Recently, a research team from the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences (CAS) prepared a kind of broadband anti-reflection (AR) laser film based on water bath treatment of electron-beam Al₂O₃ film. The work was published in Optical Materials Express. 

Broadband AR films with graded-refractive index are widely used in many applications. Researchers used many methods for preparing broadband AR coatings, including multilayer AR coatings, glancing angle deposition, biomimetic photonic nanostructure, and so on. However, these methods are limited by factors such as complex fabrication processes, high costs, and difficulty in large-scale implementation. 

Al₂O₃ film is chemically unstable in high temperature water. It has been reported that Al₂O₃ films transform into random microstructure with graded-refractive index and broadband antireflection property is obtained after high temperature water bath treatment. 

In this work, the researchers used electron beam evaporation to deposit Al₂O₃ films and systematically studied the effects of water bath treatment time on the microstructure and AR properties of Al₂O₃ films. 

Results show that the Al₂O₃ film treated in 90 ℃ deionized water for sev minutes has excellent broadband AR performance in the wavelength range of 350-1,100 nm. The average transmittance of the double-sided coated sample is about 98.8% in the wavelength range of 350-1,100 nm.

According to the fitting results by the genetic algorithm, the Al₂O₃ film has a parabola-like refractive index profile. The laser-induced damage threshold of the AR film shows the potential for application in lasers. 

This work was supported by the National Natural Science Foundation of China, the Youth Innovation Promotion Association of CAS and the Strategic Priority Research Program of CAS. 

Transmittance performance of the double-sided coated sample treated for 7 minutes. (Image by SIOM) 

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