Abstract
One of the methods used to obtain notch filters involves one or several gradient index layers. In this method, the indices are decreased and then increased step by step to create a sinusoidal-like gradient layer. This paper reports a sinusoidal-like gradient layer fabrication method based on solid state diffusion. Al2O3/MgO/Al2O3 (AMA) was deposited by electron beam evaporation and then post-annealed at 800 °C for 4 h. Through inner diffusion, the MgO layers became a low refractive index material with a porous structure (the average refractive index was 1.55) such that the MgAl2O4 spinel was formed as an inhomogeneous layer with an average refractive index of 1.69. This allowed simply using a structured multilayer, (Al2O3/MgO)8 Al2O3, and post-annealing to form a sinusoidal-like gradient layer for a UV notch filter.
Highlights
Optical coatings with a combination of high and low refractive indices have attracted a significant amount of attention for their applications in distributed Bragg reflectors, notch filters, polarizers, antireflection coatings, and band-pass filters, among others
We found that the Kirkendall effect occurs at the interface between Al2 O3 and MgO
2O3 after annealing shows that, after In theFigure annealing process, the voids appeared at MgO/Al the interface of MgO/Al
Summary
Optical coatings with a combination of high and low refractive indices have attracted a significant amount of attention for their applications in distributed Bragg reflectors, notch filters, polarizers, antireflection coatings, and band-pass filters, among others These optical coatings have excellent performance for optical system design if the refractive indices of each layer are appropriate. In 1942, the first experiment studying this phenomenon was performed by Kirkendall This effect refers to a non-reciprocal mutual solid-state diffusion process by an interface between two metals, so that vacancy diffusion occurs to compensate for the inequality of the material flow. Notch filter describes a structure exhibiting a regular cyclic variation of refractive index resembling a sine or cosine wave. Mg atoms diffused into the Al2 O3 layer and combined with the Al2 O3 molecules to form the spine of9,the
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