  We perform in-plane optical spectroscopy measurement on (00l) FeSe thin-film
grown on SrTiO$_3$ substrate by pulsed laser deposition method. The study
indicates that the low frequency conductivity consists of two Drude components,
a broad one which takes up most of the spectral weight and a very narrow one
roughly below 100-150 \cm. The narrow Drude component locates at so low
frequencies that no such behavior was observed in iron pnictides. The overall
plasma frequency is found to be smaller than the FeAs based compounds,
suggesting a stronger correlation effect. Similar to iron pnictides, a
temperature-induced spectral weight transfer is observed for FeSe. However, the
relevant energy scale is lower. Additionally, different from a recent ARPES
measurement which revealed a spin density wave (SDW) order at low temperature
for FeSe thin films grown on SrTiO$_3$ substrate, no signature of the band
structure reconstruction arising from the formation of the SDW order is seen by
optical measurement in the thick FeSe films.
