  Recently, a homogenization procedure has been proposed, based on the tight
lower bounds of the Bergman-Milton formulation, and successfully applied to
dilute ternary nanocomposites to predict optical data without using any fitting
parameters [Garcia et al. Phys. Rev. B, 75, 045439 (2007)]. The procedure has
been extended and applied to predict the absorption coefficient of a quaternary
nanocomposite consisting of Cu, Ag, and Au nanospheres embedded in a SiO2 host
matrix. Significant enhancement of the absorption coefficient is observed over
the spectral range 350-800 nm. The magnitude of this enhancement can be
controlled by varying the nanosphere diameter and the individual metal volume
fraction with respect to the host matrix. We have determined the optimal
composition resulting in enhanced broadband (350nm-800nm) absorption of the
solar spectrum using a simulated annealing algorithm. Fabricating such
composite materials with a desired optical absorption has potential
applications in solar energy harvesting.
