3PA-induced optical limiting in pure and barium borate decorated MoS2 nanocomposites

Abstract

Pure MoS2 and barium borate nanorods embedded MoS2 microspheres were successfully synthesized by adopting an environmental-friendly hydrothermal method. Powder XRD data confirmed the formation of BBO: MoS2 nanocomposite made of barium borate [2 $$\theta$$ = 22°, (018)] along with the MoS2 [2 $$\theta$$ = 14°, (002)]. The presence of few layers MoS2 was ascertained from the measured difference in characteristic $$E_{2g}^{1}$$ (379 cm−1) and A1g (407 cm−1) Raman modes of MoS2. FESEM images portrayed the formation of microspheres for pure MoS2 due to reduction in the surface energy induced by citric acid. In the nanocomposite, barium borate embedded themselves as nanorods upon MoS2 microsphere forming an urchin-like structure. Ground-state absorption studies revealed a hyperchromic shift in absorption peak due to the overlapping of transition states between highly transparent barium borate and strongly absorbing MoS2 in the visible region. Femtosecond laser pulses (800 nm, 150 fs, 80 MHz) were employed to perform the open-aperture Z-scan experiments revealing that both pure MoS2 and BBO: MoS2 nanocomposites possessed reverse saturable absorption (RSA), attributed to both 2PA and 3PA processes. Interestingly, pure MoS2 exhibited sequential 3PA (1PA + 2PA) due to the presence of near-resonant energy states, while the dominance of transparent BBO in the nanocomposite induced genuine/instantaneous 3PA process. In closed-aperture Z-scans (for determining the sign and magnitude of nonlinear refraction), the pattern switched from self-focusing (MoS2) to self-defocusing (BBO: MoS2) nature. Both pure MoS2 and BBO: MoS2 nanocomposites exhibited strong optical limiting with a lower onset limiting threshold (0.11 µJ/cm2) due to synergetic effects of nonlinear absorption (3PA) and nonlinear refraction (self-defocusing). BBO:(0.02 M) MoS2 nanocomposite possessing a urchin structure depicted strong NLO coefficients with $$\gamma_{3PA} = 2.12 \times 10^{ - 21}$$ m3/W2, $$n_{2} = - 11.1 \times 10^{ - 17} {\text{m}}^{2} /{\text{W}}$$ and χ(3) = $$14.0 \times 10^{ - 19} {\text{m}}^{2} /{\text{V}}^{2}$$ , which were higher than pure MoS2 and other composites.