Theory of nonlinear optical properties of phenyl-substituted polyacetylenes

TL;DR

This paper provides a theoretical analysis of the third-order nonlinear optical properties of phenyl-substituted polyacetylenes, focusing on third-harmonic-generation, using both independent electron and correlated-electron models to reveal anisotropic responses.

Contribution

It introduces a comprehensive theoretical framework combining different models and CI methods to study the nonlinear optical properties of PDPA, highlighting anisotropic responses at various energy scales.

Findings

Material exhibits highly anisotropic nonlinear optical response.

Low-energy response is predominantly longitudinal, similar to polyenes.

High-energy response is mainly transverse, similar to trans-stilbene.

Abstract

In this paper we present a theoretical study of the third-order nonlinear optical properties of poly(diphenyl)polyacetylene (PDPA) pertaining to the third-harmonic-generation (THG) process. We study the aforesaid process in PDPA's using both the independent electron Hueckel model, as well as correlated-electron Pariser-Parr-Pople (P-P-P) model. The P-P-P model based calculations were performed using various configuration interaction (CI) methods such as the the multi-reference-singles-doubles CI (MRSDCI), and the quadruples-CI (QCI) methods, and the both longitudinal and the transverse components of third-order susceptibilities were computed. The Hueckel model calculations were performed on oligo-PDPA's containing up to fifty repeat units, while correlated calculations were performed for oligomers containing up to ten unit cells. At all levels of theory, the material exhibits highly anisotropic nonlinear optical response, in keeping with its structural anisotropy. We argue that the aforesaid anisotropy can be divided over two natural energy scales: (a) the low-energy response is predominantly longitudinal and is qualitatively similar to that of polyenes, while (b) the high-energy response is mainly transverse, and is qualitatively similar to that of trans-stilbene.