  Optical absorption spectra of nanocrystalline cupric oxide CuO samples,
obtained using the converging spherical shock wave procedure, reveal
significant spectral weight red-shift as compared with spectra of single
crystalline CuO samples. In addition, some of these samples manifest the
remarkable temperature-dependent "peak-dip-hump" feature near 1.3-1.6 eV. The
minimal model suggested to explain both effects implies the nanoceramic CuO to
be a system of two species of identical metallic-like droplets with volume
fractions p1 >> p2 and damping parameters gamma1 >> gamma2, respectively,
dispersed in an effective insulating matrix. In other words, both effects are
assigned to the surface plasmon (Mie) resonances due to a small volume fraction
of metallic-like nanoscale droplets with Drude optical response embedded in the
bare insulating medium. Simple effective medium theory is shown to provide the
reasonable description of the experimental spectra.
