Dispersion curves of complex third-order optical susceptibilities around the surface plasmon resonance in Ag nanocrystal-glass composites

We have investigated dispersion curves of a third-order nonlinear optical susceptibility, chi((3)), around the surface-plasmon resonance energy for a Ag nanocrystal-glass composite by means of z-scan and degenerate four-wave mixing measurements. Real and imaginary parts of chi((3)) of the composite that contains Ag nano-crystals with an average diameter of 7 nm have been measured at room temperature. The imaginary part of chi((3)) exhibits a negative peak near the surface-plasmon resonance, and the largest negative value is -(1.5 +/- 0.2) X 10(-10) electrostatic units (esu). The real part of chi((3)) changes from -(4 +/- 2) X 10(-11) esu at the lower energy side of the surface-plasmon peak to +(4.4 +/- 0.5) X 10(-11) esu at the higher energy side. We calculated dispersion curves for the composite assuming that the real and the imaginary parts of the dielectric constant of metal nanocrystals change by optical excitations that are due to the creation of hot electrons. The obtained dispersion curves show a characteristic feature that differs from the exciton system in semiconductors. (C) 2003 Optical Society of America.