From Nature to Optical Nonlinearity: Ultrafast Photophysics of Biogenic Dyes, Alizarin, and Cochineal.

Natural dyes such as alizarin and cochineal are gaining renewed attention as sustainable alternatives to synthetic chromophores with promising nonlinear optical (NLO) properties. Here, we present a systematic study of their linear and NLO responses in both solution and solid-state environments. Steady-state absorption and emission spectra were recorded, and excited-state dynamics were analyzed using time-correlated single-photon counting. Absolute fluorescence quantum yields were determined with an integrating sphere. Two-photon absorption cross-sections were evaluated via two-photon excited fluorescence (TPEF) measurements, while third harmonic generation experiments on dye-doped PVA thin films enabled estimation of the effective third order NLO susceptibility χ(3). The results reveal distinct structure-property relationships driven by molecular rigidity and conjugation. Cochineal exhibits stronger two-photon activity in solution, consistent with its extended and flexible charge-transfer framework. In contrast, alizarin demonstrates a significantly enhanced third order NLO response in the solid state, with χ(3) approaching nearly twice that of fused silica, attributed to its higher rigidity and matrix-induced ordering. These findings emphasize the decisive role of molecular structure and solid-state organization in NLO performance and highlight the potential of natural dyes for multiphoton bioimaging, optical limiting, and polymer-based nonlinear photonic applications.