Near-field microscopy and spectroscopy of third-harmonic generation and two-photon excitation in nonlinear organic crystals

Near-field microscopy and spectroscopy of third-harmonic generation and two-photon excitation are demonstrated on a subdiffraction-limited scale. Near-field measurements of both the intensity dependence and the spectral dependence are performed on nonlinear organic nanocrystals of 4-(N, N-diethylamino)-β-nitrostyrene. The in-plane local anisotropies of third-order susceptibilities are observed, which is revealed to be associated to two-photon allowed π–electron transition. The local spectra exhibit the crystal symmetry on a nanometric scale.