Far-infrared probe of superconductivity in SmO$_{1-x}$F$_{x}$FeAs}
  We report far-infrared reflectance measurements on polycrystalline superconducting samples of SmO$_{1-x}$F$_{x}$FeAs ($x$ = 0.12, 0.15 and 0.2). We clearly observe superconductivity induced changes of reflectivity in a broad range of energies, which resembles earlier optical measurements on high $T_{c}$ cuprates. The superconducting-to-normal reflectivity ratio $R_{s}/R_{n}$ grows for the photon energies below 18 meV and shows a complicated structure due to the presence of a strong infrared-active phonon at about 10 meV. 
S.I. Mirzaei, V. Guritanu, A. B. Kuzmenko, C. Senatore, D. van der   Marel, G. Wu, R.H. Liu, and X.H. Chen