Quasiparticle relaxation in optically excited high-Q superconducting   resonators
  The quasiparticle relaxation time in superconducting films has been measured as a function of temperature using the response of the complex conductivity to photon flux. For tantalum and aluminium, chosen for their difference in electron-phonon coupling strength, we find that at high temperatures the relaxation time increases with decreasing temperature, as expected for electron-phonon interaction. At low temperatures we find in both superconducting materials a saturation of the relaxation time, suggesting the presence of a second relaxation channel not due to electron-phonon interaction. 
R. Barends, J. J. A. Baselmans, S. J. C. Yates, J. R. Gao, J. N.   Hovenier, and T. M. Klapwijk