The discovery of a superconducting phase in sulfur hydride under high pressure with a critical temperature above 200 K has provided a new impetus to the search for even higher Tc . Theory predicted and experiment confirmed that the phase involved is H3S with Im-3m crystal structure. The observation of a sharp drop in resistance to zero at Tc, its downward shift with magnetic field and a Meissner effect confirm superconductivity but the mechanism involved remains to be determined. Exploiting the capabilities of the AILES beamline at SOLEIL, we provide a first optical spectroscopy study of this new superconductor . Experimental results for the optical reflectivity of H3S, under high pressure of 1.5 Mbar, for several temperatures and over the range 60 to 600 meV of photon energies, are compared with theoretical calculations based on Eliashberg theory using DFT results for the electron-phonon spectral density. Two significant features stand out: some remarkably strong infrared active phonons around 160 meV and a band with a depressed reflectance in the superconducting state in the region from 450 meV to 600 meV. In this energy range, as predicted by theory, H3S is found to become a better reflector with increasing temperature. This temperature evolution is traced to superconductivity originating from the electron-phonon interaction. The shape, magnitude, and energy dependence of this band at 150 K agrees with our calculations. This provides strong evidence of a conventional mechanism. However, the unusually strong optical phonon suggests a contribution of electronic degrees of freedom.
 A. P. Drozdov, M. I. Eremets, I.A. Troyan, V. Ksenofontov and S. I.Shylin, Nature 525, 73–76 (2015)
 F. Capitani, B. Langerome, J.-B. Brubach, P. Roy, A. Drozdov,M. Eremets,E. Nicol, J. Carbotte and T. Timusk, Nat. Phys., published online, doi: 10.1038/nphys4156 (2017)