Probing interface properties of nanocomposites by third-order nonlinear optics

We describe the exploitation of third-order nonlinear optical response — particularly nonlinear absorption and the nonlinear index of refraction — to probe interface dynamics, modifications and relaxation processes in granular materials consisting of metal quantum dots embedded in such dielectrics as fused silica and sapphire. Many features of these materials can be interpreted in terms of the quantum-mechanical model of the particle-in-a-box. Electronic and thermal relaxation processes in these novel nanocomposites are dominated by interactions of conduction-band electrons at the boundary between the quantum dot and its surrounding host material. Experimental examples presented include measurements of thermal and electronic relaxation rates, dephasing due to electron collisions at the nanocluster surface, effects of local structural order, changes in the saturation parameter due to chemical modification of the substrate, and one-and two-dimensional heat-transfer effects.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994.