  We present measurements of the optical spectra on Na$_{0.7}$CoO$_2$ single
crystals. The optical conductivity shows two broad interband transition peaks
at 1.6 eV and 3.2 eV, and a weak midinfrared peak at 0.4 eV. The intraband
response of conducting carriers is different from that of a simple Drude metal.
A peak at low but finite frequency is observed, which shifts to higher
frequencies with increasing temperature, even though the dc resistivity is
metallic. We discussed the origin of the interband transitions and the
low-frequency charge dynamics based on the results of band structure
calculations and quasiparticle behavior observed in ARPES experiment.
