Here we have proposed a simple method for forming antireflective coating. By putting a rf sputtered metallic Al film into boiling ultrapure water, the film becomes transparent. Its optical transmittance exceeds the glass substrate itself. In addition, the glass substrate with such coating on both sides shows almost perfect transmittance at specific wavelengths. By electron diffraction, the crystal structure of the transformed film is confirmed to be boehmite. The refractive index of the transparent film is estimated to be one of the median values of the glass and air.
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