Lineshape of harmonic generation by metallic nanoparticles and metallic photonic crystal slabs

We study the linear- and nonlinear-optical lineshapes of metal nanoparticles (theory) and metallic photonic crystal slabs (experiment and theory). For metal nanoparticle ensembles, we show analytically and numerically that femtosecond second- or third-harmonic-generation (THG) experiments together with linear extinction measurements generally do not allow to determine the homogeneous linewidth. This is in contrast to claims of previous work in which we identify a technical mistake. For metallic photonic crystal slabs, we introduce a simple classical model of two coupled Lorentz oscillators, corresponding to the plasmon and waveguide modes. This model describes very well the key experimental features of linear optics, particularly the Fano-like lineshapes. The derived nonlinear-optical THG spectra are shown to depend on the underlying source of the optical nonlinearity. We present corresponding THG experiments with metallic photonic crystal slabs. In contrast to previous work, we spectrally resolve the interferometric THG signal, and we additionally obtain a higher temporal resolution by using 5 fs laser pulses. In the THG spectra, the distinct spectral components exhibit strongly different behaviors versus time delay. The measured spectra agree well with the model calculations.