Improved nonlinear absorption mechanism of tin oxide thin films: Role of strontium doping

Abstract We report the effect of strontium doping on third-order nonlinear optical properties of tin oxide (SnO2) thin films grown via chemical spray pyrolysis technique. The X-ray diffraction (XRD) results reveal that films exhibit a polycrystalline phase with a tetragonal crystal structure. The broadening and shifting of diffraction peak indicate that the dopant Sr2+ ion successfully replaced the Sn sites of SnO2 lattice. The reduction of crystalline size from 23.92 to 12.96 nm indicates the assimilation of point defects such as interstitial and oxygen vacancies in the SnO2 lattice upon doping. The surface morphological analysis via atomic force microscopy indicates the shattering of grains into small particles upon Sr doping. The energy band gap of films shows a substantial drop from 3.97eV to 3.03eV due to the formation of sub-band levels in the forbidden gap. Photoluminescence (PL) spectra show intense, broad asymmetric emission bands centred around 390 nm and extending up to 500 nm. The third order nonlinear absorption mechanism observed in Sr: SnO2 films were attributed to free carrier absorption (FCA) induced two-photon absorption (TPA). The third order nonlinear absorption co-efficient βeff shows a one order enhancement (0.14 × 10−1 cm/W to 3. 91 × 10−1 cm/W) which indicates that competency of grown films in nonlinear optical device applications.