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Self-organized, effective medium black silicon antireflection structures for silicon optics in the mid-infrared

: Steglich, M.; Käsebier, T.; Kley, E.-B.; Tünnermann, A.


Campo, E.M. ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Nanoengineering: Fabrication, Properties, Optics, and Devices XIII : 30-31 August 2016, San Diego, California, United States
Bellingham, WA: SPIE, 2016 (Proceedings of SPIE 9927)
ISBN: 978-1-5106-0245-8
ISBN: 978-1-5106-0246-5
Paper 992704, 6 S.
Conference "Nanoengineering - Fabrication, Properties, Optics, and Devices" <13, 2016, San Diego/Calif.>
Fraunhofer IOF ()

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.