Investigation of the nonlinear refractive index of single-crystalline thin gold films and plasmonic nanostructures

The nonlinear refractive index of plasmonic materials may be used to obtain nonlinear functionality, e. g., power-dependent switching. Here, we investigate the nonlinear refractive index of singlecrystalline gold in thin layers and nanostructures on dielectric substrates. In a first step, we implement a z-scan setup to investigate similar to 100-mu m-sized thin-film samples. We determine the nonlinear refractive index of fused silica, n(2)(SiO2) = 2.9 x 10(-20) m(2)/W, in agreement with literature values. Subsequent z-scan measurements of single-crystalline gold films reveal a damage threshold of 0.22 TW/cm(2) and approximate upper limits of the real and imaginary parts of the nonlinear refractive index, |n(2)'(Au)| < 1.2 x 10(-16) m(2)/W and |n2"(Au)| < 0.6 x 10(-16) m(2)/W, respectively. To further determine possible effects of a nonlinear refractive index in plasmonic circuitry, interferometry is proposed as a phasesensitive probe. In corresponding nanostructures, relative phase changes between two propagating near-field modes are converted to amplitude changes by mode interference. Power-dependent experiments using sub-10-fs near-infrared pulses and diffraction-limited resolution (NA = 1.4) reveal linear behavior up to the damage threshold (0.23 times relative to that of a solid single-crystalline gold film). An upper limit for the nonlinear power-dependent phase change between two propagating near-field modes is determined to Delta phi < 0.07 rad.