Donor-Acceptor Conjugated Polymers and Their Nanocomposites for Photonic Applications

Abstract

Organic materials exhibiting strong nonlinear optical (NLO) properties have attracted considerable interest in recent years because of their promising applications in opto-electronic and all-optical devices such as optical limiters, optical switches and optical modulators (Munn & Ironside, 1993; Zyss, 1994). A variety of organic materials, including conjugated molecules, polymers and dyes, have been investigated for their NLO responses (Kamanina, 1999, 2001; Kamania & Plekhanov, 2002; Kamanina et al., 2008, 2009). Conjugated organic polymers have emerged as a promising class of NLO materials because of their large nonlinear responses associated with fast response time, in addition to their structural variety, processability, high mechanical strength, and excellent environmental and thermal stability (Prasad & Williams, 1992). In contrast to misconceptions about the frailty of simple organic molecules, the optical damage threshold for polymeric materials can be greater than 10 GW/cm2. Among various conjugated materials, thiophene based polymers are currently under intensive investigation as materials for nonlinear optics because of their large third-order nonlinear response, chemical stability and their readiness of functionalization (Kishino et al., 1998; Nisoli et al, 1993; Sutherland, 1996, Udayakumar et al., 2006). It has been well-known that, the strong delocalization of -electrons along the backbone of conjugated polymers determines very high molecular polarizability and thus causes remarkable optical nonlinearities. However, a necessary step in further improving the NLO properties of conjugated polymers is to understand the fundamental relationship that exists between the molecular structure and the hyperpolarizabilities. A deeper understanding in this subject will improve the design of organic conjugated molecules and polymers by a judicious choice of functional substituents to tune their optical properties for photonic applications. Cassano et al. had reported a strategy for tuning the linear and nonlinear optical properties of soluble poly(paraphenylenevinylene) derivatives, based on the effect of simultaneous presence of electron-acceptor and electron-donor units in the conjugated backbone (Cassano, 2002). Particularly, they reported that the value of third-order nonlinear susceptibility (χ(3)) obtained for the