  We perform an in-plane optical spectroscopy measurement on high quality FeSe
single crystals grown by a vapor transport technique. Below the structural
transition at $T_{\rm s}\sim$90 K, the reflectivity spectrum clearly shows a
gradual suppression around 400 cm$^{-1}$ and the conductivity spectrum shows a
peak at higher frequency. The energy scale of this gap-like feature is
comparable to the width of the band splitting observed by ARPES. The
low-frequency conductivity consists of two Drude components and the overall
plasma frequency is smaller than that of the FeAs based compounds, suggesting a
lower carrier density or stronger correlation effect. The plasma frequency
becomes even smaller below $T_{\rm s}$ which agrees with the very small Fermi
energy estimated by other experiments. Similar to iron pnictides, a clear
temperature-induced spectral weight transfer is observed for FeSe, being
indicative of strong correlation effect.
