Details

Autor(en) / Beteiligte

• Hubbard, Steven F

Titel

Measurements of second-harmonic susceptibility in organic nonlinear optical systems

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

1997

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• We characterized the second-order nonlinear optical properties of several organic materials. This included measurements of the macroscopic nonlinear response from a fluorinated, soluble, thermally-stable poled polyimide guest-host system. We measured a second harmonic coefficient that was accurately predicted by a thermodynamic model of poled polymers based on the molecular nonlinearity of the dopant molecules. Poling-induced order was thermally stable at elevated temperature for over 300 hours. Films of the polyimide were observed to have negative birefringence and could be cast into optical waveguide structures with losses below 5 dB/cm. Additionally, a Langmuir-Blodgett polymer system was characterized by similar methods and found to have a substantial nonlinear coefficient and high transparency in robust free-standing waveguide structures. We used a high-intensity tunable picosecond infrared laser source to measure the spectral content of light scattered near the second harmonic for two well-known organic dyes, p-nitroaniline and disperse red 1, at a number of wavelengths in the near infrared. The dispersion of the magnitude of the first hyperpolarizability was determined for both dyes using a simple referencing technique and found to be consistent with a two-level dispersion model. We found in disperse red 1 that a broad two-photon fluorescence competes with the narrow hyper-Rayleigh peak. Further, dephasing of the virtual excited state led to elimination of hyper-Rayleigh scattering in favor of fluorescence when the harmonic is just slightly into the linear absorption tail. Finally, we designed a hyper-Rayleigh scattering scheme to measure 6 scalar invariants of the squared hyperpolarizability tensor, $\beta\sp2.$ Our theoretical approach expresses the rotational invariants of the irreducible $\beta$ components as scalars which eliminates the need for difficult frame transformations. We applied our scheme to several conjugated chiral molecules and found significant Kleinman-disallowed contributions to the hyperpolarizability related to three-dimensional delocalization of the $\pi$-electron system. These components indicate a handed non-planar delocalization of the charge transfer system in these chiral molecules as predicted by quantum-chemical calculations and are expected to lead to macroscopic second-harmonic generation in axially aligned polymer materials. Pseudotensor contributions to the hyperpolarizability in chiral molecules was found to be comparable with the vector contribution in p-nitroaniline. One-dimensional molecules were found to have $\beta$-components consistent with Kleinman symmetry.