Nonlinear photophysics of PDA-based nanofluid: a potential candidate for two-photon photothermal therapy.

This study reports the synthesis and nonlinear photophysical characterization of polydopamine (PDA)-based nanofluid derived from dopamine (DA) monomers. The nanofluid's nonlinear photophysical behaviour was systematically examined using the Z-scan technique under both CW and fs laser excitations. Under CW excitation, the samples exhibited strong thermo-optical nonlinearities with self-defocusing, primarily due to the high photothermal conversion efficiency of PDA, which induces significant thermal lensing. In contrast, fs laser excitation induced optical nonlinearities at both 500 nm and 660 nm, where the nanofluid demonstrated positive refractive behaviour characteristic of self-focusing, accompanied by positive absorptive responses arising from two-photon absorption (TPA). The combination of efficient TPA and superior photothermal conversion suggests promising applicability in two-photon photothermal cancer therapy, enabling precise, localized heating with reduced risk to surrounding healthy tissue. Additionally, the broad nonlinear response range positions PDA-based nanofluid as a candidate for photonic applications, including all-optical switching and modulation. Optical limiting studies further confirmed its capability to protect sensitive optical systems from high-intensity laser exposure.