  Optical reflectance R(w) of a pressure-induced superconductor SrFe2As2 has
been measured under external pressure to 6 GPa and at temperatures to 8 K.
Optical conductivity s(w) has been derived from the measured R(w). At ambient
pressure, in the antiferromagnetic state below T_N=198 K, a pronounced feature
develops in s(w) due to the opening of a spin density wave (SDW) gap, as
already reported in the literature. With increasing pressure, the SDW gap
feature in s(w) is progressively suppressed. At 4 GPa, where the sample is
superconducting, the SDW gap feature in s(w) is strongly reduced than that at
ambient pressure, but is still clearly observed. At 6 GPa, the SDW gap is
completely suppressed. The pressure evolutions of the SDW gap magnitude and the
spectral weight closely follow the pressure evolution of T_N.
