An investigation of the role of defects and band gap shrinkage in third-order nonlinear optics and enhanced optical limiting of ZnO-CeO2-rGO heterostructures

Transition metal oxide-decorated graphene nanocomposites are promising for solid-state optical limiting filters that protect the human eye and light-sensitive devices from high-intensity laser radiation. Here, ZnO-CeO 2 -rGO heterostructures were fabricated by a facile hydrothermal method. XRD, TEM, and SAED analyses confirmed that no additional crystalline impurity phases were detected showing well-dispersed oxide nanoparticles anchored on rGO sheets. Fourier-transform infrared (FT-IR), Raman, and X-ray photoelectron spectroscopy (XPS) confirmed strong interfacial chemical bonding. Optical absorption and Tauc analyses reveal that the band gap of ZnO–CeO 2 –rGO heterostructures varies from 2.85 to 2.97 eV exhibiting a non-monotonic trend with rGO content. Third-order nonlinear optical (NLO) properties were evaluated under continuous-wave (CW) 532 nm laser excitation. The open-aperture (OA) Z -scan profiles reveal strong reverse saturable absorption (RSA) driven primarily by excited-state absorption (ESA) with σ ex > σ g , assisted by defect states and thermal contributions. The T5 sample (5 wt% rGO) exhibits the highest nonlinear absorption coefficient β = 4.12 × 10 ⁻ ⁵ cm / W . Closed-aperture (CA) measurements demonstrate negative nonlinear refraction (self-defocusing) arising from thermal lensing. Optical limiting studies reveal low onset thresholds, with the T7 sample achieving a minimum onset intensity of 14.2 kW/cm 2 (1.3 kJ/cm 2 ), demonstrating excellent potential for photonic protection.