The role of graphene nanoplatelets doping in the defect induced saturable absorption in polyaniline films

The study elucidates the influence of graphene nanoplatelets (GNP) doping on the nonlinear optical (NLO) properties of polyaniline (PANI) films using a 532 nm Q-switched Nd: YAG laser. PANI films, synthesized and doped with varying GNP concentrations through drop-casting, were analysed for structural and optical properties with scanning electron microscopy (SEM), Fourier transform infrared (FTIR), and UV–visible absorption spectroscopy. The SEM images showed irregular PANI clusters on the GNPs smooth and flaky surfaces, while FTIR confirmed successful GNP integration. UV–vis absorption spectroscopy revealed a bandgap reduction in PANI from 2.65 eV to 2.13 eV with GNP doping, which is ascribed to increased defect states at the PANI-GNP interface. NLO studies revealed enhanced saturable absorption (SA) behavior in PANI-GNP composites, with notable improvements in key parameters. The nonlinear absorption coefficient (β) increased from −59.80 cm/GW (PANI) to −159.98 cm/GW (0.5 wt% GNP in PANI). The imaginary part of third-order nonlinear susceptibility ( χ i ( 3 ) ) enhanced from −0.52 × 10⁻¹⁰ esu to −1.588 × 10⁻¹⁰ esu, and the modulation depth increased from 9.3 % to 29.6 %. The saturation intensity (I s ) decreased from 150 MW/cm² to 10 MW/cm², signifying superior SA performance with increasing GNP concentration. These enhancements stem from defect induced charge carrier trapping, extended carrier lifetimes, and improved interfacial charge transfer. To the best of our knowledge, this is the first report elucidating the dopant concentration dependence of PANI’s SA properties and providing quantified NLO parameters. These findings underscore the potential of PANI-GNP composites for sophisticated photonic applications, including ultrafast laser systems and optical modulation. • Comprehensive analysis of PANI-GNP film's NLO properties with SA behavior. • Enhanced SA in PANI-GNP films achieved by varying GNP concentration via drop casting. • Prolonged carrier lifetime due to defect states in PANI-GNP composites enhances SA. • Achieved modulation depth of 29.6 % and I s of 10 MW/cm² at 0.5 wt% GNP in PANI. • First report on GNP’s tunable effect on PANI’s NLO properties for laser applications.