Wavelength-Dependent Third-Harmonic Generation in Plasmonic Gold Nanoantennas: Quantitative Determination of the d-Band Influence

Plasmonic gold nanoantennas are highly efficient nanoscale nonlinear light converters. The nano antennas provide large resonant light interaction cross sections as well as strongly enhanced local fields. The actual frequency conversion, however, takes places inside the gold volume and is thus ultimately determined by the microscopic gold nonlinearity, which has been found to significantly surpass common bulk nonlinear materials. While the influence of the nanoantenna geometry and hence the plasmonic resonance has been studied in great detail, only little attention has been paid to the microscopic material nonlinearity. Here we show that the microscopic third-order nonlinearity of gold is in fact a resonant one by virtue of interband transitions between the d- and sp-bands. Utilizing a large set of resonant nanoantennas and a fiber-feedback optical parametric oscillator as a broadband-tunable light source, we show that the radiated third-harmonic signals significantly increase at the onset of interband transitions, namely, as soon as the third harmonic becomes resonant with allowed interband transitions. With the help of an anharmonic oscillator model and independent reference measurements on a gold film we can unambiguously demonstrate that the observed third-harmonic increase is related to a strongly wavelength-dependent microscopic third-order gold nonlinearity, which is additionally underlined by quantitative agreement between simulation and measurement. This additional tuning parameter allows further manipulation and optimization of nonlinear nanoscale systems and thus renders the investigation of other plasmonic materials, especially with interband transitions located in the ultraviolet range, highly intriguing.