Resonant and off-resonant light-driven plasmons in metal nanoparticles studied by femtosecond-resolution third-harmonic generation

We report on a femtosecond-resolution study of the plasmon fields in gold n anoparticles using third-harmonic generation. Controlled resonant and off-r esonant plasmon excitation is achieved by tailoring the nanoparticle sample by an electron-beam-lithographic method. Comparing the measured third orde r interferometric autocorrelation function of the plasmon field with simula tions based on a simple harmonic oscillator model we extract the temporal c haracteristic of the plasmon oscillation. For off-resonant excitation of pa rticle plasmons we find a beating between the driving laser field and the p lasmon Field which demonstrates clearly the nature of the plasmon as a coll ective electron oscillation.