The higher symmetry selectivity of nonlinear optics allows for the specific selection and probing of a subsystem of a macroscopic ensemble otherwise not possible with linear optics. In the first example I show the use of second-harmonic generation (SHG) as an all-optical, surface specific probe of spectral hole burning to measure the ultrafast dynamics of electronic surface state excitations of the Si(111)7x7 surface. Strong excitation-induced dephasing is observed as short as 15 fs and the coupling between localized dangling bond state and associated surface phonon mode at 570 cm^-1 is revealed.

In the second part, combining the new technique of scattering near-field microscopy with SHG we achieved high resolution nonlinear imaging of ferroelectric domains by taking advantage of the optical antenna properties of the nanoscopic scanning metal tip to select the tensor component of the nonlinear suceptibility that is sensitive with respect to the ferroelectric order. This allowed for the first domain study of the magnetoelectric multiferroic YMnO_3.