  Directional harmonic generation is an important property characterizing the
ability of nonlinear optical antennas to diffuse the signal in well-defined
region of space. Herein, we show how sub-wavelength facets of an organic
molecular mesowire crystal can be utilized to systematically vary the
directionality of second harmonic generation (SHG) in the forward scattering
geometry. We demonstrate this capability on crystalline diamonoanthraquinone
(DAAQ) mesowires with subwavelength facets. We observed that the radial angles
of the SHG emission can be tuned over a range of 130 degrees. This angular
variation arises due to spatially distributed nonlinear dipoles in the focal
volume of the excitation as well as the geometrical cross-section and facet
orientation of the mesowire. Numerical simulations of the near-field excitation
profile corroborate the role of the mesowire geometry in localizing the
electric field. In addition to directional SHG from the mesowire, we
experimentally observe optical waveguiding of the nonlinear two-photon excited
fluorescence (TPEF). Interestingly, we observed that for a given pump
excitation, the TPEF signal is isotropic and delocalized, whereas the SHG
emission is directional and localized at the location of excitation. All the
observed effects have direct implications not only in active nonlinear optical
antennas, but also in nonlinear signal processing.
