  In this paper, we present a theoretical study of the nonlinear optical
response of the newly discovered conjugated polymer poly(diphenyl)polyacetylene
(PDPA). In particular, we compute the third-order nonlinear susceptibility
corresponding to two-photon absorption process in PDPA using: (a)
independent-particle H\"uckel model, and (b) using the correlated-electron
Pariser-Parr-Pople (P-P-P) model coupled with various configuration-interaction
methodologies such as the singles-configuration-interaction (SCI), the
multi-reference-singles-doubles CI (MRSDCI), and the quadruples-CI (QCI)
method. At all levels of theory, the polymer is found to exhibit highly
anisotropic nonlinear optical response, distributed over two distinct energy
scales. The low-energy response is predominantly polarized in the conjugation
direction, and can be explained in terms of chain-based orbitals. The
high-energy response of the polymer is found to be polarized perpendicular to
the conjugation direction, and can be explained in terms of orbitals based on
the side phenylene rings. Moreover, the intensity of the nonlinear optical
response is also enhanced as compared to the corresponding polyenes, and can be
understood in terms of reduced optical gap.

Mesoscale and Nanoscale Physics