2PA and 3PA induced broadband limiting of Cr3+ doped BaB2O4 nanostructures

Abstract

Broad band optical limiting action of Cr doped (γ and β)-BBO was demonstrated by Z-scan experiment using first (1064nm) and second (532nm) harmonics of Q-switched nanopulsed (9ns, 10Hz) Nd:YAG laser. The recorded OA pattern at 532nm exposes that the dispersed liquids exhibit reverse saturable absorption ascribed due to 2PA process. At 0.05M Cr doped BBO, peculiar switching of SA (0.63GW/cm2) to RSA (2.52GW/cm2) was observed under 532 nm excitation. Increase in 2PA coefficient with on-axis peak intensity and availability of Cr induced near-resonant energy states demonstrate the presence of sequential 2PA (1PA + ESA) process. Interestingly under 1064 nm excitation, Cr doped γ-BBO exhibit 3PA while Cr doped β-BBO possess 2PA process. The addition of Cr has induced transition states in visible and IR region, therefore availability of transition states confirms the involvement of excited ESA process at 1064 nm regime. All the liquids exhibit optical limiting action in which onset optical limiting threshold varies with phase, concentration of Cr and on-axis peak intensity. Comparing the NLA coefficient of Cr doped (γ and β)-BBO in both 532 nm and 1064 nm region, the involvement of trapped state of Cr at 2PA region has improved the absorption coefficient than the real 2PA state of pristine (γ and β)-BBO. Among Cr doped (γ and β)-BBO at 532 nm regime, 0.05 M Cr doped (γ and β)-BBO exhibits low threshold of 0.75 × 1012W/m2 and 0.57×1012W/m2 respectively. Interestingly at 1064nm regime, 0.03M Cr doped (γ and β)-BBO possess better onset limiting threshold of 0.986×1012W/m2 and 2.92×1012W/m2 respectively. Earlier by hydrothermal technique tunability of band structure was achieved by introducing Cr in two different phases of BBO (γ and β) at different concentration (0.01M, 0.03M and 0.05M). Incorporation of Cr without altering the crystal structure of host BBO was confirmed by XRD and Raman analysis. SEM analysis exposes the transformation of nanorod to nanowire (γ-BBO) and irregular platelet to cubic (β-BBO) morphology due to Cr incorporation in BBO. Ground state absorption studies shows Cr doped (γ and β)-BBO possess transition at 532nm and 1064nm due to 4A2g (F) → 4T1g (F), 4A2g (F) →4T2g (F) transition of Cr. Thus by Cr doping in BBO, ESA assisted 2PA and 3PA based broad band optical limiting with higher NLO coefficients was demonstrated successfully.