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

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