Excited state absorption of Cu-doped barium borate nanostructures under nanopulsed laser excitation

Abstract

Intensity- and concentration-dependent nonlinear absorption of Cu-doped ($\gamma$ and $\beta )$-$\hbox {BaB}_{2}\hbox {O}_{4}$ nanostructures was measured by a standard open aperture Z-scan setup under nanopulsed (9 ns, 10 Hz) laser excitation (532 nm) at various peak intensities (1.26–2.52 GW/$\hbox {cm}^{2})$. Intensity-dependent 2PA coefficient exposes the involvement of accumulative 2PA process rather than genuine 2PA. From ground state absorption studies, existence of new energy states due to Cu incorporation has availed a near-resonant state favoring excited state absorption leading to sequential 2PA. Among the samples, 0.05 M Cu-doped $\gamma$-$\hbox {BaB}_{2}$ $\hbox {O}_{4}$ $(2.6\times 10^{-10}$ m/W, $0.78\times 10^{12}$ $\hbox {W}/\hbox {m}^{2})$ and 0.03 M Cu-doped $\beta$-$\hbox {BaB}_{2} \hbox {O}_{4}$ $(2.5\times 10^{-10}$ m/W, $1.21\times 10^{12}\hbox {W}/\hbox {m}^{2})$ exhibit higher 2PA coefficient and lower onset limiting threshold. The presence of CT and intragap states of $\hbox {Cu}^{2+}$ ions-induced strain in visible region transformed genuine 2PA in pristine ($\gamma$ and $\beta )$-$\hbox {BaB}_{2}$ $\hbox {O}_{4}$ nanorods into sequential 2PA (1PA$+$2PA) in Cu-doped ($\gamma$ and $\beta )$-$\hbox {BaB}_{2}$ $\hbox {O}_{4}$. By simple hydrothermal process, concentration-dependent Cu-doped ($\gamma$ and $\beta )$-$\hbox {BaB}_{2}$ $\hbox {O}_{4}$ nanostructures were prepared and their structural and optical properties were studied. Thus, Cu-doped ($\gamma$ and $\beta )$-$\hbox {BaB}_{2}$ $\hbox {O}_{4}$ exhibit sequential 2PA (1PA$+$ESA)-based optical limiting with enhanced 2PA coefficient than its pristine ($\gamma$ and $\beta )$-$\hbox {BaB}_{2}$ $\hbox {O}_{4}$ nanorods.

Graphic Abstract