Large Third-Order Optical Susceptibility with Good Nonlinear Figures of Merit Induced by Octupole Plasmon Resonance of Asymmetric Au–Ag Core–Shell Nanorods

The Au–Ag core–shell nanorods with asymmetric transverse cross section are synthesized, and their tunable linear and nonlinear optical responses are investigated. After the overgrowth of the asymmetric Ag shell on the Au nanorods, (i) a strong transverse octupole (TO) plasmon resonance at 390 nm is observed; (ii) the nonlinear refractive index γ at the TO resonance wavelength significantly increases from −0.15 × 10–4 to −0.85 × 10–4 cm2/GW, and the γ at the longitudinal dipole (LD) resonance wavelength slightly increases from −0.42 × 10–4 to −0.58 × 10–4 cm2/GW; (iii) but the nonlinear absorption coefficient β induced by the TO resonance is approximately 1 order of magnitude smaller than the one induced by the LD resonance. Consequently, the asymmetric Au–Ag core–shell nanorods at the TO resonance wavelength demonstrate a very large nonlinear refractive index as well as excellent nonlinear figures of merit (W = 4.4 > 1 and T = 0.14 < 1), which satisfy the demands for waveguide all-optical switching.