Annealing Effect on Linear and Ultrafast Nonlinear Optical Properties of Bi₂Te₃ Thin Films

In recent years, the fabrication of materials with large nonlinear optical coefficients and the investigation of methods to enhance nonlinear optical performance have been in the spotlight. Herein, the bismuth telluride (Bi₂Te₃) thin films were prepared by radio-frequency magnetron sputtering and annealed in vacuum at various temperatures. The structural and optical properties were characterized and analyzed using X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, spectroscopic ellipsometry, and UV/VIS/NIR spectrophotometry. The third-order optical nonlinearities of Bi₂Te₃ thin films were investigated using the Z-scan technique, employing a 100 fs pulse width at an 800 nm wavelength. It is found that the crystallinity and the average grain size of the films increase with the annealing temperature. Meanwhile, the extinction coefficient of the annealed films increased, accompanied by a redshift in the optical bandgap. All samples exhibit pronounced saturable absorption and self-focusing behaviors. The nonlinear absorption coefficient and nonlinear refractive index of Bi₂Te₃ films annealed at 300 °C were found to be 2.44 times and 1.85 times higher than those of the as-deposited films, respectively. These findings demonstrate that annealing treatment is an effective approach to tuning the crystalline structure and linear optical properties of Bi₂Te₃ films while simultaneously enhancing their nonlinear optical performance.