Nonlinear optics on the nanoscale
The optics of media of dimensions small compared to the optical wavelength is characterized by distinctive phenomena such as optical field confinement and structural resonances. With a strong focus in research on the linear optical processes of nanostructures and colloids, the nonlinear optical properties have remained comparably unexplored. The nonlinear response, however, differs fundamentally due to its high symmetry selectivity. This can be explored, e.g., in second-harmonic generation (SHG), to probe different source polarizations which sensitively depend on dimensions and overall symmetry of the nanostructures.
In second-harmonic generation (SHG) and sum-frequency generation (SFG) both local surface dipolar and higher order nonlocal bulk terms contribute to the overall second-order souce polarization. Their separation has been a long standing problem in surface nonlinear optics because of the fundamental importance for proper signal assignment.
We demonstrated that on the nanoscale the independent emission of the different nonlinear source polarizations (nonlinear Rayleigh scattering) allows the distinct observation of local surface and nonlocal bulk contributions for partially asymmetric nanostructures. This is readily possible for approriate selection of polarization and detection directions:
Unique for the asymmetric nanostructure are configuration without any mirror plane such as this crossed saggitalin → sagittalout configuration. Here, simply by selecting the detection polarization parallel or perpendicular to the tip axis, emission from the bulk or surface term, respectively, can be selected:
The table summarizes the SH-intensities and their assignment to the respective source terms for this configuration:
We take advantage of these results for simultaneous probing of the changes in the bulk and surface response of nanoparticles during molecular adsorption. In addition, the symmetry properties of the tip-sample configuration are used for s-SNOM SHG imaging.