Organic materials for second-order nonlinear optics: Approaches to optimizing and stabilizing nonlinear optical effects

The main objective of this research has been to establish the potential of nonlinear optics (NLO) to study relationships between chemical structure and properties of organic materials. Using properly constructed molecules or photoactive dopants, phenomena such as molecular relaxation, dipolar ordering, poling dynamics, and intermolecular forces in organic materials were investigated. A second objective has been to identify mechanisms to optimize and stabilize second harmonic generation (SHG) signals through materials processing techniques. This investigation includes studies in three main topics: nonlinear optical properties of magnetically aligned alloys of nematic polymers and organic dyes, two-dimensional polymers, and novel single crystal films that exhibit zero-field SHG.; The second order nonlinear optical response of a solidified nematic polymer containing a photoactive organic dye as a dissolved solute or as phase separated crystals was investigated upon exposure to a magnetic field. The magnetically aligned nematic alloy can be regarded as an "Ising-like" medium in which the nematic and the magnetic fields confine the dipolar dye molecules along directions parallel or anti-parallel to the external DC field. An increase of the second order susceptibility by a factor of five in an idealized system has been predicted by theory. Relative to theoretical predictions, magnetically induced order in the nematic solvent was found to result in second harmonic signals which are 6-9 times more intense and the second order susceptibility of the system triples.; Due to the randomization of polar ordering after poling, the stability of SHG activity in the guest-host system was difficult to maintain. Thus, a special molecular architecture, i.e. a two-dimensional (2D) polymer, was investigated to address the stability problem. This investigation revealed that a chiral oligomer can be transformed into a two-dimensional structure through a dual chemical reaction. Homochiral recognition among strongly dipolar cyano groups was found to play an important role in the transformation of the oligomer to a 2D polymer. In contrast to its analogous comb polymer synthesized by free radical polymerization, this 2D polymer shows remarkable stability in SHG activity. Thermally stimulated discharge (TSD) results suggest that higher discharge currents correlate with higher values of {dollar}chisp{lcub}(2){rcub}{dollar}. Some derivative compounds of the 2D precursor exhibited enormous and stable SHG activity without poling, suggesting that the molecules crystallize into highly polar noncentrosymmetric structures. Through a series of analyses for the relationship between the chemical structure and SHG activity, a novel way to prepare promising SHG films was developed in one of the derivative compounds. The origin of this extraordinary NLO property was due to the formation of an active phase possibly consisting of aligned single crystals. This study shows the importance of materials processing and molecular tailoring in the development of NLO-active materials.