Liquid identification by Hilbert spectroscopy M Lyatti, Y Divin, U Poppe, K Urban Forschungszentrum Jülich, 52425 Jülich, Germany E-mail: M.Lyatti@fz-juelich.de and Y.Divin@fz-juelich.de Abstract. Fast and reliable identification of liquids is of great importance in security, biology and beverage industry. An unambiguous identification of liquids can be made by electromagnetic measurements of their dielectric functions in the frequency range of their main dispersions, but this frequency range, from a few GHz to a few THz, is not covered by any conventional spectroscopy. We have developed a concept of liquid identification, based on our new Hilbert spectroscopy and high-T c Josephson junctions, which can operate at the intermediate range from microwaves to terahertz frequencies. A demonstration setup has been developed consisting of a polychromatic radiation source and a compact Hilbert spectrometer integrated in a Stirling cryocooler. Reflection polychromatic spectra of various bottled liquids have been measured at the spectral range of 15 – 300 GHz with total scanning time down to 0.2 second and identification of liquids has been demonstrated. Keywords: Josephson junction, terahertz spectroscopy, security applications PACS: 85.25.Pb, 85.25.Cp, 07.57.Pt Submitted to Superconductor Science and Technology 1. Introduction After recently uncovered terrorist plots involving the mid-flight detonation of liquid explosives it became clear that the security measures at airports and other public places should be extended. These measures should include a specific screening of passenger’s luggage 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 in public areas. For this purpose a technique for the identification of liquids is required which is fast and so specific that the frequency of false alarms is low, so it should not disturb a normal flow of passengers through security checkpoints. Among various discussed explosives detection techniques, the techniques using electromagnetic radiation are considered as one of the most promising [1]. From the point of view of electromagnetic theory, the electric displacement-field response of any substance to a rapidly varying electrical field is defined, in linear approximation, by the complex dielectric permittivity function ε ( f ) = ε 1 ( f ) + i⋅ε 2 ( f ), (1) which is determined by the internal dynamics of the molecules [2]. Therefore, a substance can be identified by measuring its dielectric function ε(f) over a wide range of frequency f and comparing it with reference data. The internal dynamics of liquids can be considered to a first approximation as an alignment of dipoles by the local electric field followed by a Debye relaxation process [3]. In this case the dielectric function can be written as ε(f) = ε ∞ + (ε 0 - ε ∞ )/(1 + i⋅2πfτ), (2)