Facile synthesis, characterization and intensity-dependent nonlinear absorption of Ni-doped (γ and β)-BaB₂O₄ nanostructures

Intensity-dependent nonlinear optical absorption and optical limiting behavior of Ni²⁺-doped (γ and β)-BaB₂O₄ nanostructures were examined by open-aperture Z-scan technique under nanosecond pulsed green laser excitation. Observation of reverse saturable absorption (RSA) with variation in nonlinear absorption coefficient as function of on-axis peak intensity ascertains the presence of sequential 2PA process (1PA + ESA). Due to the introduced near-resonant energy state through incorporation of Ni²⁺ ions, the material exhibits excited state absorption (ESA). Here, the observed sequential 2PA in Ni²⁺-doped γ-BaB₂O₄ involves the ¹T_1g(G) states of 3d⁸–3d⁸ and ¹T_1g(D) states of Ni²⁺, while Ni-doped β-BaB₂O₄ undergoes the electronic transition involving intraionic 3d⁸–3d⁸ transition of Ni²⁺ and self-trapped excitonic state of BBO. Interestingly, as the dopant concentration and on-axis intensity increased, 2PA coefficient was found to be increased. 0.05 M Ni²⁺-doped β-BaB₂O₄ nanostructures possess higher 2PA coefficient (2.31 × 10⁻¹⁰ m/W) and lower onset limiting threshold (0.79 × 10¹² W/m²), which makes it a promising candidate for optical limiting applications. The result suggests that band structure tunability to induce excited state absorption with enhanced nonlinear absorption coefficient is possible through Ni doping in β-BaB₂O₄ nanostructures.