Facile synthesis, characterization and intensity-dependent nonlinear absorption of Ni-doped (γ and β)-BaB_2O_4 nanostructures

摘要

Intensity-dependent nonlinear optical absorption and optical limiting behavior of Ni~(2+)-doped (γ and β)-BaB_2O_4 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 Ni~(2+) ions, the material exhibits excited state absorption (ESA). Here, the observed sequential 2PA in Ni~(2+)-doped γ-BaB_2O_4 involves the ~1T_(1g)(G) states of 3d~8-3d~8 and ~1T_(1g)(D) states of Ni~(2+), while Ni-doped β-BaB_2O_4 undergoes the electronic transition involving intraionic 3d~8-3d~8 transition of Ni~(2+) 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 Ni~(2+)-doped β-BaB_2O_4 nanostructures possess higher 2PA coefficient (2.31 × 10~(-10) m/W) and lower onset limiting threshold (0.79 × 10~(12) W/m~2), 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-BaB_2O_4 nanostructures.