Fig. 1. Images of optical constant fitting of germanium substrate: (a) The coincidence between the transmittance curve and the fitting result; (b) Fitting results of optical constants of germanium substrate

Fig. 2. Thin film transmission spectrum measurement results: (a) No.1-5 sample; (b) No.6-10 sample

Fig. 3. SEM images of cross-section of YbF₃ film

Fig. 4. Comparison of transmission spectra of sample No.4 and sample No.8

Fig. 5. The coincidence between the measured results and the fitting results of the transmittance curve of No.1-10 thin film samples

Fig. 6. Fitting results of refractive index of samples No.1-10: (a) electron beam heating evaporation; (b) resistance heating evaporation

Fig. 7. Comparison of n values of samples No.2 and No.7

Fig. 8. Fitting results of extinction coefficient of samples No.1-10：（a） electron beam heating evaporation；（b） resistance heating evaporation

Fig. 9. Comparison of k values between sample No.3 and sample No.7

Fig. 10. Stress test results：（a） transverse stress；（b） longitudinal stress

Fig. 11. XRD diffraction spectrum：（a） 1-5 sample；（b） 6-10 sample

Fig. 12. AFM test results of sample No.1-10

Fig. 13. Film transmittance curve：（a） theoretical value；（b） measured value