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Plasmonic nanoantennas offer the possibility to confine light in sub-wavelength volumes and to strongly enhance local fields. The latter is highly beneficial for nonlinear optics, as the efficiency of second- and third-order nonlinear processes increases with increasing field strength. While third-order processes may occur in any material, symmetry breaking is required for second-order processes. This is achieved by utilising materials with non-centrosymmetric unit cells or by exploiting the symmetry breaking of surfaces when using centrosymmetric materials. In this work, gold nanoantennas with a strong surface nonlinearity and broad plasmonic resonances at 1500 nm and 750 nm are combined such that they efficiently couple to far-field radiation, even for second-order processes, thanks to a non-centrosymmetric arrangement of the antennas. The key idea of this work is to use this structure to support two nonlinear processes of different order for characterising two unknown pulses at the same time, which is in contrast to conventional techniques where nonlinear crystals that are optimised for only one specific process allow for the characterisation of only one pulse at a time, either by nonlinear interaction with itself or with another known reference pulse. Here, Sum-Frequency generation (SFG) and Four-Wave-Mixing (FWM) spectra from the structure were plotted against the time delay between the interacting input pulses to yield a double spectrogram with enough independent information to simultaneously characterise both pulses. Advantages of pulse characterisation with our plasmonic nanoantennas are the broad spectral range - the pulses were separated by almost an octave - and relaxed phase-matching constraints in the sub-wavelength interaction volume. The main limitations are the damage threshold of the particles and the plasmonic dephasing time of about 10 fs.
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