Towards a microscopic description of the optical nonlinearities of gold-based plasmonic devices

Surface plasmon polaritons (SPPs) constitute the best candidates for manipulating light on the nanoscale and for the development of subwavelength all-optical devices. Experimental studies on metallic surfaces indicate strong third-order nonlinear susceptibilities that vary by several orders of magnitude. Such an enormous spectral variation suggests that the underpinning nonlinear mechanism is resonant at optical frequencies. Theoretical and experimental confirmation of this hypothesis is to be found in the results of Rosei et al. on the thermo-modulational reflection spectra of thin films of noble metals. The light-induced heating of the conduction electrons smears out their energy distribution, affecting the resonant interband absorption and hence the dielectric susceptibility. This process is intrinsically nonlinear, since the increase of temperature, which modulates the dielectric response of gold, depends on the optical power. Very recently, a complete analysis of the nonlinear optical response of noble metals, leading to the first theoretical derivation of a consistent model for the third-order nonlinear susceptibility of gold. Following the semi-classical approach, the spectral dependence of the thermo-modulational interband nonlinear susceptibility can be calculated directly from the band structure of gold. The nonlinear optical propagation along a gold nanowire can be modeled by a nonlinear Schrodinger equation for the field envelope ψ.