  Optical conductivity s(w) of PrRu4P12 has been studied under high pressure to
14 GPa, at low temperatures to 8 K, and at photon energies 12 meV-1.1 eV. The
energy gap in s(w) at ambient pressure, caused by a metal-insulator transition
due to an unconventional charge-density-wave formation at 63 K, is gradually
filled in with increasing pressure to 10 GPa. At 14 GPa and below 30 K, s(w)
exhibits a pronounced Drude-type component due to free carriers. This indicates
that the initial insulating ground state at zero pressure has been turned into
a metallic one at 14 GPa. This is consistent with a previous resistivity study
under pressure, where the resistivity rapidly decreased with cooling below 30 K
at 14 GPa. The evolution of electronic structure with pressure is discussed in
terms of the hybridization between the 4f and conduction electrons.
