Preparation and Properties of Sol-Gel ZrO₂/SiO₂ Broadband Antireflective Film

Objective Preparing chemical films using the sol-gel method is very important in inertial confinement fusion laser devices. The antireflective (AR) films could be coated on glass elements such as target mirror, anti-splash plate, beam sampling grating, and potassium dihydrogen phosphate/deuterated potassium dihydrogen phosphate (KDP/DKDP) crystal elements to achieve optical performance enhancement at specific wavelengths. The optical elements coated with AR films have a high laser-induced damage threshold (LIDT) due to the porous properties of the films. KDP/DKDP nonlinear optical elements for frequency doubling conversion need to simultaneously and efficiently transmit light with mixed wavelengths, which can be realized by coating different chemical AR films on them. Then, the system energy loss caused by the terminal components will be reduced during the operation of high-power laser devices. Methods ZrO2 and SiO2 sols were prepared by the sol-gel method with zirconium n-propoxide and tetraethoxysilane as precursors. The three-layer "wide-M-type" first harmonic and second harmonic AR film of ZrO2/SiO2 was simulated by TFCalc optical film software and prepared by the dip coating method, and SiO2 double-layer broadband AR film was prepared by the same method for performance comparison. The optical property, refractive index, micro-morphology, and other characteristics of the three-layer "wide-M-type" ZrO2/SiO2 film were measured and analyzed by a UV-Vis spectrometer, a spectroscopic ellipsometer, a scanning electron microscope, and other equipment. Results and Discussions On the basis of an optical principle, the non-quarter-wavelength three-layer "wide-M-type" film system with a refractive index combination of 1. 65/1.42/1.2 on K9 substrate (refractive index is 1. 52) was simulated by TFCalc (Fig. 1). This study selected ZrO2 and SiO2 as the materials of the three-layer sol-gel film, considering the existing sol-gel technology of the research group and simulation results. The transmittance of the uniform three-layer AR film at 527 nm and 1053 nm was about 99. 5%, and the wavelength range where the transmittance was greater than 99% exceeded 150 nm (Fig. 3). This optical performance was significantly better than that of the two-layer AR film, which improved the fault tolerance of the film thickness during the film preparation (Fig. 4). The surface of three-layer AR film was smooth after heat treatment whose root-mean-square roughness was 1. 34 nm (Fig. 5), which could reduce the influence of scattering formed on the film surface on the luminous flux, energy loss, and beam quality in the laser device system. The zero-probability LIDT of the three-layer AR film reached 36. 8 J . cm(-2) (1064 nm, 10. 7 ns) measured by the 1-on-1 LIDT test method, and the result was similar to that of the double-layer film (Fig. 7). The porous property of randomly stacked sol-gel film during film formation made the LIDT of the three-layer film not decrease, though a layer of ZrO2 film was added (Fig. 6). Conclusions ZrO2 sol was prepared with zirconium n-propoxide as the precursor by the Stober method. Based on the existing mature SiO2 sol technology of the research group, the three-layer ZrO2/SiO2 film was prepared on a K9 substrate through simulation and experiment, which could give consideration to the high antireflection at both 1053 nm and 527 nm. The three-layer ZrO2/SiO2 film had good optical properties, surface roughness, and LIDT, which could broaden the range of selecting chemical film materials suitable for high-power devices. In future work, the stability of the three-layer ZrO2/SiO2 film under different environmental conditions needs to be systematically studied for a better understanding of it.