Tunable structural and optical properties of Bi₂Se_<i>x</i>Te_3−<i>x</i> thin films: implications for nonlinear optical applications

In this work, we successfully fabricated a series of Bi₂Se_<i>x</i>Te_3-<i>x</i> 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 Bi₂Te₃ thin film exhibits the largest modulation depth, with its nonlinear optical absorption coefficient approximately eight times that of the amorphous Bi₂Se₃ thin film. However, the amorphous Bi₂Se₃ thin film possesses the highest damage threshold. All Bi₂Se_<i>x</i>Te_3-<i>x</i> 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 Bi₂Se_<i>x</i>Te_3-<i>x</i> 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.