Femtosecond pump–probe experiments performed on squaraines demonstrate strong couplings between the first excited state and high-lying two-photon states. The experimental data agree well with our earlier quantum many-electron calculations based on multiple-excited configuration interactions. We show that high-lying two-photon states in squaraines are critically important to understanding the observed third-order optical properties and that two-level models are inadequate even for molecules with negative third-order optical susceptibilities.
© 1994 Optical Society of America
Original Manuscript: January 3, 1994
Published: June 1, 1994
Y. Z. Yu, C. H. Grossman, R. F. Shi, and A. F. Garito, "Origin of negative χ(3) in squaraines: experimental observation of two-photon states," Opt. Lett. 19, 786-788 (1994)