By applying the Maxwell Garnett model to gold nanoparticles in water we deduce a value of Im i (3) ϭ 1.1 ϫ 10 Ϫ7 esu for the imaginary part of the cubic susceptibility for gold corresponding to a Fermi smearing mechanism. We also demonstrate a sign reversal in the nonlinear absorption for gold particles in 1, 1Ј, 3, 3, 3Ј, 3Ј-hexamethylindotricarbocyanine iodide. Although the imaginary part of (3) is positive for each component by itself, remarkably the imaginary part of (3) is negative for the

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... colloid as a whole. We show that the nonlinearity of the host must be considered and that the sign reversal in (3) is a result of the fact that at the surface plasmon resonance the local field factor has an imaginary component that arises from a phase shift between the applied field and the local field inside the particle. © 1997 Optical Society of America [S0740-3224(97)03807-1] Smith et al.