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
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