Double-band enhancement of effective third-order nonlinear susceptibility in gold-dielectric-gold multilayer nanoshells

The effective third-order nonlinear susceptibility χ_eff ⁽³⁾ of gold-dielectric-gold three-layer nanoshells was theoretically studied using the quasi-static approximation. Because of the physical origin from the SPR-induced local field enhancement, both the real and imaginary parts of the enhancement factor of χ_eff ⁽³⁾ present maximum responses near the plasmon wavelengths. By altering the geometry parameters and local dielectric environment of the nanoshells, the intensity, wavelength position, and band number of the χ_eff ⁽³⁾ could be fine tuned. The mode transformation of the χ_eff ⁽³⁾ from single band to double band has been observed as the separate layer dielectric constant is increased or the inner gold sphere radius is increased. The band number of χ_eff ⁽³⁾ also depends on the thickness of the outer gold shell. However, only single maximum response band of χ_eff ⁽³⁾ could be observed when the shell thickness is too small or too large.