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, c hi-(ZZZZ)(3)(-3-omega; omega, omega, omega), of all-trans beta-caroten e, doped in a polystyrene thin film, have been measured by the third-h armonic generation Maker fringe technique for incident laser wave-leng ths in the 0.96-1.01-mu-m range. Multiple reflections of the fundament al electric field in the thin film are included in the analysis. The m agnitude of chi-(ZZZZ)(3)(-3-omega; omega, omega, omega) was determine d to be 1.1-1.2 x 10(-11) esu and relatively constant in this wave-len gth 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 o f 21 300 cm-1 for the location of a one-photon forbidden, two-photon a llowed n 1A(g) excited state in beta-carotene that plays a central rol e in the microscopic theory of chi-(ZZZZ)(3)(-3-omega; omega, omega, o mega).