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)-BaB2O4 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/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-BaB2O4 (2.6 x 10(-10) m/W, 0.78 x 10(12) W/m(2)) and 0.03 M Cu-doped beta-BaB2O4 (2.5 x 10(-10) m/W, 1.21 x 10(12) W/m(2)) exhibit higher 2PA coefficient and lower onset limiting threshold. The presence of CT and intragap states of Cu2+ ions-induced strain in visible region transformed genuine 2PA in pristine (gamma and beta)-BaB2O4 nanorods into sequential 2PA (1PA-F2PA) in Cu-doped (gamma and beta)-BaB2O4. By simple hydrothermal process, concentration-dependent Cu-doped (gamma and beta)-BaB2O4 nanostructures were prepared and their structural and optical properties were studied. Thus, Cu-doped (gamma and beta)-BaB2O4 exhibit sequential 2PA (1PA-FESA)-based optical limiting with enhanced 2PA coefficient than its pristine (gamma and beta)-BaB2 O-4 nanorods.