3RD-HARMONIC GENERATION FROM ALL-TRANS BETA-CAROTENE IN POLYSTYRENE THIN-FILMS - MULTIPLE REFLECTION EFFECTS AND THE ONSET OF A 2-PHOTON RESONANCE

The magnitude and phase of the third-order nonlinear susceptibility, chi-(ZZZZ)(3)(-3-omega; omega, omega, omega), of all-trans beta-carotene, doped in a polystyrene thin film, have been measured by the third-harmonic generation Maker fringe technique for incident laser wave-lengths in the 0.96-1.01-mu-m range. Multiple reflections of the fundamental electric field in the thin film are included in the analysis. The magnitude of chi-(ZZZZ)(3)(-3-omega; omega, omega, omega) was determined to be 1.1-1.2 x 10(-11) esu and relatively constant in this wave-length range, and the phase was found to vary between 207-degrees and 233-degrees. The dispersion of the phase was used to obtain an estimate of 21 300 cm-1 for the location of a one-photon forbidden, two-photon allowed n 1A(g) excited state in beta-carotene that plays a central role in the microscopic theory of chi-(ZZZZ)(3)(-3-omega; omega, omega, omega).