15 September 2016 Self-organized, effective medium black silicon antireflection structures for silicon optics in the mid-infrared
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Thanks to its high quality and low cost, silicon is the material of choice for optical devices operating in the mid-infrared (MIR; 2 μm to 6 μm wavelength). Unfortunately in this spectral region, the refractive index is comparably high (about 3.5) and leads to severe reflection losses of about 30% per interface.

In this work, we demonstrate that self-organized, statistical Black Silicon structures, fabricated by Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE), can be used to effectively suppress interface reflection. More importantly, it is shown that antireflection can be achieved in an image-preserving, non-scattering way. This enables Black Silicon antireflection structures (ARS) for imaging applications in the MIR. It is demonstrated that specular transmittances of 97% can be easily achieved on both flat and curved substrates, e.g. lenses. Moreover, by a combined optical and morphological analysis of a multitude of different Black Silicon ARS, an effective medium criterion for the examined structures is derived that can also be used as a design rule for maximizing sample transmittance in a desired wavelength range. In addition, we show that the mechanical durability of the structures can be greatly enhanced by coating with hard dielectric materials like diamond-like carbon (DLC), hence enabling practical applications.

Finally, the distinct advantages of statistical Black Silicon ARS over conventional AR layer stacks are discussed: simple applicability to topological substrates, absence of thermal stress and cost-effectiveness.
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Martin Steglich, Martin Steglich, Thomas Käsebier, Thomas Käsebier, Ernst-Bernhard Kley, Ernst-Bernhard Kley, Andreas Tünnermann, Andreas Tünnermann, "Self-organized, effective medium black silicon antireflection structures for silicon optics in the mid-infrared", Proc. SPIE 9927, Nanoengineering: Fabrication, Properties, Optics, and Devices XIII, 992704 (15 September 2016); doi: 10.1117/12.2236293; https://doi.org/10.1117/12.2236293

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