Terahertz Electrodynamics and Superconducting Energy Gap of NbN

We have measured the transmission spectra of the conventional Bardeen-Cooper-Schrieffer (BCS) superconductor niobium nitride (NbN) thin films (T-c = 11 K) using terahertz time-domain spectroscopy (THz-TDS) over the spectral range of 10 - 110 cm(-1) and the temperature range of 3.9 - 295 K. We extracted both the real part, sigma(1), and the imaginary part, sigma(2), of the optical conductivity, (sigma) over tilde = sigma(1) + i sigma(2), independently and simultaneously, without a Kramers-Kronig analysis. The superconducting gap Delta(T) was observed in the real part of the conductivity, sigma(1), below T-c = 11 K with a maximum value of 2 Delta(0) = 30 cm(-1) and the gap ratio 2 Delta(0)/k(B)T(c) = 3.92.