Fig. 1.
Characterization of spectral amplitude and phase in nano-antenna hotspots. (A) A gold nanoantenna (Inset) is excited with a Fourier limited 15-fs pulse with depicted spectrum and residual phase Φresidual that is flat within 0.05π rad. (B) A series of cosinusoidal phases with shifting offset δ is added to the pulse in double-pass 4f-pulse shaper. (C) The antenna resonance adds a particular phase to the field in a hotspot. (D) The shifting cosinusoidal phase samples the antenna phase, which gives an antenna signature that can be fitted to retrieve the phase profile (see also Fig. S1).