A macroscopic enhancement of surface second harmonic generation in polymer nanofibers with embedded para-nitroaniline nanocrystals

Assembling nanomaterials with a strong nonlinear optical (NLO) response is the key to many photonic applications. Currently the majority of the nonlinear photonic components are based on inorganic optical materials. However,... [ view full abstract ]

Assembling nanomaterials with a strong nonlinear optical (NLO) response is the key to many photonic applications. Currently the majority of the nonlinear photonic components are based on inorganic optical materials. However, problems persist with these materials such as the magnitude of their optical nonlinearities together with the cost of fabrication. Organic systems are an attractive alternative because of their low cost, fast and intense nonlinear response over a broad frequency range and the wide scope for molecular tunability.
The challenge for organic NLO materials has been to transfer the strong individual molecular NLO response into a macroscopic material that is stable and resistant to intense laser irradiation. For molecules like para-nitroaniline (pNA) that have a large molecular first hyperpolarizability quadratic nonlinear optical phenomena are forbidden in bulk, due to the crystalline centrosymmetric structure.The strong interaction between the large dipole moments, typical of these molecules, favours a pairwise antiparallel configuration that cancels out the second order NLO response. However, a medium that is centrosymmetric in the bulk necessarily loses that symmetry at the surface.
The particular case of SHG generated at surface of pNA has been described, theoretically, by Munn and his co-workers. To promote the enhancement of the surface second order NLO effects, polymeric nanofibers are used as scaffolds where the organic molecules are arranged into suitable organizations. These hybrid organic nanofibers created by electrospinning are extremely promising in terms of amount of produced nanomaterials, low cost, and operational convenience. These supramolecular systems are fabricated on a 1D sub-wavelength scale, where the strong electric field, used in the deposition, and the rapid solidification forces the alignment of the organic material to add up constructively. Polarimetry analysis (Figure 1) demonstrates well-oriented dipole moments inside the nanostructure, while the spectrum of the SHG is wide-band indicating the extreme thinness of the active sections. X-Ray analysis and the analytical data fit, according to Munn approach, were used as tools to reveal the molecular arrangement within the symmetry plane which induces a strong macroscopic surface SHG response.