Finding materials with third-order nonlinearity is one of the most important research areas in the field of optical materials. Because of its high refractive index, high nonlinearity, and adjustable properties with components, chalcogenide glass has obtained many excellent results in all optical switching, optical limiting, optical communication, and other fields. Ultrafast dynamics is the exploration of the microscopic state of materials, and the change of the state of microscopic particles is the direct cause of the change in the macroscopic properties of materials. The study of ultrafast dynamics can observe the change of microscopic particle motion, clarify the mechanism of optical nonlinearity, and obtain the dynamics parameters. However, studies on the ultrafast dynamics of chalcogenide glass are still lacking. Therefore, on the basis of enhancing the nonlinearity of chalcogenide glass, the ultrafine dynamic process of chalcogenide glass is discussed in this paper, which provides an important reference for further understanding of its optical nonlinearity mechanism and developing related devices.

The experimental samples in this paper were prepared by co-evaporation technique. In the evaporation process, we set different evaporation rates of Ge₂₈Sb₁₂Se₆₀(GSS) powder and Bi particles to obtain (Ge₂₈Sb₁₂Se₆₀)_100-xBi_x chalcogenide glass films with different Bi content and thickness of 50 nm. The elemental composition of each sample with a specific composition was measured by energy disperse spectroscopy (EDS). The transmission and absorption spectra of each sample in the wavelength range of 400-2000 nm were measured and calculated using a UV-3150 spectrometer. The optical band gap (E_g) of the prepared sample is calculated according to the classical Tauc equation, and the nonlinear absorption coefficient of the sample is measured at 532 nm by the picosecond Z-scan method. Finally, the effect of Bi doping on the ultrafast dynamics process of GSS in chalcogenide glass was investigated by the PO pump-probe method and the introduction of a three-energy level system. The optical nonlinear mechanism was studied, and the related dynamics parameters were obtained.

In the wavelength range of 400-2000 nm, with the increase in Bi element content from 0% to 19.4%, the transmission curve (Fig. 3) of the samples decreases significantly and accompanies by redshift, while the absorption curve (Fig. 3) of the samples increases gradually, and the absorption coefficients in the visible wavelength region (the strong absorption region) reach the magnitude of 10⁴-10⁵ cm^-1. The results of the calculation of Tauc's equation show that the optical band gap of the samples decreases from 1.81 eV to 1.14 eV with the increase in Bi content (Table 2), which is caused by the broadening of the energy band of the samples and the decrease in the cohesive energy of the system. The nonlinear absorption curves of the samples obtained from the picosecond Z-scan experiment all show a "valley" shape (Fig. 5), which is a typical feature of the reverse saturation absorption behavior, and the increase in Bi doping leads to the decrease in the "valley depth", which indicates that Bi doping plays an obvious enhancement role in the nonlinear absorption. Finally, the maximum nonlinear absorption coefficient β=3.78×10^-6 m/W is obtained, which is nearly four times higher than that of the original chalcogenide glass (GSS) film. Subsequently, the absorption dynamics curves of the samples obtained by the PO pump-probe method (Fig. 7) demonstrate that the optical nonlinear absorption mechanism of the samples is excited state absorption, and the doping of Bi elements leads to an increase in the first excited state absorption cross-section of the samples up to the order of 10^-19, which is the main reason for the enhancement of the nonlinear absorption, and it leads to the increase in the excited state lifetimes.

GSS thin films with different Bi contents were prepared by co-evaporation technique, and their transmission and absorption spectra showed a trend of redshift. It was found that the inclusion of Bi element significantly enhanced the reverse saturation absorption characteristics of GSS, and its nonlinear absorption coefficient β increased from 0.98×10^-6 m/W to 3.78×10^-6 m/W, which enhanced nearly four times. The enhancement effect was realized by broadening the energy band and reducing the cohesive energy of the system after doping Bi into the GSS system, thus reducing the optical band gap. On this basis, the ultrafast dynamics were investigated by using the PO pump-probe technique and introducing a simplified three-energy-level system. The measured dynamics curves show no "sharp valley" at the zero moment and a long trailing tail afterward, indicating that the nonlinear absorption mechanism of the Bi doped GSS film was excited state absorption, and the excited state lifetime was on the order of ns. The specific value of the first excited state absorption cross section of the samples was also obtained, which reached the order of 10^-19, and the doping of Bi elements, which were less electronegative and weakly electron-binding, increased to achieve the nonlinear enhancement. These results provide a valuable reference for the study of ultrafast dynamics of chalcogenide glass and the development of chalcogenide glass-related photonics devices.