Size-and shape-dependent nonlinear optical response of Au nanoparticles embedded in sapphire

Nonlinear optical response of Au metallic nanoparticles, synthesized and embedded in sapphire by using ion implantation, as a function of their size and shape is studied. The size of the Au NPs was varied by controlling the annealing time of the gold-irradiated sapphire in a reducing atmosphere. Their shape was changed from approximately spherical to prolate by swift heavy-ion irradiation using Si3+, obtaining an anisotropic composite consisting in deformed NPs, all oriented in the direction of the Si beam irradiation. At 532 nm and 26 ps pulses, the isotropic system shows negative nonlinear absorption increasing with size, and positive nonlinear refraction. On the other hand, prolate nanoparticles show negative (null) absorption and null (positive) refraction for the minor (major) axis. This kind of system also shows figures of merit and relaxing times in the order of the picoseconds, appropriate for all-optical switching applications.