Figure 13: (a) THz transients transmitted through a thin niobium film (  K) on quartz in the normal state (dashed red line) and the superconducting state (solid blue line). (b) Real and imaginary parts of the normalized complex conductivity, and , of Nb in the superconducting state evaluated from the above data (after [96]). The dashed black line indicates the predictions by the BCS theory using the Mattis-Bardeen equations (1) and (2). The energy gap is given by .