Abstract

Metal nanocomposites have been studied intensively as media possessing high optical nonlinearities with short response times.^1–5 The response time was measured by the pulse delay technique.^1–3 The authors⁴ observed a sign reversal of nonlinear absorption of gold particles in dye solutions when the concentration of gold increased. High values of the nonlinear susceptibility, χ⁽³⁾, are the result of local field effects. The local field is enhanced at the surface plasmon resonance of a single metal nano-particle.^1,3,4 Much greater enhancement factor can be obtained when particles aggregate into fractal clusters.² In fractals, collective plasmon oscillations are strongly affected by the fractal morphology, leading to the existence of localized eigenmodes which appear as areas of high and low local fields.⁵ In many cases, local enhancements in "hot" spots exceed the average surface enhancement by many orders of magnitude. The localized eigenmodes provide optical absorption in a wider spectral range than single-particle plasmon resonance. The enhancement factor was predicted to increase in the longwavelength range.^2,5

© 1999 Optical Society of America