Investigation of the Degenerate Two-Photon Absorption Cross-Section in all-trans Retinal: Nonlinear Spectrum and Molecular Structure

Abstract – Retinal is a chromophore of foremost relevance for potential applications in bio-photonics devices. Here, we study the degenerate two-photon absorption spectrum of all-trans retinal in ethanol. We observed two peaks; one associated to the twophoton dipolar contribution to a higher energy state, which contributes to the monotonic increase of the nonlinearity below 600 nm. The other one is a two-photon transition to a lower excited-singlet state (Ag-). We modeled the two-photon absorption spectrum using a sum-over-states model. Using Density Functional theoretical calculations we show that due the presence of the β-ionone ring there is a break of planarity of the molecule which decreases the nonlinear effect.