Design and Analysis of Two-dimensional Antireflective Sub-wavelength Structured Surface on Sapphire

Two-dimensional sub-wavelength structured surfaces are insensitive to polarized light and can be etched on the surface of substrates for antireflective purposes.Two-dimensional antireflective sub-wavelength structured surface was designed with effective medium theory(EMT) and optical film theory.The impacts of the structure parameters,such as period,filling factor and etching depth,on the reflectance were analyzed.The results showe that the structured surface is of excellent antireflective properties when its effective refractive index and etching depth are equivalent to those of the ideal single-layer anti-reflective thin film respectively.As an example,the sub-wavelength structured surface is designed on sapphire for antireflection over 3 μm-5 μm waveband,and the optimum structure parameters are obtained.With double surfaces designed the anti-reflective sub-wavelength structured surface,the average transmittance over 3 μm-5 μm waveband of the sapphire is as high as 99.25%.