Molecular Optics Nonlinear Optical Processes in Organic and Polymeric Crystals and Films. Part 2
Final rept. 15 Jul 1990-14 Jan 1991,
PENNSYLVANIA UNIV PHILADELPHIA DEPT OF PHYSICS
Pagination or Media Count:
Comprehensive theoretical and experimental studies of the magnitude, sign, dispersion, and length dependence of the third order molecular susceptibility gamma ijkl-omega 4 omega 1, omega 2, omega 3 demonstrate that the microscopic origin of the nonresonant third order nonlinear optical properties of conjugated linear chains is determined by the effects of electron correlation due to electron-electron repulsion. Multiple-excited configuration interaction calculations of gamma jkl-omega 4 omega 1, omega 2, omega 3 for the archetypal class of quasi-one dimensional conjugated structures known as polyenes reveal for the first time the principal role of strongly correlated, energetically high-lying, two photon 1 Ag virtual states in the largest of the two dominant, competing virtual excitation processes that determine gamma ijkl- omega 4 omega 1, omega 2, omega 3. It is also found in studies of the effects of conformation on gamma ijkl-omega 4 omega 1, omega 2, omega 3 that the origin of the third order optical properties remains basically the same for the all-trans and cis-transoid polyenes, and the results for the two conformations are unified by a common power law dependence of the dominant tensor component gamma xxxx-omega 4 omega 1, omega 2, omega 3 on the physical end-to-end length L of the chain with an exponent of 3.5. Calculations for a noncentrosymmetric conjugated chain demonstrate that virtual excitation processes involving diagonal transition moments that are forbidden in centrosymmetric structures lead to a more than an order of magnitude enhancement in gamma xxxx-omega 4 omega 1, omega 2, omega 3 compared to the analog centrosymmetric structure.