Terahertz transmission of NbN superconductor thin film R. Tesar ˇ a,b, * , J. Kolác ˇek a , Z. Šimša a , M. Šindler a,b , L. Skrbek b , K. Il’in c , M. Siegel c a Institute of Physics ASCR, v. v. i., Cukrovarnická 10, CZ-162 53 Praha 6, Czech Republic b Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, CZ-121 16 Praha, Czech Republic c Institute for Micro- and Nanoelectronic Systems, University of Karlsruhe, Hertzstr. 16, 76187 Karlsruhe, Germany article info Article history: Available online 21 February 2010 Keywords: Far-infrared transmission NbN Superconducting film Magnetic vortices Terahertz waves abstract Transmission of terahertz waves through a thin layer of the NbN superconductor deposited on a sapphire substrate was studied as a function of temperature in zero field as well as in magnetic field perpendicular to the sample. For photon energies lower than optical gap, detailed temperature measurements in zero field provide BCS-like curves with a pronounced peak below the critical temperature. In accordance with the BCS model, the temperature peak disappears as the energy of incident radiation is increased above the gap. In non-zero field, the temperature behavior of transmission is modified because the gap is sup- pressed and vanishes at upper critical field. In addition, the presence of quantized vortices in the super- conducting film substantially changes shape of the temperature curves. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Vortex dynamics in superconductors is experimentally most effectively studied in the terahertz frequency region. A number of experiments have already been performed, nevertheless, some questions still remain open. Recently Ikebe et al. [1] used terahertz time-domain spectroscopy to study vortex dynamics in the NbN thin film at 3 K. They used the Maxwell Garnett theory [2] and the modified Coffey–Clem self-consistent theory [3] to reveal how vortices contribute to ac conductivity in the THz frequency range. In this paper, we describe our experimental setup, report first observed temperature and magnetic field dependences of ter- ahertz wave transmission through a NbN thin film sample depos- ited on a sapphire substrate and compare these results with available theories. 2. Experimental setup Our experimental setup is schematically shown in Fig. 1. Its essential parts are the CO 2 laser pumping the far-infrared (FIR) la- ser source [4], the SM4000 Spectromag cryomagnetic system [5] and the helium cooled bolometer [6]. The FIR laser generates a coherent, linearly polarized, mono- chromatic radiation at discrete wavelengths in the range from 40 lm to 1 mm. From a number of the available far-infrared lines only few sufficiently intense lines have so far been exploited in our experiment, as listed in Table 1. To achieve a successful laser action, several conditions must be satisfied. First, the waveguide tube of the FIR laser is filled with the operating gas at a low pres- sure. Appropriate excitation energy emitted from the strong infra- red CO 2 laser is injected into the resonator cavity with 130 Hz repetition frequency. The length of the FIR resonator is fine tuned to meet the lasing condition. Part of the FIR output beam reflected by a mylar beamsplitter (BS) is focused by a mirror (M2) to a pyro- electric detector (PD) which is used for monitoring the laser power. The beam transmitted through the beamsplitter is concentrated on the sample using a gold coated off-axis parabolic mirror (M4). Radiation transmitted through the sample is collected by mirrors (M4 0 , M5) to the bolometer detector. Transmission of the sample is proportional to the ratio of signals from bolometer and pyrode- tector. This method effectively eliminates any possible time insta- bility in the laser power. The Spectromag cryomagnetic equipment enables to measure optical transmission at temperatures ranging from 3 K to 300 K in magnetic fields up to 11 T. The system stands on a simple non-magnetic rotation stage designed for an easy change of the field configuration as indicated by the curved arrow in Fig. 1. The horizontal split pair of superconducting coils provides optical ac- cess in two perpendicular directions through four pairs of mylar windows. The sample rod can be rotated manually about vertical axis to adjust position of the sample perpendicular or parallel with the magnetic field. The magnet coils are supplied by a bipolar cur- rent source which allows energizing the magnet in both field directions. The temperature of the sample can be controlled automatically by a dynamic cooling and heating. Liquid helium flows from the main bath into the sample space through a needle valve and evaporates in a heat exchanger. A stabilized flow rate is adjusted 0921-4534/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2010.02.077 * Corresponding author. Address: Institute of Physics ASCR, v. v. i., Cukrovarnická 10, CZ-162 53 Praha 6, Czech Republic. E-mail address: tesar@fzu.cz (R. Tesar ˇ). Physica C 470 (2010) 932–934 Contents lists available at ScienceDirect Physica C journal homepage: www.elsevier.com/locate/physc