Effects of highly conducting interface and particle size distribution on optical nonlinearity in granular composites

The optical nonlinearity has been investigated in granular metal/dielectric composites taking the effects of highly conducting interfaces between the constituent phases as well as the distribution of particle sizes into account. We compute analytically the spectral function for composites with a binary distribution of particle sizes. For a log-normal distribution of width sigma, numerical results show that the spectral density m(s) changes from a delta function for zero width to a prominent peak, accompanied by a broad spectrum for a finite width sigma. As a result, the locations of the nonlinearity enhancement peak and the absorption spectrum shift to small frequencies with the increase of the interfacial factor I. The strength of the absorption and the optical nonlinearity are always decreased near resonance with the increase of sigma, while the absorption peak and optical nonlinearity peak shift to smaller volume fraction f as I is increased. Moreover, the variation of I will further increase the optical absorption (optical nonlinearity) by more than one (three) order of magnitude, depending on the volume fractions. (C) 2000 American Institute of Physics. [S0021-8979(00)04216-X].