A photo-controllable third-order nonlinear optical (NLO) switch based on a rhodamine B salicylaldehyde hydrazone metal complex

In this work, a novel photo-controllable third-order nonlinear optical (NLO) switch based on a rhodamine B salicylaldehyde hydrazone metal complex was developed. The complex was found to undergo reversible intramolecular spirolactam ring-opening reaction upon UV irradiation, which led to a distinct change in third-order NLO properties. Before light irradiation, the complex in its spirolactam ring-closed form exhibited no NLO properties. After light irradiation, the complex turned to its spirolactam ring-opened form and exhibited strong NLO properties of self-focusing refraction and saturable absorption. The third-order NLO responses of the complex were investigated systematically by top-hat Z-scan and time-resolved pump-probe with phase object (PO-pump-probe) techniques. The results showed that the p-conjugation of the rhodamine xanthen part is indispensable for third-order NLO properties. More importantly, the first excited singlet state lifetime of the complex is remarkably shorter than that of the corresponding rhodamine B dye, which suggest that the complex could not only be used as a photo-controlled switch with excellent reversibility and fatigue resistance, but also be used as a third-order NLO switch with a fast recovery rate.