Boosting third-order nonlinearities in rutile TiO₂ by chromium doping

In this study, we investigate the impact of chromium-induced point defects on the nonlinear optical properties and electric-field-induced second harmonic generation (EFISH) in rutile titanium dioxide (TiO₂). Chromium thin films were deposited by electron beam evaporation on (001)-oriented bulk TiO₂ substrates and subsequently diffused into the lattice in a tube furnace under a nitrogen atmosphere at 900 °C. The introduction of chromium significantly enhanced the third harmonic generation (THG) of a 1560 nm laser, with an amplification factor of up to 8.3, indicative of an enhanced third-order nonlinear susceptibility, χ⁽³⁾. Moreover, the application of an external voltage induced a pronounced EFISH signal in the chromium-doped samples, further confirming the enhanced nonlinear response. These results demonstrate that defect engineering via chromium doping in rutile TiO₂ offers a promising pathway for the development of high-performance nonlinear optical devices.