Measurement of electrodynamic parameters of superconducting films in the far-infrared and submillimeter frequency ranges

Electrodynamics of Metallic Superconductors

“…While the previous experiments have been performed utilizing Fourier-transform spectrometers, Gorshunov et al employed a THz spectrometer on the basis of tunable coherent monochromatic backward wave oscillators [93][94][95] to measure 115Å thin NbN films on sapphire substrate [91,92]. They demonstrated that for frequencies below 2Δ the error bars are too large to draw conclusions about the superconducting state and in-gap states, for instance.…”

“…(a) Transmission spectra Tr (]) of an asymmetrical Fabry-Perot resonator, formed by a NbN film ( 2 = 115Å) on a sapphire substrate (0.43 mm), measured at two temperatures above and below critical temperature ≈ 12 K. (b) Transmission spectra Tr (]) of a plane parallel sapphire plate (0.41 mm) covered with NbN films on both sides ( 2 = 250Å and 4 = 60Å), again measured at 300 K and 5 K.The solid lines represent the theoretical fit. (c) For the latter case the temperature dependence of the optical conductivity 1 ( ) of NbN film is calculated for two frequencies, 8 cm −1 and 29 cm −1 (after[91,92]). …”

Modulation of terahertz radiation using optical control of Fabry–Perot resonances

“…For a practical realisation of the broadband, THz antireflection coating conductive materials with tuning capabilities are desirable. A fairly easy way to realise the impedance tuning is to adjust the metal layer thickness [2], doping or temperature of vanadium dioxide [4]. In a recent publication it was shown [5] that by increasing surface conductivity of the metal layer, the inversion of the FPR fringes can be achieved with the interchanged position of maxima and minima interference peaks.…”

“…Due to the refractive index mismatch at the interfaces, the plate causes reflections giving rise to unwanted Fabry-Perot resonances (FPR). Since the standard dielectric quarter-wave antireflection coatings are not suitable for broadband THz radiation, a thin metallic layer is usually used for the impedance matching between the dielectric plate and free space leading to the elimination of the FPR fringes in a wide frequency range [2,3]. For a practical realisation of the broadband, THz antireflection coating conductive materials with tuning capabilities are desirable.…”

Subterahertz spectroscopy at He-3 temperatures

“…Several groups employed semicoherent submillimeter sources such as backward wave oscillators and far-IR lasers. The scope of the experimental information extracted from these measurements is dramatically enhanced through the use of Mach-Zehnder 25 or Fabry-Pérot 26 interferometer schemes permitting evaluation of the optical constants without KK analysis. Yet another approach is based on the use of synchrotron light sources instead of conventional black-body sources in Fourier transform spectrometers.…”