Second-harmonic near-field imaging of ferroelectric domain structure of YMnO3

Near-field tip-enhanced and symmetry-selective nonlinear light scattering is demonstrated for imaging nanoscopic ferroelectric order. The surface topology of antiparallel ferroelectric domains in multiferroic YMnO3 is spatially resolved by second-harmonic generation (SHG). Optical phase contrast is obtained by interference between the local near-field SHG and a self-homodyne far-field reference SHG field. Ferroelectric domains anisotropically elongated along the hexagonal crystallographic axis were found, with typical dimensions of similar to 1000x200 nm(2). The irregular domain shapes seem to contrast the behavior of ordinary ferroelectrics. Relying on symmetry properties only, SHG scattering-type scanning near-field optical microscopy can be extended to probe any ferroic order.