Effect of the annealing temperature on structural, morphological, and nonlinear optical properties of TeO2 thin films used for efficient THz generation

Abstract

This paper reports the effect of annealing on TeO2 films deposited using the thermal evaporation technique. TeO2 films of 120 nm thickness were grown on a glass substrate at room temperature (RT) and annealed at 400°C and 450°C. The structure of the film and the influence of the annealing temperature on the crystalline phase change were examined using the X-ray diffraction method. Optical properties such as transmittance, absorbance, complex refractive index, and energy bandgap were determined between ultraviolet-visible to terahertz (THz) range. These films have a direct allowed transition with the optical energy bandgap of 3.66, 3.64, and 3.54 eV at as-deposited temperatures (RTs) of 400°C and 450°C. The effect of the annealing temperature on the morphology and surface roughness of the films was investigated using atomic force microscopy. The nonlinear optical parameters, which are the refractive index and absorption coefficients, were calculated using THz time domain spectroscopy. The microstructure variation of the TeO2 films in terms of surface orientation plays an important role in understanding the change in the nonlinear optical properties of the films. Finally, these films were subjected to 800 nm wavelength of 50 fs pulse duration obtained from a Ti:sapphire amplifier at a 1 kHz repetition rate for efficient THz generation. The power of incidence of the laser beam was tuned at a range between 75 and 105 mW; the highest power of the generated THz signal was of the order of 210 nW for 450°C annealed film with respect to the incident power of 105 mW. The conversion efficiency was found to be ∼0.22×10−5%, which is 2.025 times more than the film annealed at 400°C.