Giant dispersive and absorptive optical nonlinearities in TiO2 thin films

We report the experimental observation of giant dispersive and absorptive optical nonlinearities in direct current magnetron sputtered as-grown and annealed TiO2 thin films. The TiO2 film was deposited in a mixture of argon and oxygen. The as-grown film and the film annealed below 250 degrees C demonstrated amorphous behavior having particle size variation in the range of 15-20 nm, while the films annealed above temperature > 300 degrees C exhibited crystalline structure with larger particle size (50-80 nm). The enhancement in optical nonlinearity in amorphous films can be attributed to small particle sizes, while in crystalline thin films, the enhancement in optical nonlinearity is attributed to phonon-assisted resonant transitions. The films are characterized by the x-ray diffraction technique, atomic force microscopy, UV-visible spectroscopy, and Raman spectroscopy for their structural properties, morphology, bandgap, and presence of phonon modes, respectively. The intensity-dependent nonlinear optical properties of the films are analyzed by using the Z-scan technique. (C) 2020 Optical Society of America