Optical bistability and tristability in nonlinear metal/dielectric composite media of nonspherical particles.

Based on a spectral representation method and a self-consistent mean field theory, we present a general framework to investigate the optical bistability in a nonlinear two-phase composite, where spheroidal metallic inclusions are randomly oriented and embedded in a dielectric host. The relation between the spatial average of the local field squared <[absolute value of E](2)>(i) (i=1,2) and the external field squared E20 is obtainable through the spectral density function which is predicted from our recently derived Maxwell-Garnett approximation. In addition to single optical bistability (OB), the appearance of double OB and optical tristability (OT) is reported, and the corresponding phase diagram is given. We find that the regions of the single OB, the double OB, and the OT are dependent on the shape and volume fraction of the metallic particles. Our method allows us to take one step forward to study some field-dependent effective optical properties, such as the refractive index, extinction coefficient as well as reflectance. The general framework is also applied to investigate exactly the solvable composites consisting of nonlinear spheroidal inclusions and linear dielectric host in the dilute limit. To this end, the present method is shown to be in excellent agreement with the exact solution. In addition, the present method predicts a larger threshold intensity than the variational approach.