Energy-gap dynamics of superconducting NbN thin films studied by   time-resolved terahertz spectroscopy
  Using time-domain Terahertz spectroscopy we performed direct studies of the photoinduced suppression and recovery of the superconducting gap in a conventional BCS superconductor NbN. Both processes are found to be strongly temperature and excitation density dependent. The analysis of the data with the established phenomenological Rothwarf-Taylor model enabled us to determine the bare quasiparticle recombination rate, the Cooper pair-breaking rate and the electron-phonon coupling constant, \lambda = 1.1 +/- 0.1, which is in excellent agreement with theoretical estimates. 
M. Beck, M. Klammer, S. Lang, P. Leiderer, V.V. Kabanov, G. N.   Gol'tsman, and J. Demsar
cond-mat.supr-con cond-mat.str-el