Doping dependence of the optical spectra in Fe$_{1.02}$Te$_{1-x}$Se$_{x}$

We measured the optical reflectivity and extracted the optical conductivity over a broad spectral range of the Fe-chalcogenide Fe$_{1.02}$Te$_{1-x}$Se$_{x}$, at various doping levels, from $x$=0.12 to $x$=0.45. Except for the highest concentration of Se ($x$=0.45), optical properties show unusual frequency and temperature dependence. The zero frequency Drude peak is absent in optical conductivity, a plasma edge cannot be unambiguously defined and both reflectivity and optical conductivity have non-monothonic temperature dependence. For the superconducting samples, we found a weak enhancement of the reflectivity in the far-infrared region upon cooling below Tc and its possible association with the superconducting gap will be discussed.