Nonlinear optical materials through weak interactions and their application in 3D printing

Abstract

Nonlinear optical (NLO) materials are highly coveted for their ability to change the properties of incident radiation as this feature has a variety of applications in laser technology and optoelectronics. The primary focus of this study was second harmonic generation (SHG), also known as frequency doubling, which is one of the most sought-after and widely studied NLO phenomena. In this work, several different approaches were used to study nonlinear optical materials and their potential applications. First, noncovalent interactions were investigated as a way to influence the packing of the molecules within crystals to achieve favourable, non-centrosymmetric structures necessary for second harmonic generation. Halogen bonding was studied via reactions between simple di- and interhalogens and 4-aminopyridine. Instead of forming co-crystal adducts, the halogens were observed to favour the formation of iodonium-bridged bis(pyridines) or protonation of the pyridine ring, leading to solely centrosymmetric structures. Next, a variety of dipolar stilbenes and their diaromatic derivatives were synthesized to investigate the effects of structural changes like doubling the length of the ethylene bridge and replacement of one aromatic ring with thiophene. While most of the modifications led to centrosymmetric structures, the simple substituent changes afforded a compound with a significantly higher second harmonic generation intensity than the reference material urea. Lastly, a novel method of utilizing optically active materials within transparent 3D printed objects was investigated. Powdered NLO active materials were mixed with a photopolymerizable resin, which was then used to print simple lenses via stereolithography. The lenses were able to generate the second harmonic, and its intensity was found to strengthen almost linearly with the increasing thickness of the 3D printed lens. This method offers a fast and simple way of manufacturing highly customizable functional objects for optical applications.