Abstract

Squaraines have attracted much research interest due to their large negative third order optical susceptibilities γ(−ω₄; ω₁, ω₂, ω₃)³ in the zero frequency limit.^2,3 Evidence has accumulated that a two-level model for γ(−ω₄; ω₁, ω₂, ω₃) is not adequate even for molecules with negative γ(−ω₄; ω₁, ω₂, ω₃). For centrosymmetric conjugated organic molecules, where γ(−ω₄; ω₁, ω₂, ω₃) arises mainly from delocalized π-electrons, there are usually two types of virtual transition sequences which give the dominant contributions to γ(−ω₄; ω₁, ω₂, ω₃).

© 1994 Optical Society of America