Fig. 1. Refractive index of different thin film materials. (a) YbF₃; (b) ZnS; (c) Ge

Fig. 2. Theoretical transmittance spectral curve

Fig. 3. Test results. (a) Result after adhesion test; (b) comparison of spectra before and after coating of substrate

Fig. 4. The refractive index of ZnSe

Fig. 5. Design curve and adhesion test results. (a) Design transmittance curve; (b) test result of flat adhesion; (c) test result of concave adhesion

Fig. 6. Schematic of film stress state

Fig. 7. The simulation results of thermal stress on the substrate under different heating modes. (a) Gradient heating for 130 min, and constant temperature for 35 min; (b) direct heating for 130 min and constant temperature for 35 min; (c) direct heating for 165 min; (d) direct heating for 130 min to 200 ℃, constant temperature for 25 min, and start to cool for 10 min

Fig. 8. The simulation results of thermal stress on the substrate under different cooling modes. (a) Vacuum gradient annealing; (b) direct cooling; (c) constant temperature for 20 min, start to cool down; (d) after the coating is finished, start to cool down, keep the temperature at 100 ℃ for 40 min, and continue to cool down

Fig. 9. The simulation results of thermal stress on the thin film under different cooling modes. (a) Vacuum gradient annealing; (b) direct cooling; (c) constant temperature for 20 min, start to cool down; (d) after the coating is finished, start to cool down, keep the temperature at 100 ℃ for 40 min, and continue to cool down

Fig. 10. Sample physical map. (a) Concave result; (b) convex result

Fig. 11. Temperature of thin film changed with time

Fig. 12. Comparison of anti-reflection coating design and test curves