Extraordinarily Large Third-Order Optical Nonlinearity in Au Nanorods under Nanowatt Laser Excitation

Size and operating energy are two key parameters that constrain the integration capability of a nonlinear optical device. Here, we report a 60 nm thin film made of randomly distributed gold nanorods that demonstrates extraordinarily large third-order nonlinear optical absorption under an excitation power of as low as 100 nW, where the effective nonlinear optical absorption (NLA) coefficient is calculated to be 8 x 10(9) cm/GW with the corresponding optical susceptibility (Im chi((3))) of 7.6 x 10(-2) esu, which is 3 to 4 orders of magnitude larger than those of other metallic nanostructures. The sign of the NLA could be controlled by changing the excitation wavelength or varying the pump power. The optical Kerr effect measurement also shows that the intrinsic chi((3)) of our sample is quite large compared to the values of other nanostructures. Our work eliminates the requirements of strong operation power and the large physical size of nonlinear optical materials and offers new opportunities for plasmonic nanostructures in applications of on-chip nonlinear optical devices.