Investigation of the linear and nonlinear optical properties of La2−xSrxCoO4 (x = 0.5, 0.7, 0.9, 1.1, 1.3 and 1.5) nanoparticles

In this research, the linear and nonlinear optical properties of La2−xSrxCoO4 (x = 0.5, 0.7, 0.9, 1.1, 1.3 and 1.5) nanoparticles were investigated. The structure and nanoscale morphology of the synthesized samples were confirmed by XRD analysis and FE-SEM imaging, respectively. Optical transmission, reflection and absorption studies were conducted to analyze the optical constants of the nanopowders. The obtained results revealed that the optical band gap energy decreased with the increase of Sr doping from 4.25 eV for x = 0.5 to 3.65 eV for x = 0.9. The enhancement trend is observed for the refractive and extinction coefficients with increasing Sr contents from x = 0.5 to x = 0.9 using the Kramers–Kronig approach. Third-order nonlinear optical susceptibility, nonlinear refractive (NLR) index, and nonlinear absorption (NLA) values were evaluated using the Z-scan technique. The NLA (as two phonon absorption (TPA)) and NLR responses have the highest amount of transmittance (62.28 × 10−7 cm W−1 and 8.16 × 10−11 cm2 W−1) for x = 0.9 and the lowest amount of transmittance (7.52 × 10−7 cm W−1 and 2.7 × 10−11 cm2 W−1) for x = 0.5, respectively. These results could be due to the strong hybridization of Co-3d and O-2p orbitals which is caused by enhancement of Co3+-LS populations with increasing Sr doping content from x = 0.5 to 0.9. As the Sr doping content increased from x = 0.9 to x = 1.5, the Co3+ spin state transition from LS to IS leads to decrease of the refractive index, extinction coefficient, and NLA and NLR responses of La2−xSrxCoO4 compounds. The excellent nonlinear properties of La2−xSrxCoO4 nanoparticles indicate that these compounds are good candidates for optical devices and can also provide a reference for the synthesis of new optical materials.