Third-order nonlinear optical properties of TiO₂ nano colloidal synthesized via sol–gel and hydrothermal methods: A comparison

Titanium dioxide (TiO₂) exhibits high nonlinear refractive index. However, the comparison of nonlinear optical properties of TiO₂ nano colloidal, synthesized via hydrothermal and sol–gel is not available. Considering the gap, this work aims to compare the third-order nonlinear susceptibility χ⁽³⁾ of TiO₂ nano colloidal solution, using z-scan technique under continuous wave He-Ne laser. TiO₂ nanoparticles (NPs) were synthesized via hydrothermal and sol–gel and calcinated at different temperatures to achieve anatase and rutile phases and same was confirmed from x-ray diffraction. The particle size increased from 25 to 98 nm and 18 to 109 nm for 300 °C, 700 °C and 900 °C samples and bandgap decreased from 3.13 to 2.98 eV and 3.18 to 2.99 eV, when TiO₂ phase changes from anatase to rutile. All samples exhibit self-defocusing and reverse saturable absorption behavior. High χ⁽³⁾ and optical clamping occur for sol–gel anatase TiO₂ (1.72 × 10⁻⁶ esu and 3.9 mW) rather than hydrothermal.