Abstract
Intensity-dependent nonlinear optical absorption and optical limiting behavior of Ni2+-doped (gamma and beta)-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 gamma-BaB2O4 involves the T-1(1g)(G) states of 3d(8)-3d(8) and T-1(1g)(D) states of Ni2+, while Ni-doped beta-BaB2O4 undergoes the electronic transition involving intraionic 3d(8)-3d(8) 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 beta-BaB2O4 nanostructures possess higher 2PA coefficient (2.31 x 10(-10) m/W) and lower onset limiting threshold (0.79 x 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 beta-BaB2O4 nanostructures.