Interband transition enhanced third harmonic generation from nanoplasmonic gold

We measure third harmonic generation (THG) as a function of infrared fundamental wavelength for gold nanoparticles over a gold film. An order of magnitude enhancement in THG is found at ~2.5 eV. We ensure that this enhancement is away from plasmonic resonances, so that it may be attributed directly to the ultrafast third-order susceptibility of gold. Using a simple relation between linear and third-order susceptibilities in conjunction with the linear response of gold, we show that the experimental results agree well with the enhancement from interband transitions. This result is interesting for potential applications leveraging telecom-band sources, such as third harmonic deep-tissue imaging with plasmonic nanoparticle markers.