Nonlinear optical performance of poled liquid crystalline azo-dyes confined in SiO₂ sonogel films

The catalyst-free sonogel route was implemented to fabricate highly pure, optically active, hybrid azo-dye/SiO₂-based spin-coated thin films deposited onto ITO-covered glass substrates. The implemented azo-dyes exhibit a push–pull structure; thus chromophore electrical poling was performed in order to explore their quadratic nonlinear optical (NLO) performance and the role of the SiO₂ matrix for allowing molecular alignment within the sonogel host network. Morphological and optical characterizations were performed to the film samples according to atomic force microscopy (AFM), ultraviolet-visible (UV-Vis) spectroscopy and the Maker-fringe technique. Regardless of absence of a high glass transition temperature (T _g) in the studied monomeric liquid crystalline azo-dyes, some hybrid films displayed stable NLO activity such as second harmonic generation (SHG). Results show that the chromophores were homogeneously embedded within the SiO₂ sonogel network, where the guest–host molecular and mechanical interactions permitted a stable monomeric electrical alignment in this kind of environment.

Keywords

• hybrid materials,
• thin films,
• azobenzene,
• sol-gel,
• sonogel,
• nonlinear optics