Characterization of superconducting NbTiN films using a dispersive Fourier transform spectrometer

We have built a terahertz dispersive Fourier transform spectrometer [Birch, Microchim. Acta 93, 105-122 (1987)] to study frequency properties of superconducting films used for fabrication of THz detectors. The signal reflected from the tested film is measured in time domain, which allows to separate it from the other reflections. The complex conductivity of the film depends on frequency and determines the reflection coefficient. By comparing the film reflection in the superconducting state (temperature is below Tc) with the reflection of the normal state, we characterize the film quality at terahertz frequencies. The method was applied to 70 and 200 nm thick Nb films on a silicon wafer and to 360 nm thick NbTiN films on silicon and quartz wafers. The strong-coupling coefficient, α, was found to be 3.52 for Nb and 3.71-4.02 for the NbTiN films. The experimental results were fitted using extended Mattis-Bardeen theory [Noguchi et al., Phys. Procedia 36, 318-323 (2012)] and show good agreement.