OHD 2007 LE 19EME COLLOQUE INTERNATIONAL "OPTIQUE HERTZIENNE ET DIELECTRIQUES" DU 5 AU 8 SEPTEMBRE 2007 VALENCE - FRANCE Probe design for electromagnetic near field THz mappings. A. Penarier, P. Nouvel, J.-P. Guillet, T. Laurent, J. Torres, R. Adam, S. Jarrix, L. Chusseau IES, UMR CNRS 5214, Université Montpellier II, 34095 MONTPELLIER Cedex 5 Email: annick.penarier@ies.univ-montp2.fr Abstract: An electromagnetic near field continuous wave set-up working in the 75 – 110 GHz frequency range with a subwavelength resolution is presented. Two probes are particularly studied. They are both based on waveguides and optimized via calculations using EM 3D CST Microwave Studio Software. One is made of a waveguide with an integrated wire while the other consists of a tapered waveguide filled with a dielectric material. To prevent any wave propagation cutoff, two faces of the taper are metallized. Both probes preferentially collect the electric field. Electrical field mapping of a bow-tie antenna were undertaken at 100 GHz and compared to calculations. Key words: Electromagnetic Near field, Terahertz, Probe. 1. Introduction The terahertz (THz) frequency domain has long remained unexplored because of a lack of efficient techniques, either for generation or detection of signal. Since the past twenty years, considerable improvements have been brought to THz systems. The major motivation of this evolution is dictated by the characterization of materials, and the spectroscopy of chemical species and biological objects [1]. In materials, the knowledge of physical properties like complex conductivity is expected to open new fields of applications. Of particular biological interest are the resonance frequencies related to the frequencies of rotation and vibration of molecular arrangements or molecules. THz waves have also shown to be of a large interest in medical applications because of a larger penetration in the human body as compared to visible optics waves. Traditional intensity imagery in the THz frequency range is intrinsically of low spatial resolution because of the diffraction limit that makes the minimal focal spot size of the order of a few hundred of micrometers. The best solution involves the transposition of the well known optical near-field observation techniques to the THz domain. Moreover, to increase the experimental spectroscopic selectivity a monochromatic continuous THz source is chosen. Electromagnetic compatibility problems are becoming more important in the elaboration of electronic systems. Hence a near field characterization allows a non-destructive analysis of systems like that already realized for frequencies up to 20 GHz [2]. The purpose of this experiments is to carry out near-field microscope configurations allowing sample characterizations for all the electromagnetic field components. Micrometer displacements combine with a near-field collection technique may allow subwavelength spatial resolution corresponding to one thousands of the wavelength. In this paper, the electromagnetic near field THz experiments working in the 75 – 110 GHz frequency range is presented. We particularly study several types of probes, inspired by Scanning Near Field Optical Microscopy (SNOM) [3, 4]. To fully characterize the inspected sample, the polarization properties of specific probes as well as their ability to enhance the electric or magnetic fields are considered. The optimal combination of continuous wave (CW) THz imaging with a polarization selective probe will give a polarization contrast analogue to that of an optical microscope. 2. Experimental set-up The set-up is composed of three different parts: the source, the detector and one or two probes involved in the coupling of the sample with the 100 GHz field produced by the source or captured by the detector. The mm-wave beam is focused on the device under test (DUT) using two parabolic mirrors. It allows an optimal use of the source power on the sample, provided that the parabolic mirror numerical aperture matches that of the horn. The DUT is thus in the waist of the THz beam whose size has been measured to 1 cm.