Theoretical calculation of enhancement factor of third-order nonlinear susceptibility in gold nanowire and nanotube

The third-order nonlinear optical properties in gold nanowire and nanotube were theoretical studied by using quasi-static approximation. The enhancement factor patterns of the third-order nonlinear optical susceptibility in both gold nanowire and nanotube are anisotropic. For gold nanowire and the symmetric mode of nanotube, the "hot spots" of positive or negative nonlinear enhancement factors occur close to the poles of the wire or tube along the incident polarization direction. Whereas for antisymmetric mode of nanotube, the "hot spots" of positive or negative nonlinear enhancement factors occur within the gold tube and locate close to the poles of the tube along or perpendicular to the incident polarization direction. These calculation results show that the pattern of the third-order nonlinear enhancement factor is controlled by the fluctuation in surface electrons and the distribution of local electric field. These tunable third-order nonlinear optical characteristic will help us to select the best morphology of gold nanotube for the fabrication of smart optical nano-sensors.