DISPERSION MEASUREMENTS OF ELECTRIC-FIELD-INDUCED 2ND-HARMONIC GENERATION AND 3RD-HARMONIC GENERATION IN CONJUGATED LINEAR-CHAINS

Experimental measurements of the frequency dependence of the isotropically averaged, molecular dc-induced second-harmonic susceptibility <gamma(-2-omega; omega, omega, 0)> and third-harmonic susceptibility <gamma(-3-omega; omega, omega, omega)> of conjugated linear-chain structures are in excellent agreement with the results of many-electron, multiple-excited-configuration-interaction calculations. For hexatriene, the N = 6 carbon site linear polyene chain, we demonstrate the first successful comparison, to our knowledge, between experiment and theory for the magnitude, sign, and dispersion of <gamma(-2-omega; omega, omega, 0)> and <gamma-3-omega; omega, omega, omega)>. These results provides experimental verification of the importance of electron-correlation effects in the third-order nonlinear-optical properties of conjugated structures. Measurements of <gamma(-2-omega; omega, omega, 0)> and <gamma(-3-omega; omega, omega, omega)> for beta-carotene, the intermediate length, N = 22, conjugated chain, are also reported, and the frequency dependence is well described by a three-level model that is developed based on the results of the electron-correlation description of gamma-ijkl(-omega-4; omega-1, omega-2, omega-3) for shorter chains. Finally, through the detailed comparisons of <gamma(-2-omega; omega, omega, 0)> with <gamma(-3-omega, omega, omega, omega)> and of experiment with theory, as well as through examination of previously reported values of the macroscopic third-order susceptibility chi(3)(-omega-4; omega-1, omega-2, omega-3) of glasses by other nonlinear-optical techniques, it is demonstrated that the common reference standard for chi(3)(-omega-4; omega-1, omega-2, omega-3) of glass is inaccurate, and an improved value is recommended.