LAUSR

Plasmonic gold nanoantennas are highly efficient nanoscale nonlinear light converters. The nanoantennas 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 tran...