Directional Second Harmonic Generation Controlled by Sub-wavelength Facets of an Organic Mesowire

TL;DR

This study demonstrates how sub-wavelength facets of organic mesowires can be used to control the directionality of second harmonic generation, with potential applications in nonlinear optical devices and signal processing.

Contribution

We show systematic control of SHG directionality in organic mesowires through facet engineering and provide numerical and experimental insights into the underlying mechanisms.

Findings

Radial angles of SHG emission can be tuned over 130 degrees.

Directional SHG is influenced by mesowire geometry and facet orientation.

Optical waveguiding of TPEF is observed, with SHG being localized and directional.

Abstract

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.