Third-Order Optical Nonlinearities of Novel Phthalocyanines and Related Compounds

Abstract

The field of nonlinear optics has been developing for a few decades as a promising field with important applications in the domain of optoelectronics and photonics. Materials that exhibit nonlinear optical (NLO) behavior are useful because they allow manipulation of the fundamental properties of laser light beams, and are hence of great technological importance in areas such as photonic switching, optical computing and other optical data processing systems (Perry et al., 1994, 1996; Shirk et al., 2000). NLO activity was first found in inorganic crystals (Zyss, 1994), such as LiNbO3, but the choice of these materials is rather limited. Also, most of them have either low NLO responses or important drawbacks for processing into thin films and being incorporated into micro-optoelectronic devices. By the mid-1980s, organic materials emerged as important targets of choice for nonlinear optical applications because they exhibit large and fast nonlinearities and are, in general, easy to process and integrate into optical devices. Moreover, organic compounds offer the advantage of tailorability: a fine-tuning of the NLO properties can be achieved by rational modification of the chemical structure. Finally, they are ideal to achieve the ultimate goal of device miniaturization by going into the molecular level. Large optical nonlinearities in organic molecules usually arise from highly delocalized electron systems. During the last two decades, phthalocyanines (Pcs) have been intensively investigated for their third-order NLO properties both in solutions and as thin films because of their extensively delocalized two dimensional 18-electron system (Ho, 1987; Shirk, 1989; Kambara, 1996; Ma, 2003; Zhou, 2004; He, 2007). They also exhibit other additional advantages, namely, exceptional stability, versatility, and processability features. The architectural flexibility of phthalocyanines is well exemplified by the large number of metallic complexes described in the literature, as well as by the huge variety of substituents that can be attached to the phthalocyanine core. Furthermore, some of the four isoindole units can be formally replaced by other heterocyclic moieties, giving rise to different phthalocyanine analogues. All these chemical variations can alter the electronic structure of the macrocyclic core, and therefore, they allow the fine-tuning of the nonlinear response. Aside from their practical interest, Pc-related molecules present very attractive features for fundamental NLO studies. Since the unsubstituted and many substituted compounds are planar (2D, two dimensional), they offer the possibility of investigating of the role of