Ultrafast Nonlinear Plasmonic Spectroscopy: From Dipole Nanoantennas to Complex Hybrid Plasmonic Structures

The key property of metal nanostructures is their unique ability to channel far-field radiation to subwavelength dimensions. The resulting strongly confined and enhanced electromagnetic fields boost nonlinear optical effects at the nanoscale. In this Review article we highlight and summarize the recent most important investigations and advances in the field of nonlinear plasmonic spectroscopy, and we present results of second- and third-harmonic spectroscopy experiments of plasmonic nanoantenna arrays that consist of different unit cell elements, ranging from dipole nanoantennas to complex hybrid plasmonic structures. The experiments on dipole nanoantennas show that nonlinear optical processes can be enhanced by plasmonic resonances either at the fundamental laser wavelength or at the spectral position of the harmonic signal. Furthermore, we investigate the quadrupolar third-harmonic response of dolmen-type plasmonic Fano structures and find that the third harmonic polarization field of the quadrupolar mode does not radiate to the far-field due to destructive interference. Finally, we incorporate indium tin oxide nanocrystals into the hot-spot of plasmonic gap-antennas and find a doubling of the third-harmonic response of the hybrid antennas when compared to bare gold gap-antennas. The experimental results of the nanoantenna arrays can be modeled and understood using a classical model of anharmonic oscillators and are supported by finite element simulations. Parts of this Review article are based on previous publications.(1-4)