Far-infrared studies of superconducting thin films and Fabry-Perot resonators made of such films.

摘要

Using Fourier transform interferometers, we studied high-temperature superconducting YBa₂Cu₃O_7−δ (YBCO) thin films on silicon substrates in the far-infrared spectral range. We measured front-side reflectance, back-side reflectance (first measurement of this kind, with the incident ray coming from the substrate side), and transmittance at several temperatures in both the normal and superconducting state. Two select samples with closest matching characteristics were mounted facing each other in order to form Fabry-Perot resonators. The resonator gap was changed by spacers of various thickness. The resonator transmission was measured.; Initially, several samples were measured within the spectral range 10–100 cm−1 (wave number) or 1000–100 μm (wavelength). We could fit the results using the Drude model for the normal state, and the two-fluid model for the superconducting state of the YBCO film, a constant permittivity for the Si substrate, and ignoring the two thin buffer layers of YSZ and CeO₂.; Interference, optical resonance, and antireflection on the infrared properties of high-temperature superconducting (HTSC) films were observed and quantitatively analyzed. At certain frequencies the back-side reflectance can be deeply modulated by oscillating the temperature above and below a critical value—enabling the use of the film as a far-infrared intensity modulator. Analysis of the absorptance, calculated from the measured reflectance and transmittance, showed that the absorptance from the back (substrate) side is higher. This fact demonstrates potential for designing radiation detectors based on superconducting thin films.; The complex refractive index of the films was determined. The effect of partial coherence on the Fabry-Perot resonator transmittance was studied. A complex interplay among the cavity length, substrate thickness, and phase shift upon reflection affecting the magnitude of the resonant transmittance and finesse, was studied thoroughly to deduce an optimization method.; Recently, new samples were measured over the range 17–600 cm −1. This wider spectral region allows a greater fit accuracy. We included phonons within the SiO₂ layer on the substrate back surface, and phonons of the YSZ buffer layer and the YBCO film, in order to fit the measured results. Spectral beating caused by close-in-frequency phonon oscillators in opposite layers was observed. Two strong and wide oscillators indicate that the SiO₂ and YSZ layers are amorphous.