Tunable structural and optical properties of BiSeTe thin films: implications for nonlinear optical applications.

In this work, we successfully fabricated a series of BiSeTe thin films with similar thickness using radio-frequency magnetron sputtering. The influence of elemental composition on the films' structure, optical constants, optical band gap, and nonlinear absorption properties was studied by techniques such as X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, spectroscopic ellipsometry, spectrophotometry, and open-aperture Z-scan. Linear optical property measurements indicate that as the proportion of Te increases, the metallic characteristics of the films are enhanced, which is reflected in a decrease in the refractive index, an increase in the extinction coefficient, and a redshift of the optical bandgap. Regarding nonlinear absorption properties, the annealed crystalline BiTe thin film exhibits the largest modulation depth, with its nonlinear optical absorption coefficient approximately eight times that of the amorphous BiSe thin film. However, the amorphous BiSe thin film possesses the highest damage threshold. All BiSeTe thin films display saturable absorption behavior and exhibit large third-order nonlinear optical absorption coefficients, suggesting their potential as promising materials in ultrafast nonlinear optics. Our findings indicate the optical properties of BiSeTe thin films can be effectively tuned through composition adjustment, demonstrating the potential device application in mode-locked lasers, Q-switching, all-optical diodes, and so on.