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 depen dence are performed on nonlinear organic nanocrystals of 4-(N, N-diethylami no)-beta -nitrostyrene. The in-plane local anisotropies of third-order susc eptibilities are observed, which is revealed to be associated to two-photon allowed pi -electron transition. The local spectra exhibit the crystal sym metry on a nanometric scale. (C) 2001 American Institute of Physics.