  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.
