Origin of negative chi((3)) in squaraines: experimental observation of two-photon states.
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 twophoton 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.
Yu
Y Z
YZ
Shi
R F
RF
Garito
A F
AF
Grossman
C H
CH