Nonlinear Optical Properties

Abstract

In the presence of a sufficiently intense light, the optical properties of a polymer may become measurably nonlinear, leading to field‐dependent changes in the frequency response, the refractive index, and/or the absorption coefficient of the polymer. This article introduces nonlinear optical phenomena and the structural properties that enhance a material's nonlinear optical response. Second‐and third‐order nonlinear optical (NLO) phenomena are discussed, with a primary emphasis on harmonic generation, intensity‐dependent refractive index, and nonlinear, multiphoton, absorption. Macroscopic nonlinear phenomena are related to the molecular properties of the materials. Sum‐over‐states models are used to understand the fundamental molecular parameters affecting the nonlinear response, particularly π‐electron conjugation, bond length alternation, and donor–acceptor substitution. Measurement techniques used to characterize a material's nonlinear response are surveyed.

Wavelength Dependence of the Complex Third-Order Nonlinear Optical Susceptibility of Poly(3-hexylthiophene) Studied by Femtosecond Z-Scan in Solution and Thin Film

“…In addition, the effective conjugation length of the polymer strongly affects its absorption spectra and the NLO properties. − It was shown that for oligothiophenes the hyperpolarizability γ increases exponentially with the chain length; however, the NLO response undergoes fast saturation as the number of the repeat units increases further. − From DFWM measurements of various length oligothiophenes, Zhao et al predicted that to achieve optimally high values of χ (3) it is sufficient to use oligomers with 9–10 repeating units. Later this conclusion was verified by Thienpont et al who measured a series of oligothiophenes with side alkyl chains, with various number of repeat units.…”