Measuring plasmon-resonance enhanced third-harmonic χ(3) of Ag nanoparticles

By coinciding the plasmon frequency with the third-harmonic frequency of the excitation light, the authors determined the plasmon-resonance enhanced optical third-harmonic-generation (THG) susceptibility of a polyvinylpyrrolidone-coated Ag nanoparticle with a $5–7\mathrm{nm}$ diameter. With dispersed Ag nanoparticles on a quartz surface and through measuring the frequency dependent THG intensities, interface THG showed evident enhancement when the third harmonic of excitation matched the Ag-nanoparticle’s plasmon-resonant frequency. According to the effective medium theory and by analyzing the interface THG under focused Gaussian beams, the ensemble-averaged ${\chi }^{\left(3\right)}(3\omega :\omega ,\omega ,\omega )$ of a Ag nanoparticle can be estimated to be on the order of $2\times {10}^{-11}\mathrm{esu}$ .