (i.e. microwave and terahertz imaging and spectroscopy) are considered as having great potential by experts of the National Research Council of US National Academy [1]. Reliable identification of liquids with low false alarms rate could be made by electromagnetic measurements of their dielectric functions at the frequency range of their main dispersions. Therefore, a substance can be identified by measuring its dielectric function over a wide range of frequencies and comparing it with reference data. However, the spectral range of dispersion for pure liquids is rather broad, from a few gigahertz (GHz) to a few terahertz (THz), and not covered by any single conventional spectroscopic technique. In principle, Hilbert-transform spectroscopy, based on high-T c Josephson junctions, having a broad frequency range from a few GHz to 5 THz, a large power dynamic range of 50 - 60 dB and a shot response time, can be used for quick identification of liquids. To prove this idea we suggested a concept of a liquid identifier based on a high-T c Josephson detector [2]. The liquid identifier consists of a broadband radiation source, a radiation coupling unit, the Josephson detector and a data acquisition system, controlled by a computer as shown in fig. 1a. The radiation of the radiation source is coupled to liquid by the radiation coupling unit. Reflected radiation collected and guided to the Josephson detector. A response of the Josephson detector is digitized by a data acquisition system and compared with liquid signatures from a data base. Due to broad frequency bandwidth required for the liquid identification, all major parts of the liquid identifier such as the broadband radiation source, the radiation coupling unit and the Josephson detector could not bought, but had to be developed from scratch. Following the concept, two demonstrators of liquid identifier were developed. The first one had a quasioptical coupling unit. A photo of the liquid identifier with quasioptical coupling is shown in fig. 1b. In this demonstrator, radiation from a broadband polychromatic source was focused by a gold-plated elliptical mirror on a bottle with liquid, and the radiation, reflected from the bottle, was focused on the Josephson detector with the help of the second elliptical mirror. The broadband radiation source consisted of several frequency multipliers and could deliver polychromatic radiation in the frequency range of 30–500 GHz. A characterization of the frequency multipliers and the chains of frequency multipliers was done by a Hilbert spectrometer [3]. The detailed description of the experimental setup can be found in [4]. The second liquid identifier had a radiation coupling unit based on a dielectric waveguide. A photograph of the liquid identifier with waveguide coupling is shown in fig. 2c. The radiation coupling unit based on the dielectric waveguide was used to extend frequency range down to 1 GHz. In this demonstrator, the radiation of the broadband radiation source was coupled to a polyethylene (PE) waveguide, directed to a PE prism, reflected from an interface between the prism and liquid and guided by another PE waveguide to the Josephson detector. The broadband radiation source consisted of several frequency multipliers and a frequency