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VACUUM ›› 2022, Vol. 59 ›› Issue (6): 22-28.doi: 10.13385/j.cnki.vacuum.2022.06.04

• Thin Film • Previous Articles     Next Articles

Study on Gradient Index Films Prepared by Thermal Evaporation Technology

LIU Qi1, XU Jun-qi1, SU Jun-hong1, HAN Gang2, LI Yang1, YUAN Song-song1   

  1. 1. Shaanxi Province Key Laboratory of Thin Films Technology and Optical Test, Xi′an Technological University, Xi′an 710021, China;
    2. Xi′an Northern Photoelectric Co., Ltd., Xi′an 710043, China
  • Received:2022-03-11 Online:2022-11-25 Published:2022-12-05

Abstract: The intermediate refractive index material required for preparing gradient refractive index films is obtained by adopting the method of dual-source co-evaporation of MgF2 and ZnS and controlling the deposition rate ratio of both materials. The G|HL|A double-layer anti-reflection film is graded into 5 layers, and keeping the total optical thickness unchanged, the gradient index anti-reflection film with a gradient of 1.91,1.77, 1.64, 1.48, and 1.38(wavelength of 532nm) along the thickness direction is prepared. All samples are tested for optical and laser damage characteristics. The results show that the refractive index of the composite film obtained by double source co-evaporation of MgF2 and ZnS and the deposition rate ratio of the two materials satisfy the relationship deduced according to Drude theory. The refractive index of the composite film accords with the normal dispersion. Except for the material with the refractive index of 1.91, of which the absorption is large(extinction coefficient at 532nm is 0.01), the other composites show small absorption(extinction coefficients at 532nm are less than 0.002). When all the monolayer films are irradiated with 100mJ energy laser, the composite film with the refractive index of 1.64 has the lowest laser damage, showing higher resistance to laser damage than single-component ZnS and MgF2 films. When the double-layer anti-reflection film is equivalent to five layers with graded refractive index, the passband range is extended from 205nm to 380nm, and the laser damage threshold is increased from 2.0J/cm2 to 3.7J/cm2, which is increased by 85%. It can be seen that it is feasible to prepare thin films with graded refractive index by adjusting the evaporation rate ratio of the two materials. As long as the ratio of the two materials is appropriate, using co-evaporation technology can obtain a composite film that has higher resistance to laser damage than a single-component film. The graded index anti-reflection film has better optical properties and higher laser damage threshold than double-layer anti-reflection film.

Key words: graded refractive index, dual-source co-evaporation, anti-reflection film, composite film, LIDT

CLC Number: 

  • O484
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